Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations

You should read the following discussion and analysis of our financial condition and results of operations in conjunction with our consolidated financial statements and related notes appearing elsewhere in this Annual Report on Form 10-K. Some of the information contained in this discussion and analysis or set forth elsewhere in this Annual Report on Form 10-K, such as statements regarding our plans, objectives, expectations, intentions and projections, includes forward-looking statements that involve risks and uncertainties. As a result of many factors, including those factors set forth in the ‘‘Risk Factors’’ section of this Annual Report on Form 10-K, our actual results could differ materially from the results described in, or implied by, these forward-looking statements.

A discussion regarding our financial condition and results of operations for the years ended December 31, 2025 and 2024, including a year-to-year comparison between 2025 and 2024, is presented below. For a full discussion of changes from the year ended December 31, 2024 to the year ended December 31, 2023, refer to Management’s Discussion and Analysis of Financial Condition and Results of Operation in Part II, Item 7. of our Annual Report on Form 10-K for the year ended December 31, 2024 (filed with the SEC on March 13, 2025).

Overview

We are a clinical-stage company focused on improving patient outcomes in medically vulnerable populations through discovery and development of novel live biotherapeutic product, or LBP, candidates. We led the successful development and approval of VOWST, the first FDA-approved orally administered microbiome biotherapeutic and a Breakthrough Therapy designated drug, which was sold to Société des Produits Nestlé S.A., or SPN, and with certain of its affiliates, collectively, Nestlé Health Science, in September 2024. We have established field-leading capabilities and platforms that are powered by best-in-field human data sets to advance a portfolio of products that can uniquely address diseases by targeting host pathways that are modulated by microbes in the human body, and, in particular, diseases associated with mucosal barrier-immune interface targets. We believe clinical and nonclinical data across our programs support the development of LBPs to target the prevention and treatment of a broad swath of infections, and to treat inflammatory and immune, or I&I, diseases. Our pipeline consists of SER-155, SER-603, and SER-147, as well as other potential candidates in earlier development.

Our LBP candidates are consortia of bacteria designed to optimize specific, targeted pharmacological properties, and are formulated for oral delivery. We maintain a differentiated live biotherapeutics drug discovery and development platform that includes good manufacturing practices, or GMP, manufacturing capabilities for this novel drug modality. Our pipeline assets, including SER-155 and SER-603, are designed to target multiple disease-relevant pathways and are manufactured from standard clonal cell banks via cultivation. Our knowledge base and platforms enable selection of bacteria for precision consortia design to drive specific clinical outcomes and further provide unique insights on microbe-associated disease targets. We are designing LBP candidates to modulate host function to increase epithelium integrity and to induce immune homeostasis and tolerance, as well as to prevent the colonization and overgrowth of pathogens in the gastrointestinal, or GI, tract. We believe that the scientific and clinical data from the development of VOWST (our then product candidate SER-109 program) and the data from the SER-155 Phase 1b study (described below) validate our novel therapeutic approach in both infectious disease and I&I diseases. In the context of infection, we believe that our technology may be replicable across different bacterial pathogens with the potential to develop live biotherapeutics to protect a range of medically compromised patients at risk of antimicrobial resistance, or AMR, infections and bloodstream infections, or BSIs, that can result from a compromised epithelial barrier and that can be a major cause of mortality.

SER-155, our most advanced LBP candidate, is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve GI epithelial barrier integrity, and induce immune homeostasis to prevent bacterial BSIs, including those that can harbor antimicrobial resistance, as well as other pathogen-associated negative clinical outcomes in patients undergoing allogeneic hematopoietic stem cell transplantation, or allo-HSCT. In December 2023, we received Fast Track Designation for SER-155 to reduce the risk of infection and GvHD in patients undergoing allo-HSCT, and in December 2024, the FDA granted Breakthrough Therapy designation to SER-155 for the reduction of BSIs in patients 18 years and older undergoing allo-HSCT. In our placebo-controlled Phase 1b study of SER-155 in allo-HSCT, SER-155 was associated with a 77% relative risk reduction in bacterial BSIs and a significant reduction in systemic antibiotic exposure as well as a lower incidence of febrile neutropenia, as compared to placebo through day 100 post HSCT. SER-155 was generally well tolerated, with no observed treatment-related serious adverse events. Importantly, we observed clinical translation on the drug candidate's mechanisms of action, including improvements in epithelial integrity and immune homeostasis. Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT, including the submission of a final protocol to the FDA in January 2026, study site evaluation and qualification with our CRO, and manufacturing of drug substance, we have paused additional investment in that program, preserving the optionality to efficiently restart the study, while continuing to seek funding for the Phase 2 study.

Our current strategy prioritizes advancing our programs that target I&I indications. We have meaningfully advanced over the past decade our scientific understanding of how microbes in the GI functionally modulate pathways at the mucosal barrier-immune interface that are associated with inflammatory and immune-related disease. The clinical data from our SER-155 Phase 1b study in allo-HSCT, along with our extensive preclinical and translational clinical data compiled over the past decade support and inform the advancement of our earlier stage programs targeting I&I diseases. We are evaluating SER-155 in immune checkpoint-related

enterocolitis, or irEC, and we are developing SER-603, broadly in inflammatory bowel disease, or IBD, including ulcerative colitis, or UC, and Crohn's disease. We believe that our LBPs could represent a non-immunosuppressive treatment option for I&I diseases that are linked to colitis and could broadly address immune therapy toxicities, both of which represent significant unmet medical needs and potential commercial opportunities. We are currently exploring potential collaborations related to those I&I disease programs.

We have been collaborating with Memorial Sloan Kettering Cancer Center for over a decade on the impact of the GI microbiome on immune related diseases and cancer; recently this long-standing collaboration included an investigator-sponsored trial, or IST, evaluating SER-155 in 15 participants with irEC. irEC is among the most frequent and severe immune-related adverse events, or irAEs, in recipients of immune checkpoint inhibitor, or ICI, therapy and can be observed in up to 50% of patients, with rates varying based on cancer drug and treatment regimen. ICIs can cause a wide range of irAEs with links to T cell biology and epithelial barrier inflammation, both of which are biological functions shown in our preclinical and clinical pharmacology data to be positively impacted by SER-155. We expect to report initial clinical results, including preliminary safety, efficacy, pharmacology, and exploratory biomarker data in the second quarter of 2026. We believe data from this IST could further support the potential for live biotherapeutics to address a significant unmet need among the large population of cancer patients receiving ICIs and may further support evaluation of our biotherapeutic approach in this setting.

SER-603 is a novel, LBP candidate optimized to address disruptions in the GI microbiome and to improve GI mucosal barrier integrity through the inhibition of inflammatory bacteria and associated metabolites, the promotion of epithelial barrier integrity to reduce the translocation of inflammatory molecules and barrier inflammation, and to induce immune homeostasis through non-immunosuppressive regulatory T cell, or T-reg, induction via T cell signaling. Our research on SER-603 has been primarily supported through a partnership with the Crohn’s and Colitis Foundation, or CCF. These efforts aim to (i) confirm the functional phenotype and inflammatory state of patient subpopulations observed in our prior ulcerative colitis, or UC, clinical trials, and (ii) prioritize inflammatory targets and evaluate the potential to utilize biomarker-based patient selection and stratification for future studies. Many IBD patients experience an efficacy ceiling due to non-response or poor durability of response to existing therapies, and further, most advanced therapies target downstream inflammatory and immune responses and are immunosuppressive leading to toxicities and limitations with respect to combination therapies. IBD is a heterogeneous disease with both disruptions in the GI microbiome and epithelial barrier compromise being important drivers of disease that are not addressed by existing IBD therapies.

We believe that SER-155 and other cultivated live biotherapeutic candidates could be developed in additional patient populations to address barrier compromise and bloodstream and AMR infections beyond allo-HSCT, including autologous-HSCT patients, cancer patients with neutropenia, chimeric antigen receptors therapy, or CAR-T, recipients, individuals with chronic liver disease, or CLD, solid organ transplant recipients, as well as patients in the intensive care unit, or ICU, and long-term acute care facilities. We continue to develop another proprietary live biotherapeutic composition, SER-147, designed to prevent bacterial bloodstream and spontaneous bacterial peritonitis, or SBP, infections in patients with metabolic disease, including CLD. Additionally, we are developing an oral liquid formulation based on SER-155 strains, for dosing in patients who cannot take oral capsules, such as intubated patients in the ICU, and other medically vulnerable patients at high risk of AMR infections, supported by a grant from CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator). We continue to leverage microbiome pharmacokinetic and pharmacodynamic data from across our clinical and preclinical portfolios, using our reverse translational development platform to prioritize future drug targets and to identify opportunities for monotherapy treatment and in combination with existing therapies across various indications, including inflammatory and immune diseases, cancer, and metabolic diseases.

We have built and deploy a reverse translational platform and knowledge base, which we call our MbTx Platform, for the discovery and development of live biotherapeutics, and maintain extensive proprietary know-how that may be used to support future research and development efforts. This platform incorporates high-resolution analysis of human clinical data to identify microbiome biomarkers associated with disease and non-disease states; preclinical screening using human cell-based assays and in vitro/ex vivo and in vivo disease models customized for live biotherapeutics; and a strain library and associated microbiological capabilities that spans broad biological and functional breadth. This platform and knowledge base are integrated through a proprietary knowledge graph and agentic artificial intelligence, enabling rapid identification of specific microbes, microbial genes, and microbial metabolites/peptides associated with disease and the design of therapeutic consortia of bacteria for specific pharmacological properties to restructure the gut microbiome and modulate functional pathways associated with disease. In addition, we own a valuable intellectual property estate related to the development and manufacture of live biotherapeutics.

Since our inception in October 2010, we have devoted substantially all of our resources to developing our programs, platforms, and technologies, building our intellectual property portfolio, developing our supply chain, business planning, raising capital and providing general and administrative support for these operations.

Our product candidates are in early-stage clinical or preclinical development. Our ability to achieve profitability will depend heavily on the successful development and eventual commercialization of one or more of our product candidates. Since our inception, we have incurred significant operating losses. Our net loss from operations was $94.0 million, $121.3 million, and $195.1 million for the years ended December 31, 2025, 2024, and 2023, respectively. As of December 31, 2025, we had an accumulated deficit of $972.4 million.

While we plan to focus our investment on advancing our early-stage live biotherapeutic candidates, starting with SER-603, in the near-term and, subject to receiving additional funding, progressing the development of SER-155 into a Phase 2 study, our expenses may increase in connection with these future activities. See “Risk Factors—Risks Related to Our Financial Position and Need for Additional Capital—We are a clinical-stage company and have incurred significant losses since our inception. We expect to incur losses for the foreseeable future and may never achieve or maintain profitability.”

In addition, if we obtain marketing approval for any of our product candidates, we expect to incur costs related to product manufacturing and commercialization, including marketing, sales and distribution. Furthermore, we expect to continue to incur additional costs associated with operating as a public company.

As a result, we will need additional financing to support our continuing operations. Until such time as we can generate significant revenue from product sales, if ever, we expect to finance our operations through a combination of collaborations with third parties, public or private equity or debt financings or other sources. Adequate additional financing may not be available to us on acceptable terms, or at all. For example, the trading prices for our and other biopharmaceutical companies’ stock have been highly volatile as a result of factors such as the impacts of pandemics, increases in inflation rates, interest rates and tariffs. As a result, we may face difficulties raising capital through sales of our common stock and any such sales may be on unfavorable terms. Our inability to raise capital or secure a partnership as and when needed would have a negative impact on our financial condition and our ability to pursue our business strategy. We will need to generate significant revenue to achieve profitability, and we may never do so.

As of December 31, 2025, we had cash and cash equivalents totaling $45.8 million. Based on our currently available cash resources, and considering our future operating plans, we anticipate that we will require additional funding following the third quarter of 2026. In accordance with applicable accounting standards, we evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about our ability to continue as a going concern within 12 months after the date of the issuance of the consolidated financial statements included elsewhere in this Annual Report on Form 10-K. In performing this analysis, we excluded certain elements of our operating plan that cannot be considered probable of occurring. Under the applicable accounting standards, any future transactions or equity issuances cannot be considered probable, as these events are outside our control. Accordingly, management has concluded that substantial doubt exists about our ability to continue as a going concern for 12 months from the date the consolidated financial statements included elsewhere in this Annual Report on Form 10-K, are issued. See “Risk Factors— Risks Related to Our Financial Position and Need for Additional Capital —We have identified conditions and events that raise substantial doubt regarding our ability to continue as a going concern.”

On September 23, 2025, we announced that we implemented cost reduction actions, including decreasing our workforce by approximately 25%. During the year ended December 31, 2025, we incurred approximately $1.0 million in restructuring costs, primarily related to severance costs. These costs were paid out in the fourth quarter of 2025.

Immunology and Inflammation

Irritable Bowel Disease and Ulcerative Colitis

UC, a form of IBD, is a relapsing-remitting chronic inflammatory disorder affecting the mucosal surface of the colon, leading to episodes of bloody diarrhea, urgency and mucosal inflammation (Danese and Fiocchi, 2011), which generally begins in young adulthood and endures for life. The incidence of UC is rising worldwide, and the prevalence of the disease is highest in the United States, Canada, and Europe. In the United States alone, the prevalence of UC is estimated to be 378 per 100,000, or approximately 1.25 million Americans (Lewis et al., 2023). The severity, extent, and duration of disease are also risk factors for developing colon cancer, which occurs at a rate as high as 0.5-1.0% per year, an important complication given the young age at which the disease strikes. Patients with UC also experience increased risk of Clostridioides difficile infection, or CDI, and primary sclerosing cholangitis, compared to the general population (Dlalal & Allegretti, 2022).

The majority of current medical therapies for the treatment of UC suppress the immune system rather than target reducing the triggers of immune activation and promoting immune tolerance. We believe there remains an unmet need for safer agents with novel non-immunosuppressive mechanisms of action. Moreover, alternative therapy is needed for patients with UC who experience frequent flares, are intolerant to the aminosalicylate class of medication, or where there are safety concerns relating to the use of immunomodulator or steroid therapy.

Current IBD therapies primarily suppress immune responses rather than address upstream drivers of immune activation. Despite advances in biologics and small molecules, many patients fail to achieve durable remission, and a substantial subset experience incomplete response, loss of response over time, or safety limitations associated with chronic immunosuppression. Additionally, current therapeutic approaches in IBD do not address the potential role of microbiome functional disruptions in causing or aggravating disease in IBD. However, not all patients with IBD present with microbiome disruption; many patients with IBD demonstrate comparable taxonomic and functional microbiome diversity to healthy subjects (Lloyd-Price 2019). Similarly, pre-clinical models have shown that microbiomes from patients with IBD drive variable immune responses, with only a subset of microbiomes resulting in inflammation (Hart et al. 2017; Britton et al. 2019). These data suggest that the microbiome may play a role in a subset of subjects with IBD.

Data from our SER-287 Phase 2b study and the first cohort of subjects from our SER-301 Phase 1b study in patients with mild-to-moderate UC suggest that the pharmacodynamic effects observed for SER-287 and SER-301 were greater in a subset of patients. Based on these results, we continue to advance research and development activities supported by partnerships to evaluate the potential to utilize biomarker-based patient selection and stratification in future clinical development efforts in IBD, and to further optimize our live biotherapeutic lead candidates. In October 2023, we were awarded a $500,000 grant from the CCF to leverage our clinical results and biological mechanism insights to functionally characterize subpopulations and to define associated biomarkers for IBD patient selection and stratification of patients where the GI microbiome plays an active role in inflammation and could be modified to reduce colitis. Our preclinical studies conducted to date have recapitulated the patient subpopulation observations from the previously run trials and progressed associated biomarker delineation of the populations, as well as confirmed a microbiome-driven functional link to disease. These research efforts aim to prioritize inflammatory targets for future clinical trials and evaluate the potential to utilize biomarker-based patient selection and stratification for these future studies.

SER-603

SER-603 is an investigational live biotherapeutic designed to improve response rates and durability of remission in patients with IBD, including UC and Crohn's disease. SER-603 is being developed for use as either a stand-alone therapy in mild-to-moderate disease or as a mechanism-distinct adjunctive therapy in combination with biologics or small molecules in moderate-to-severe disease. The program leverages our prior clinical experience in UC, including learnings from SER-287 and SER-301, and incorporates biomarker-driven patient stratification and optimized microbiome conditioning strategies.

SER-603 is in preclinical development, with IND-enabling activities ongoing. The program is supported by translational analyses from prior clinical studies and by preclinical studies that have recapitulated microbiome-defined patient subpopulation observations. We continue to evaluate biomarker-based patient selection approaches and functional characterization strategies to inform future clinical development.

Emerging clinical and preclinical evidence suggests that microbiome functional disruption contributes to disease activity in a defined subset of IBD patients. Observational and translational data from our prior SER-287 Phase 2b study and SER-301 Phase 1b study indicate that pharmacodynamic responses were greater in microbiome-defined subpopulations. These findings, along with external data demonstrating heterogeneity in microbiome-associated inflammatory responses, support the development of targeted, biomarker-informed live biotherapeutic strategies. We have discovered more than fifty GI bacterial features linked to inflammatory outcomes and have nominated and validated microbe associated biomarkers that can predict a response to current advanced therapies for IBD. Leveraging these biomarkers and our integrated preclinical and clinical data sets, SER-603 is optimized to address epithelial barrier dysfunction and microbiome-driven inflammation without systemic immunosuppression.

Given the chronic nature of IBD and continued unmet need for safe, durable, and non-immunosuppressive treatment options, we believe SER-603 has the potential to address a substantial commercial opportunity. SER-603 targets multiple segments of the IBD market, including moderate-to-severe UC (approximately 300,000-400,000 patients in the United States) as combination therapy with current standards of care, moderate-to-severe Crohn's disease in combination therapy setting, and mild-to-moderate UC and Crohn's disease as potential monotherapy.

Immune checkpoint-related enterocolitis (irEC)

IrEC is among the most frequent and severe irAEs in recipients of immune checkpoint-inhibitor therapy and can be observed in up to 50% of patients with rates varying based on cancer drug and treatment regimen. Immune checkpoint inhibitors can cause a wide range of irAEs with links to T cell biology and epithelial barrier inflammation, both of which are biological functions shown in our preclinical and clinical pharmacology data to be positively impacted by SER-155. We believe that supportive data from this study could provide further support for the expansion of indications that may be well suited for our biotherapeutic approach.

Immune checkpoint inhibitors are a commonly used class of drugs that promote anti-tumor immune activity for cancer treatment. irEC represents a growing and underserved market driven by the rapid expansion of ICI use across oncology. Importantly, irEC shares substantial pathophysiologic overlap with inflammatory bowel disease (IBD), including epithelial barrier dysfunction, dysregulated mucosal immunity, and microbiome perturbation. This biological convergence enables clinical, regulatory, and commercial synergies between irEC and IBD development programs. With approximately 500,000 patients using ICI globally, approximately 50,000 to 100,000 are anticipated to experience clinically significant irEC. irEC rates vary, with rates highest in CTLA-4-based and combination therapies (~50%); grade 2+ irEC can lead to discontinuation of therapy, risking further cancer progression.

SER-155 in irEC

SER-155 is currently being evaluated as a first-in-class live biotherapeutic candidate for the treatment of Grade 2–3 immune checkpoint inhibitor-related enterocolitis (irEC) as a result of ICI therapy. The program is designed to promote mucosal healing and modulate inflammation without systemic immunosuppression, with the goal of reducing or eliminating the need for high-dose corticosteroids, which carry the risk of toxicity and ICI efficacy impact. We believe SER-155 could be a first-in-class therapy; current approaches manage toxicity reactively with immunosuppression, which can negatively impact cancer treatment.

We are nearing completion of a single-arm, open-label Phase 1b investigator-sponsored trial conducted with Memorial Sloan Kettering Cancer Center evaluating SER-155 as first-line treatment of irEC. The study was fully enrolled (n=15) in early 2026, and we expect to report initial clinical results, including preliminary safety, efficacy, pharmacology, and exploratory biomarker data in the second quarter of 2026.

Infection Risk Reduction

We continue to be invested in the infectious disease space, with a renewed focus on leveraging our existing clinical data, translational insights, and manufacturing capabilities to support targeted development efforts across a defined set of related indications. We believe that the scientific and clinical data from our VOWST program (our then product candidate SER-109 program) validate our novel approach of using live biotherapeutics to decolonize pathogens and improve epithelial barrier integrity, resulting in reduced rate of infections in medically compromised patients. Data from the ECOSPOR III and ECOSPOR IV Phase 3 trial published in the New England Journal of Medicine (Feuerstadt et al., 2022) and Journal of the American Medical Association(Sims et al., 2023) suggest that live biotherapeutics have the potential to restructure the gut microbiome and shift the gut metabolic landscape. Additional data show that VOWST rapidly reduced the abundance of bacteria associated with common antibiotic resistance genes, or ARGs, and reduced ARG abundance in the gut (Straub et al., 2023). Collectively, we believe these data suggest the potential for live biotherapeutics to prevent the colonization and overgrowth of pathogens that can establish in the gut and ultimately to reduce infections. We believe that reducing pathogen colonization in the GI and improving GI epithelial barrier integrity to reduce the risk of infection may be replicable in a range of medically compromised patients, protecting them from infections and resulting downstream clinical sequelae.

We believe this approach may also enable us to reduce AMR, which the World Health Organization declared as a top ten global public health threat facing humanity, and with estimates that yearly deaths may reach 10 million by 2050, putting mortality due to AMR on par with deaths due to cancer. Recently, two manuscripts were published in Nature Medicine (Bryant et al. 2026) and the Journal of Infectious Diseases (Bryant et al. 2025) highlighting new insights into the functional mechanism and clinical impact of VOWST. The Nature Medicine article, titled “The impact of an oral purified microbiome therapeutic on the GI microbiome”, confirmed our pharmacological hypotheses from earlier VOWST studies, with higher VOWST dosing associated with enhanced pharmacokinetics, as assessed by faster and more robust therapeutic species engraftment in the gut. Treatment also significantly altered the composition of the intestinal microbiome and microbe-associated metabolites, including decreased primary and increased secondary bile acids, as well as elevated short- and medium-chain fatty acids, functional changes that inhibit C. difficile spore germination and vegetative growth. Further, in vitro analyses confirmed that VOWST batches induced production of these metabolites that disrupt C. difficile life cycle and growth. Collectively, these findings support VOWST’s role in restoring microbe-associated metabolic functions critical to preventing CDI recurrence. A complementary publication in the Journal of Infectious Diseases, titled “Comparability of Gastrointestinal Microbiome and Bile Acid Profiles in Patients With First or Multiply Recurrent Clostridioides difficile Infection”, reported a post hoc analysis of the ECOSPOR IV Phase 3 trial, evaluating differences in gastrointestinal microbiome and bile acid profiles between patients experiencing a first recurrence C. difficile infection (frCDI) versus multiply recurrence infection (mrCDI). These data demonstrate that the underlying functional disease etiology is consistent in both first and multiply recurrent CDI patient populations, with VOWST demonstrating similar efficacy and drug pharmacology across the broad patient population.

We believe these data provide important clinical translation and further demonstrate the potential of live biotherapeutics to target specific microbiome functions that are linked to serious disease, including those that are not effectively treated with other drug modalities. The underlying data supporting these publications was developed using Seres MbTx platform, which provides high-resolution assessment of drug pharmacology and functional mechanism of action. These data on bacterial function and pharmacology anchored the preclinical development of SER-155 and inform the continued development of Seres’ pipeline of next-generation live biotherapeutic products.

SER-155 in allo-HSCT

We are developing SER-155, an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve GI epithelial barrier integrity, and induce immune homeostasis to prevent bacterial BSIs as well as other pathogen associated negative clinical outcomes in patients undergoing allo-HSCT. SER-155 is a live biotherapeutic candidate designed to prevent frequent, expensive, and fatal infections in blood cancer patients.

SER-155 contains 16 bacterial strains selected using our reverse translation discovery and development platform technologies to optimize SER-155’s targeted profile. The design incorporates biomarker data from human clinical data and screening data from nonclinical human cell-based assays and in vivo disease models. The bacteria consortia is designed to optimize: (i) the prevention of the growth of various Enterococcaceae and Enterobacteriaceae species known to potentially dominate the GI tract and lead to downstream negative clinical outcomes in medically compromised patients and that can harbor antibacterial resistance, (ii) the production of multiple bacterial metabolites that can promote mucosal and epithelial barrier integrity with the goal of reducing the likelihood of harmful bacteria translocating from the gut to the bloodstream through a compromised epithelium, and (iii) the

production of multiple bacterial metabolites that can modulate immune pathways to induce immune tolerance with a potential impact on GvHD.

The rationale for this program is based in part on published clinical evidence from our collaborators at Memorial Sloan Kettering Cancer Center showing that allo-HSCT patients with decreased diversity of commensal microbes and pathogen domination in the GI tract were significantly more likely to die due to infection and/or lethal GvHD (Peled et al., 2020). There are an estimated 40,000 allo-HSCT procedures annually worldwide, and infection is one of the most common causes of mortality in these patients. The Center for International Blood & Marrow Transplant Research, or CIBMTR, reports that 19-28% of deaths in allo-HSCT patients over 18 years of age within 100 days post-transplant are caused by infections and 5-14% by GvHD. In December 2023, we received Fast Track Designation for SER-155 to reduce the risk of infection and GvHD in allo-HSCT patients. In December 2024, the FDA granted Breakthrough Therapy designation for SER-155 for the reduction of BSIs in patients 18 years and older undergoing allo-HSCT.

SER-155 Phase 1b Study (including placebo-controlled Cohort)

SER-155 has been evaluated in a Phase 1b study in patients undergoing allo-HSCT. The SER-155 Phase 1b study included two cohorts. Cohort 1 was designed to assess safety and drug pharmacology, specifically the drug strain engraftment in the GI tract. Cohort 1 included 13 subjects who received any dosing of the SER-155 regimen, with 11 subjects subsequently receiving an allo-HSCT. Results from this cohort, announced in May 2023, showed SER-155 was generally well tolerated and resulted in successful drug strain engraftment and a reduction in pathogen domination in the GI microbiome relative to a historical control cohort.

Study Cohort 2 utilized a randomized, double-blinded 1:1 placebo-controlled design to further evaluate safety and drug strain engraftment, as well as key secondary and exploratory endpoints such as the incidence of bacterial bloodstream infections and related medical consequences such as febrile neutropenia and antibiotic use. Cohort 2 included 45 patients in the intention-to-treat (ITT) population. Of the ITT population, 20 received SER-155 and 14 received placebo, each of whom subsequently received an allo-HSCT, with data available for clinical evaluation through day 100, the study’s prespecified primary observation point. Exploratory hypothesis testing was conducted at the two-sided α=0.05 level. Ninety-five percent (95%) 2-sided confidence intervals (CIs) were determined, where specified. No adjustment for multiplicity was done. A subset of patient samples was available for drug pharmacology analysis.

The median age in Cohort 2 was 63, and most subjects had acute myeloid leukemia, acute lymphocytic leukemia, myelodysplastic syndrome or myeloproliferative neoplasia as their primary disease and received reduced-intensity conditioning pre-transplant. Most patients received peripheral blood stem cells from a matched unrelated donor. A majority received post-transplant cyclophosphamide as part of their GvHD prophylaxis.

Results from Cohort 2, announced in September 2024, were consistent with the observations from Cohort 1. SER-155 was generally well tolerated, and no treatment-emergent serious adverse events related to drug were observed. SER-155 bacterial strains engrafted into the gastrointestinal tract of patients following the administration of SER-155.

The incidence of BSIs was significantly lower in the SER-155 arm compared with the placebo arm (2/20 (10%) vs. 6/14 (42.9%), respectively; [Odds Ratio: 0.15; 95% CI: 0.01, 1.13, p=0.0423]), which represents a relative risk reduction of approximately 77% and an absolute risk reduction of approximately 33%. In addition, while treatment antibiotic starts were similar in each arm, patients administered SER-155 were treated with antibiotics for a significantly shorter cumulative duration compared to patients in the placebo arm (9.2 days vs. 21.1 days, respectively, with a mean difference of -11.9 days [95% CI: -23.85, -0.04; p=0.0494]). The incidence of febrile neutropenia was lower in patients administered SER-155 compared to placebo (65% vs. 78.6%, respectively; [Odds Ratio: 0.51; 95% CI: 0.07, 2.99; p=0.4674]). Six cases of GI infections (C. difficile infections) were observed in the study, with four cases (20%) in the SER-155 arm and two cases (14.3%) in the placebo arm.

Recent changes in the allo-HSCT standard of care and the increasing use of post-transplant cyclophosphamide as part of prophylactic therapy for GvHD have reduced rates of GvHD overall in this patient population. The rates of GvHD in the study were low, with two cases of grade 2 GvHD observed in each arm, and no cases of grade 3 or 4 GvHD were observed.

In Cohort 2, the ability to detect pathogen domination (i.e., relative abundance in the GI ≥30%) in the placebo arm, and differences between the study arms, was constrained due to the limited number of placebo stool samples and an imbalance in the number of available stool samples between the arms. Observed pathogen domination events were low in the placebo and SER-155 arms with no significant differences identified. In a comparison of the prevalence of pathogen domination versus a larger allo-HSCT historical control cohort, pathogen domination in SER-155 subjects was substantially lower, providing further evidence of SER-155 activity.

We believe the available study data from Cohort 1 suggest that SER-155 administration results has the potential to significantly lower incidence rates of gastrointestinal dominations with pathogens of clinical concern, such as Enterococcaceae, Enterobacteriaceae, Streptococcaceae, and Staphylococcaceae. We further believe the resulting Cohort 2 data, together with the Cohort 1 SER-155 Phase 1b study results provide encouraging evidence to support further development of SER-155 to potentially reduce GI associated bloodstream and AMR infections as well as increase immune tolerance in individuals undergoing allo-HSCT for cancers and other serious conditions.

Proposed SER-155 Phase 2 Study

The SER-155 Phase 2 study will incorporate a well-powered, placebo-controlled design, which provides for a planned interim analysis to enable an expedited initial data readout. The SER-155 Phase 2 study is expected to enroll approximately 248 participants and incorporate an adaptive design and an interim data analysis when approximately half of the enrolled participants have reached the primary endpoint. We expect to obtain the interim clinical results within twelve months following study initiation, which we believe will facilitate timely engagement with the FDA on the design of a Phase 3 study and inform development in adjacent medically vulnerable patient populations. We believe that positive results, if achieved, from the Phase 2 study could enable advancement into a single Phase 3 trial to support registration.

Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT, including the submission of a final protocol to the FDA in January 2026, study site evaluation and qualification with our CRO, and manufacturing of drug substance, we have paused additional investment in that program while continuing to seek funding for the Phase 2 study.

We continue our efforts to obtain capital and other resources to support further development of SER-155 and our broader portfolio of live biotherapeutic product candidates with applications for inflammatory diseases. We are evaluating a range of potential deal structures that we believe could leverage our live biotherapeutics expertise and success, as demonstrated by bringing VOWST from early development through FDA approval.

In October 2025, we presented new post hoc data from our SER-155 Phase 1b trial in an oral presentation at IDWeek in Atlanta, Georgia. The presentation included new post-hoc analysis from the completed SER-155 Phase 1b study describing differences between the SER-155 and placebo groups, including the bacterial and fungal organisms causing BSIs, BSI event clinical outcomes, antibacterial prophylaxis use, and patterns of AMR among the bacterial BSI organisms. These new data illustrated that BSIs occurred despite antibacterial prophylaxis, and that BSI bacteria exhibited AMR. Resistance to multiple antibacterial agent classes was observed only in the BSI bacteria from placebo-treated participants, two of whom had fatal outcomes related to their BSIs. These new data further support the potential of SER-155 as an innovative alternative approach to the significant unmet medical need for prevention of BSIs in HSCT patients, especially those BSIs associated with AMR that increases the risk of morbidity and mortality.

Exploratory biomarker data

In January 2025, we reported exploratory translational biomarker data from the SER-155 Phase 1b study which provided evidence supporting the intended therapeutic mechanisms, including promotion of intestinal epithelial barrier integrity to reduce the potential of bacterial translocation into the bloodstream, and reduction of systemic inflammatory responses. Results from this exploratory biomarker analysis showed that SER-155 was associated with lower levels of fecal albumin and lower concentrations of various plasma biomarkers associated with systemic inflammation (i.e., IFN-y, TNF-α, IL-17, and IL-8) in the HSCT peri-transplant period, the period from the end of the first SER-155 treatment course through to neutrophil engraftment. The results support SER-155’s intended mechanisms of action and reinforce the previously reported promising clinical study efficacy and safety data. These systemic inflammatory response observations further support the potential to develop our live biotherapeutics to address inflammatory and immune diseases, including ulcerative colitis and Crohn’s disease.

In February 2025, clinical and biomarker results from our biotherapeutic programs were presented as a poster at the 2025 Tandem Transplantation & Cellular Therapy Meetings of the American Society for Transplantation and Cellular Therapy (ASTCT) and Center for International Blood and Marrow Transplant Research (CIBMTR). SER-155 Phase 1b clinical study data were also featured in an oral presentation in the Best Abstracts in Infectious Diseases track at the Tandem meeting.

In April 2025, we presented SER-155 Phase 1b clinical and exploratory biomarker results at the 51st annual meeting of the European Society for Blood and Marrow Transplantation, or EBMT. Our presented poster was recognized by the EBMT scientific organizing committee and obtained the “Best Clinical Poster Award.”

In May 2025, we presented data at the Digestive Disease Week, or DDW conference highlighting preclinical and clinical data that enable identification of patients with a disease etiology linked to the gastrointestinal microbiome, and the identification of microbiome-based biomarkers that are predictive of response and suitable for patient selection and stratification in clinical trials. Our poster, entitled “Candidate Biomarkers of Microbiome Disruption for Patient Selection or Stratification in Clinical Trials of Microbiome Therapies in Ulcerative Colitis” received a Poster of Distinction award in the Microbiome and Microbial Therapies subgroup. We believe that the data generated suggest that live biotherapeutics could provide a novel treatment modality that could benefit patients living with gut-related inflammatory and immune diseases that are not effectively addressed today. Furthermore, research indicates that specific patient subpopulations optimally suited for biotherapeutic-based treatments may be identifiable. We are exploring options, including potential partnerships, to advance the development of our biotherapeutics in inflammatory and immune diseases, including ulcerative colitis and Crohn’s disease.

In May 2025, we presented new exploratory biomarker data from the SER-155 Phase 1b study in a poster session at the 2025 American Society of Clinical Oncology, or ASCO, Annual Meeting. The biomarker data presented at ASCO demonstrate the potential of SER-155 to promote immune reconstitution following allo-HSCT by modulating homeostatic cytokines and peripheral T-cell expansion. In post hoc analyses from the SER-155 Phase 1b study, significantly higher levels of the homeostatic cytokine IL-7 were

observed both after the second course of SER-155 (administered after neutrophil recovery) and at HSCT Day 100, as compared to placebo. Additionally, a higher frequency of CD4+ T cells was observed in peripheral blood at these same timepoints in the SER-155 arm. We believe the results support the ability of SER-155 to promote peripheral T-cell recovery and immune reconstitution to support favorable outcomes post allo-HSCT.

We believe that exploratory biomarker data presented at recent medical meetings have supported the intended mechanisms of SER-155 and demonstrated the broader potential of live biotherapeutics in inflammatory and immune mediated diseases.

Oral Live Biotherapeutic Product - Liquid Formulation (LBP-LF)

In July 2025, we were awarded a grant from CARB-X to support the development of an oral liquid formulation of an LBP based on SER-155 (LBP-LF) for medically vulnerable patient populations at risk of BSIs, including AMR infections, who cannot be dosed with oral capsules, such as intubated patients in the ICU. The CARB-X grant provides us with up to $3.6 million of funding for research, manufacture, and design of a Phase 1 clinical trial in ICU patients. LBP-LF is designed to target the prevention of bloodstream infections in medical ICU patients by Escherichia coli and other gut-derived bacteria capable of harboring antibiotic resistance. Up to 50% of all preventable medical ICU deaths have been attributed to infections with E. coli and other gut-derived bacteria (Mayr, 2006). These infections are also the leading cost in the medical ICU (Neidell, 2012). When ICU patients with multidrug resistant, or MDR, infections survive hospitalization, they have high long-term morbidity with over 20% 30-day readmission rates (Chang, 2015; Mayr, 2017). Over 5 million patients are admitted to ICUs in the U.S. annually, and these admissions account for approximately 20% of all acute care hospitalizations (Barrett et al. 2024).

Infections with pathogenic, often MDR, bacteria are the leading cause of mortality in the medical ICU, causing up to 9 deaths for every 100 ICU patients admitted (Vincent, 2009). Most patients are admitted to the medical ICU with a known or suspected infection (i.e., sepsis) but, with targeted or empiric antibiotics, most recover from this initial infection. Once in the ICU, secondary, healthcare-associated infections frequently develop during the prolonged recovery from sepsis and are a significant driver of mortality. LBP-LF is a novel approach that addresses both gut colonization and subsequent translocation by E. coli and other gut-derived pathogens to prevent a significant proportion of these secondary hospital acquired infections. LBP-LF is in preclinical development, with IND-enabling activities ongoing and IND-readiness targeted by the end of 2026. Development of LBP-LF is ongoing with grant support from CARB-X and in collaboration with Columbia University.

SER-147

We are also developing another proprietary live biotherapeutic composition, SER-147, designed to prevent bacterial bloodstream and spontaneous bacterial peritonitis, or SBP, infections in patients with metabolic disease, including chronic liver disease, or CLD. SER-147 was designed and optimized using our reverse translational therapeutics development platform. CLD is a progressive condition marked by deterioration of liver function and is reaching epidemic proportions affecting nearly 1.7 billion people worldwide, causing substantial health burden on afflicted countries (GBD 2017 Cirrhosis Collaborators, 2020, Clinical Liver Disease, 2021).

In the advanced stages of CLD, known as decompensated cirrhosis, patients exhibit significant immune dysfunction, microbiome disruption, and increased contact with the healthcare system, all of which drive increased susceptibility to bacterial infections such as SBP and BSIs (Bajaj et al., 2021, Albillos et al., 2022). Over 40% of patients with decompensated cirrhosis experience an infection within the first year of diagnosis. Antibiotics are the only prophylactic option for patients at high risk of infections like SBP, resulting in exposure to antibiotics for months or years. To combat increasing rates of AMR, antibacterial prophylaxis for primary SBP is no longer recommended for the majority of patients outside of very high-risk, leaving significant unmet need. Many cultivated live biotherapeutics currently in clinic are constrained by formulation technologies incompatible with concomitant medications commonly used in CLD.

SER-147 is in preclinical development. The program is ready to progress to IND-enabling activities, including manufacturing, in order to advance to clinical development.

Nasdaq Notice and Compliance

On November 7, 2024, we received a letter from the Listing Qualifications Department of The Nasdaq Stock Market LLC, or Nasdaq, notifying us that, for the last 30 consecutive business days, the bid price for our common stock had closed below the $1.00 per share minimum bid price requirement for continued inclusion on The Nasdaq Global Select Market pursuant to Nasdaq Listing Rule 5450(a)(1), or the Bid Price Requirement.

On April 21, 2025, we effected a 1-for-20 reverse stock split of our common stock. Trading of our common stock on The Nasdaq Global Select Market commenced on a split-adjusted basis on April 22, 2025 and we regained compliance with the Bid Price Requirement.

All shares of our common stock, stock-based instruments, and per-share data included in this Annual Report on Form 10-K have been retroactively adjusted as though the reverse stock split had been effected prior to all periods presented.

Intellectual Property

Patent Portfolio

We have an extensive patent portfolio directed to rationally designed ecologies of spores and microbes. The portfolio includes both company-owned patents and applications, and those that we have rights to as licensee. The patents and applications included in our portfolio cover both composition of matter and methods (e.g., method of treating). Our intellectual property rights related to SER-155 and SER-147 extend through 2043 (not including any potential term extension). We plan on continuing to broaden our patent portfolio. Currently, we have 22 active patent families, which includes 18 nationalized applications and three at the provisional stage. To date, we have obtained issuance of 33 U.S. patents (which includes three as licensee). Of the issued U.S. patents, 13 U.S. patents (including one as licensee) have been assigned to Nestlé Health Science as part of its purchase of VOWST.

In connection with the Transaction and pursuant to the Purchase Agreement, we transferred certain patents and trademarks affiliated with the VOWST Business to SPN at Closing. In addition, in connection with Closing, we entered into a cross-license agreement, or the Cross-License Agreement, with SPN. Under the Cross-License Agreement, we granted to SPN a perpetual, worldwide, non-exclusive, fully paid-up license under certain Seres patents that have been issued or will issue in the future and current know-how controlled by us that was not transferred to SPN pursuant to the Purchase Agreement. In the field of the treatment of CDI and recurrent CDI and associated complications, or collectively, the CDI Field, the license to SPN under such Seres patents and know-how is exclusive to SPN for five years after the Closing and co-exclusive between SPN and Seres following that five year period. The license from Seres to SPN is to issued Company patents that currently or in the future cover the Product or improvements thereof and know-how that is used or reasonably useful in connection with the exploitation of the VOWST Business. We also granted SPN an exclusive, perpetual, worldwide, fully paid-up license under issued Seres patents that currently or in the future cover the Product and improvements thereof and know-how that is used or reasonably useful in connection with the exploitation of the Product to exploit SER-262 in the CDI Field. SPN granted to us a perpetual, worldwide, non-exclusive license under the patents and know-how that are transferred to SPN pursuant to the Purchase Agreement or developed under the TSA, for Seres' products for use outside of the CDI Field, and after five years from Closing for Seres products containing designed, cultivated, bacterial consortia not manufactured using human stool (excluding SER-262) in the CDI Field. From and after Closing, certain license agreements between us, SPN, and/or their respective affiliates terminated and are of no further force or effect, except as contemplated by the Purchase Agreement.

Regulatory Exclusivity

If we obtain marketing approval for any of our product candidates, we expect to receive reference product exclusivity against biosimilar products.

Financial Operations Overview

Revenue

To date we have not generated any revenues from the sale of products. Our revenues have been derived primarily from our agreements with our collaborators. See “–Liquidity and Capital Resources.”

Operating Expenses

Our operating expenses since inception have consisted primarily of research and development activities and general and administrative costs. In connection with the TSA we entered into with NESA following the sale of the VOWST Business during the third quarter of 2024, our operating expenses also consisted of certain passthrough costs incurred in performing duties under the TSA and manufacturing services related to the VOWST Business and operations.

Research and Development Expenses

Research and development expenses consist primarily of costs incurred for our research activities, including our discovery efforts and the development of our product candidates and other obligations, which include:

•

expenses incurred under agreements with third parties, including contract research organizations, or CROs, that conduct research, preclinical activities and clinical trials on our behalf as well as contract manufacturing organizations and other third parties that manufacture or test drug products for use in our preclinical and clinical trials;

•

salaries, benefits and other related costs, including stock-based compensation expense, for personnel in our research and development functions;

•

costs of outside consultants, including their fees, stock-based compensation and related travel expenses;

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the cost of laboratory supplies and acquiring, developing and manufacturing preclinical study and clinical trial materials;

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costs related to compliance with regulatory requirements;

•

facility-related expenses, which include direct depreciation costs and allocated expenses for rent and maintenance of facilities and other operating costs; and

•

labor and passthrough costs, reimbursable by Nestlé, incurred in performing duties under the TSA.

We expense research and development costs as incurred. We recognize external development costs based on an evaluation of the progress to completion of specific tasks using information provided to us by our vendors and our clinical investigative sites. Payments for these activities are based on the terms of the individual agreements, which may differ from the pattern of costs incurred, and are reflected in our consolidated financial statements as prepaid or accrued research and development expenses.

Our primary focus of research and development since inception has been on our reverse translational platform and the subsequent development of our product candidates. Our direct research and development expenses are tracked on a program-by-program basis and consist primarily of external costs, such as fees paid to investigators, consultants, CROs in connection with our preclinical studies and clinical trials, lab supplies and consumables, and regulatory fees. We do not allocate employee-related costs and other indirect costs to specific research and development programs because these costs are deployed across multiple product programs under development.

Research and development activities are central to our business model. Product candidates in later stages of clinical development generally have higher development costs than those in earlier stages of clinical development, primarily due to the increased size and duration of later-stage clinical trials. We anticipate an overall decrease in research and development expenses in 2026 as compared to 2025, following the conclusion of the TSA, completion of the Phase 1b study of SER-155 in allo-HSCT, and the cost reduction actions that were implemented in September 2025 and February 2026, including pausing investment in the SER-155 Phase 2 study and headcount reductions. Research and development expenses may increase in the future if and as we resume development of any clinical or preclinical programs. In 2025, research and development expenses included labor and passthrough costs, reimbursable by Nestlé, incurred in performing obligations under the TSA. Given the conclusion of the TSA as of December 31, 2025, these costs and related reimbursements are not expected to reoccur in 2026.

General and Administrative Expenses

General and administrative expenses consist primarily of salaries and other related costs, including stock-based compensation for personnel in our executive, finance, commercial, business development and administrative functions. General and administrative expenses also include legal fees relating to patent and corporate matters; professional fees for accounting, auditing, tax and consulting services; insurance costs; travel expenses; and facility-related expenses, which include direct depreciation costs and allocated expenses for rent and maintenance of facilities and other operating costs. In 2025, general and administrative expenses included labor and passthrough costs, reimbursed by Nestlé, incurred in performing obligations under the TSA.

We expect that our general and administrative expenses will decrease in 2026 as compared to 2025, following the conclusion of the TSA, reduction of our workforce and overall cost containment efforts. General and administrative expenses may increase in the future as we continue to incur increased expenses associated with being a public company, including increased costs of accounting, audit, legal, regulatory and tax-related services associated with maintaining compliance with exchange listing rules and the requirements of the SEC, director and officer insurance costs and investor and public relations costs.

Manufacturing Services

Under the TSA with NESA, beginning in the fourth quarter of 2024, we provided certain manufacturing services and related functions of the VOWST Business and operations. Expenses associated with the manufacturing services include certain facility-related, labor, lab supplies and consumables, and other manufacturing costs that would have been capitalized into inventory prior to the sale of VOWST Business.

We provided the manufacturing services until December 31, 2025. We do not expect to incur any expenses related to manufacturing services related to the VOWST business in the future, as the TSA and other Transaction-related obligations concluded on December 31, 2025.

Other (Expense) Income, Net

Interest Income

Interest income consists of interest earned on our cash, cash equivalents and investments.

Interest Expense

Interest expense consists of interest incurred under our loan and security agreement, or the Hercules Loan and Security Agreement, with Hercules Capital, Inc., or Hercules.

Other (Expense) Income, Net

Other (expense) income, net primarily consists of:

•

sublease income;

•

amortization of premiums or accretion of discounts on investments;

•

gains and losses on foreign currency transactions;

•

changes in the fair values of our warrant liabilities associated with our prior credit facility with Oaktree;

•

loss associated with the extinguishment of our prior credit facility with Oaktree;

•

the amount Nestlé paid for costs associated with PRMS manufacturing; and reimbursement for certain labor and other passthrough costs of the transition services performed by the Company under the TSA; and

•

gains or losses associated with the change in the Company's accrued liabilities due to SPN - related party.

Discontinued Operations

We completed the sale of the VOWST Business to SPN on September 30, 2024. The financial results of the VOWST Business have been classified as discontinued operations in the consolidated statements of operations and the related assets and liabilities of the VOWST Business have been classified as assets and liabilities of discontinued operations in the consolidated balance sheets. Unless otherwise noted, amounts and disclosures in this section relate to our continuing operations (except for the Liquidity and Capital Resources section).

Income Taxes

Since our inception in 2010, we have not recorded any U.S. federal or state income tax benefits for the net losses we have incurred in each year or our earned research and development tax credits, due to our uncertainty of realizing a benefit from those items. As of December 31, 2025, we had federal and state NOL carryforwards of $616.4 million and $597.8 million, respectively, both of which begin to expire in 2035. As of December 31, 2025, we also had federal and state research and development tax credit carryforwards of $46.2 million and $10.0 million, respectively, net of uncertain tax position reserves, which begin to expire in 2031 and 2028, respectively. The federal research and development tax credits include an orphan drug credit carryforward of $25.9 million.

Critical Accounting Policies and Significant Judgments and Estimates

Our consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States. The preparation of our consolidated financial statements requires the application of appropriate technical accounting rules and guidance, as well as the use of estimates. The application of these policies necessarily involves judgments regarding future events. These estimates and judgments, in and of themselves, could materially impact the consolidated financial statements and disclosures based on varying assumptions. We believe that the estimates and assumptions involved in the accounting policies described below may have the greatest potential impact on our consolidated financial statements and, therefore, consider these to be our critical accounting policies. We evaluate our estimates and assumptions on an ongoing basis. Our actual results may differ from these estimates under different assumptions and conditions.

Accrued Research and Development Expenses

As part of the process of preparing our consolidated financial statements, we are required to estimate our accrued research and development expenses. This process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated costs incurred for the services when we have not yet been invoiced or otherwise notified of the actual costs. The majority of our service providers invoice us in arrears for services performed, on a pre-determined schedule or when contractual milestones are met; however, some require advance payments. We make estimates of our accrued expenses as of each balance sheet date in our financial statements based on facts and circumstances known to us at that time, which include information from our CROs and CMOs reported to us on a periodic basis. Examples of estimated accrued research and development expenses include fees paid to:

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CROs in connection with performing research services on our behalf and clinical trials;

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investigative sites or other providers in connection with clinical trials;

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vendors in connection with preclinical and clinical development activities; and

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vendors related to product manufacturing, development and distribution of preclinical and clinical supplies.

We base our expenses related to preclinical studies and clinical trials on our estimates of the services received and efforts expended pursuant to quotes and contracts with multiple CROs that conduct and manage clinical trials on our behalf. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to our vendors will exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of patients and the completion of clinical trial milestones. In accruing service fees, we estimate the time period over which services will be performed, enrollment of patients, number of sites activated and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from our estimate, we adjust the accrual or amount of prepaid expense accordingly. Although we do not expect our estimates to be materially different from amounts actually incurred, our understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and may result in us reporting amounts that are too high or too low in any particular period. To date, we have not made any material adjustments to our prior estimates of accrued research and development expenses.

Results of Operations

Comparison of the Years Ended December 31, 2025 and 2024

The following table summarizes our results of operations for the years ended December 31, 2025 and 2024.

Year EndedDecember 31,

(in thousands)

Revenue:

Operating expenses:

Other income:

Revenue

Total revenue was $0.8 million for the year ended December 31, 2025 consisting of costs reimbursable under the CARB-X grant that we were awarded in the third quarter of 2025.

Research and Development Expenses

Year Ended December 31,

(in thousands)

Personnel-related (including stock-based compensation) 21,553 28,689 (7,136 )

Research and development expenses were $49.1 million for the year ended December 31, 2025, compared to $64.6 million for the year ended December 31, 2024. The decrease of $15.5 million was due primarily to the following:

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a decrease in personnel-related costs of $7.1 million, primarily due to a decrease in salaries, bonuses, employee benefits expenses, and stock-based compensation expense due to lower headcount,

•

a decrease of $5.4 million in expenses related to our live biotherapeutics platforms and research and development operations comprised of a decrease of $3.3 million of facilities and depreciation expense primarily due to the impairment charge that we recorded in the first quarter of 2024 related to our idled donor collection facility in Cambridge, Massachusetts, as well as decreases of $0.7 million of consulting and other costs, $0.6 million of clinical storage costs, and $0.8 million of lab assays; and

•

a decrease of $3.1 million in expenses related to our SER-155 program due to lower clinical trial costs of $3.7 million as the Phase 1b study has been completed, partially offset by an increase of $0.3 million in lab supplies and consumables due to the commencement of manufacturing of clinical trial material for the planned Phase 2 study and an increase of $0.3 million of regulatory consulting.

General and Administrative Expenses

Year EndedDecember 31,

(in thousands)

General and administrative expenses were $39.2 million for the year ended December 31, 2025, compared to $53.2 million for the year ended December 31, 2024. The decrease of $14.0 million was primarily due to the following:

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a decrease in personnel-related costs of $8.3 million primarily due to a decrease in salaries, bonus, employee benefit expenses and stock-based compensation expenses due to lower headcount;

•

a decrease in professional fees of $0.8 million primarily due to a decrease of $1.1 million related to lower audit, tax, and other consulting services, partially offset by a $0.3 million increase in legal fees; and

•

a decrease in facility-related and other costs of $4.9 million primarily related to headcount-based information technology costs that were reduced as a result of lower headcount, as well as impairment charges recorded in the first quarter of 2024.

Manufacturing Services

Manufacturing services were $6.5 million for the year ended December 31, 2025, compared to $3.5 million for the year ended December 31, 2024. The TSA with Nestlé was effective in the fourth quarter of 2024, while 2025 reflects a full year of manufacturing services. The expenses are associated with the PRMS manufacturing performed on behalf of Nestlé, including labor, materials, allocated facility-related, lab supplies and other manufacturing costs that would have been capitalized into inventory prior to the sale of VOWST Business.

Other Income (Expense), Net

Other income (expense), net was $99.7 million of income for the year ended December 31, 2025 compared to $4.5 million of expense for the year ended December 31, 2024. The change in other income (expense), net was primarily due to the $75.0 million of installment payments received from Nestlé in January and July 2025, which were conditioned on our material compliance with obligations under the TSA, an increase of $7.0 million of reimbursement income from Nestlé associated with the performance of TSA services, and $0.6 million increase in sublease income. The increases were partially offset by a decrease in interest income of $1.7 million due to our lower cash balance. Additionally, the year ended December 31, 2024 included a $23.4 million loss associated with the extinguishment of the prior credit facility with Oaktree recorded in connection with the sale of the VOWST Business to SPN.

Liquidity and Capital Resources

Since our inception, we have generated revenue only from collaborations and have incurred recurring net losses from operations. We anticipate that we will continue to incur losses for at least the next several years. We will need additional capital to fund our operations, which include our research and development and general and administrative expenses, which we may obtain from additional financings, public offerings, research funding, additional collaborations, contract and grant revenue or other sources.

On August 5, 2024, we entered into the Purchase Agreement with SPN, pursuant to which we agreed to sell our VOWST Business to SPN and its designated affiliates, and SPN and its designated affiliates assumed certain liabilities from us. The Transaction closed on September 30, 2024 following stockholder approval. As consideration for the Transaction, SPN agreed to pay us:

(i)

a cash payment, which was paid at Closing, of $100 million, less approximately $17.9 million owed by us to an affiliate of SPN as of March 31, 2024 under the prior license agreement between us and the SPN affiliate, less approximately CHF 2.0 million in satisfaction of fees due under an existing manufacturing agreement between us and Bacthera;

(ii)

cash installment payments of $50 million, which was received on January 15, 2025 and $25 million, which was received on July 1, 2025, (offset by $1.4 million paid by us to Nestlé on July 1, 2025 related to certain employment obligations assumed by SPN, as described below), conditioned on our material compliance with obligations under the TSA entered into at Closing between us and NESA;

(iii)

prepayment of the $60 million Prepaid Milestone tied to the achievement of the First Sales Milestone of worldwide annual net sales of the Product of $150 million, which was paid in cash at Closing, which Prepaid Milestone will accrue interest at a fixed rate of 10% per annum until the First Sales Milestone is achieved and 5% per annum thereafter until the earlier of (x) the date on which the Prepaid Milestone, plus accrued interest thereon, has been repaid in full by set-off and (y) the last day of the Milestone Period; and

(iv)

future Milestone Payments of (x) $125 million tied to the achievement of worldwide annual net sales of the Product of $400 million and (y) $150 million tied to the achievement of worldwide annual net sales of the Product of $750 million, during the Milestone Period from Closing until December 31 of the calendar year in which the tenth anniversary of Closing occurs.

As they are earned, the Milestone Payments will be satisfied as follows: (i) first, by set-off against all accrued interest on the Prepaid Milestone until the amount of such accrued interest has been paid in full, (ii) second, by set-off against the outstanding balance of the Prepaid Milestone until the Prepaid Milestone has been repaid in full and (iii) thereafter, in cash. If any amount of the Prepaid Milestone (and any accrued interest thereon) remains outstanding as of following the last day of the Milestone Period (defined below), the balance thereof (together with any interest accrued thereon) will be forgiven and the right of set-off of SPN with respect thereto will be deemed forfeited. The installment payment received on July 1, 2025 was offset by the $1.4 million we paid to Nestlé on the same date related to certain employment obligations assumed by SPN through the period prior to the Closing Date.

As a condition to Closing, we and SPN entered into the Securities Purchase Agreement, pursuant to which SPN purchased 714,285 shares of Common Stock at Closing, at a purchase price per share of $21.00, for an aggregate purchase price of $15.0 million.

In May 2021, we entered into a Sales Agreement, or the Sales Agreement, with Cowen and Company, LLC, or Cowen, to sell shares of our common stock with aggregate gross sales proceeds of up to $150.0 million, from time to time, through an "at-the-market" equity offering program under which Cowen acts as sales agent. During the year ended December 31, 2025, we sold 739,545 shares of common stock under the Sales Agreement, at an average price of approximately $18.48 per share, raising aggregate net proceeds of approximately $13.2 million after deducting an aggregate commission of approximately 3%. During the year ended December 31, 2024, we sold 1,030,846 shares of common stock under the Sales Agreement, at an average price of approximately $23.67 per share, raising aggregate net proceeds of approximately $23.5 million after deducting an aggregate commission of approximately 3% and other issuance costs.

As of December 31, 2025, we had cash and cash equivalents totaling $45.8 million and an accumulated deficit of $972.4 million. For the year ended December 31, 2025, we incurred a net loss from operations of $94.0 million, and used cash in operations of $73.9 million, net of $75.0 million in milestone payments received from Nestlé. We expect that our operating losses and negative cash flows will continue for the foreseeable future.

Under applicable accounting standards, we have the responsibility to evaluate whether conditions or events raise substantial doubt about our ability to meet our future financial obligations as they become due within 12 months after the date the consolidated financial statements are issued. The ability to obtain sufficient proceeds from additional equity offerings, collaborations or other financing with terms favorable or acceptable to us cannot be considered probable, as these events are outside of our control. Based on our currently available cash resources, and considering our future operating plans, we anticipate that we will require additional funding following the third quarter of 2026. Accordingly, management has concluded that these circumstances raise substantial doubt about our ability to continue as a going concern. Substantial doubt about our ability to continue as a going concern may materially and adversely affect the price per share of our common stock, and it may be more difficult for us to obtain financing or enter into a partnership. If potential collaborators or partners decline to do business with us or potential investors decline to participate in any future financings due to such concerns, our ability to increase our cash position may be limited. We will need to generate significant revenues to achieve profitability, and we may never do so. Because of the numerous risks and uncertainties associated with the development of our current and any future product candidates, the development of our platform and technology and because the extent to which we may enter into collaborations with third parties for development of any of our product candidates is unknown, we are

unable to estimate the amounts of increased capital outlays and operating expenses required for completing the research and development of our product candidates.

Cash Flows

The following table summarizes our sources and uses of cash, cash equivalents and restricted cash for the years ended December 31, 2025 and 2024.

Year Ended December 31,

(in thousands)

Cash provided by (used in) operating activities $ 1,117 $ (148,609 )

Cash (used in) provided by investing activities $ (42 ) $ 142,293

Cash provided by (used in) financing activities $ 13,898 $ (90,372 )

Operating Activities

During the year ended December 31, 2025, net cash provided by operating activities was $1.1 million, primarily due to net income of $5.7 million and non-cash charges of $23.2 million, partially offset by changes in our operating assets and liabilities of $27.8 million. Non-cash charges consisted of stock-based compensation expense of $10.8 million, $8.4 million related to the amortization of right-of-use assets, $4.1 million of depreciation and amortization, partially offset by a $0.2 million gain on sale of property and equipment. Changes in our operating assets and liabilities during the year ended December 31, 2025 consisted of a decrease in accrued liabilities due to SPN of $14.5 million, a decrease in operating lease liabilities of $8.7 million, a decrease in accrued expenses and other liabilities of $6.5 million, a decrease in accounts payable of $2.4 million, and an increase in accounts receivable relating to the CARB-X program of $0.1 million, partially offset by a decrease in prepaid expenses and other current and other non-current assets of $2.7 million and a decrease in accounts receivable due from SPN - related party of $1.7 million.

During the year ended December 31, 2024, net cash used in operating activities was $148.6 million, primarily due to non-cash charges of $83.2 million and changes in our operating assets and liabilities of $65.5 million. Non-cash charges consisted of the $146.7 million gain on sale of VOWST Business and $0.5 million decrease in the fair value of warrants, partially offset by stock-based compensation expense of $21.0 million, $9.2 million related to the amortization of right-of-use assets, $5.5 million of depreciation and amortization, $1.4 million of amortization of debt issuance costs, $23.4 million of loss associated with the extinguishment of the prior credit facility with Oaktree, $0.3 million loss on disposal of fixed assets, and $3.3 million of impairment charges related to our long-lived assets. Changes in our operating assets and liabilities during the year ended December 31, 2024 consisted of a $1.0 million decrease in prepaid expenses and other current and non-current assets, a decrease in inventories of $33.8 million in connection with the sale of VOWST Business to SPN, a $2.9 million decrease in accounts payable, a decrease in deferred income - related party of $4.1 million, a decrease in collaboration receivable - related party of $8.7 million, a decrease in operating lease liabilities of $6.3 million, and a $10.2 million decrease in accrued expenses and other liabilities, partially offset by an increase in accrued liabilities due to SPN - related party of $15.7 million and an increase in accounts receivable due from SPN - related party of $2.1 million. The decrease in inventories, increase in accrued liabilities due to SPN - related party, increase in receivables due from SPN - related party, decrease in deferred income – related party and decrease in collaboration receivable – related party were all related to the sale of the VOWST Business. The decrease in operating lease liabilities was due to the cash payment of lease obligations and the decrease in accounts payable and accrued expenses were the results of payments to vendors and changes in the business following the sale of the VOWST Business.

Investing Activities

During the year ended December 31, 2025, net cash used in investing activities was $0.1 million, consisting of $0.3 million of purchases of property and equipment, partially offset by $0.2 million proceeds from the sale of property and equipment.

During the year ended December 31, 2024, net cash provided by investing activities was $142.3 million, primarily due to $141.3 million of proceeds from the sale of the VOWST Business and $1.4 million sale of a restricted investment relating to a security deposit on one of our leases that was reclassified as restricted cash, partially offset by $0.4 million of purchases of property and equipment.

Financing Activities

During the year ended December 31, 2025, net cash provided by financing activities was $13.9 million, consisting of $13.2 million from the issuance of common stock under our at the market equity program, net of issuance costs, $0.5 million from the exercise of stock options, and $0.2 million from the issuance of common stock under our 2015 Employee Stock Purchase Plan, or ESPP.

During the year ended December 31, 2024, net cash used in financing activities was $90.4 million, consisting of $127.9 million repayment of the prior credit facility with Oaktree, partially offset by $23.5 million from the issuance of common stock under our at the market equity program, net of issuance costs, $13.5 million from the issuance of common stock in connection with the sale of the VOWST Business to SPN, and $0.5 million from the issuance of common stock under our 2015 ESPP.

Funding Requirements

Our expenses may increase in connection with our ongoing clinical development activities and research and development activities. In addition, we expect to continue to incur additional costs associated with operating as a public company. We anticipate that our future expenses will increase if and as we:

•

invest in our early-stage pipeline product candidates, including SER-603 in IBD and SER-155 in irEC;

•

conduct clinical trials for SER-155 in allo-HSCT and other medically vulnerable populations;

•

make strategic investments in manufacturing capabilities;

•

maintain and augment our extensive proprietary live biotherapeutic drug development know-how that may be used to support future research and development efforts, including our intellectual property portfolio and intellectual property that we may opportunistically acquire;

•

establish a sales and distribution infrastructure and scale-up manufacturing capabilities to commercialize any other products for which we may obtain regulatory approval;

•

perform our obligations under any agreements with collaborators;

•

seek to obtain regulatory approvals for our product candidates; and

•

experience any delays or encounter any issues with any of the above, including but not limited to failed studies, complex results, safety issues or other regulatory challenges.

Because of the numerous risks and uncertainties associated with the development of our product candidates, we are unable to estimate the amounts of increased capital outlays and operating expenses associated with completing the research and development of our product candidates. Our future capital requirements will depend on many factors, including:

•

the cost of conducting clinical trials for our lead candidate, SER-155 in allo-HSCT and other targeted indications, and other product candidates in our pipeline;

•

the total amount of the Milestone Payments we may receive from the Transaction,

•

the cost of manufacturing our product candidates;

•

the scope, progress, results and costs of preclinical development, laboratory testing and clinical trials for our product candidates;

•

the costs, timing and outcome of regulatory review of our product candidates and research activities;

•

the costs, timing and revenue, if any, of potential future commercialization activities, including manufacturing, marketing, sales and distribution, for any of our product candidates for which we receive marketing approval;

•

the costs and timing of preparing, filing and prosecuting patent applications, maintaining and enforcing our intellectual property rights and defending any intellectual property-related claims;

•

the effect of competing technological and market developments; and

•

the extent to which we acquire or invest in businesses, products and technologies, including entering into licensing or collaboration arrangements for product candidates.

Identifying potential product candidates and conducting preclinical testing and clinical trials is a time-consuming, expensive and uncertain process that takes years to complete, and we may never generate the necessary data or results required to obtain marketing approval for our current or future product candidates and achieve product sales. In addition, our product candidates, if approved, may not achieve commercial success. Additionally, part of our commercial revenues, if any, will be derived from sales of product candidates that we do not expect to be commercially available for many years, if ever. Accordingly, we will need to obtain substantial additional funds to achieve our business objectives.

Adequate additional funds may not be available to us on acceptable terms, or at all. Additionally, market volatility resulting from macroeconomic conditions, or other factors could also adversely impact our ability to access capital as and when needed. To the

extent that we raise additional capital through the sale of equity or convertible debt securities, our stockholders’ ownership interest will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect our stockholders’ rights as common stockholders. Any debt financing and preferred equity financing, if available, may involve agreements that include, covenants limiting or restricting our ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends. Additional debt or preferred equity financing may also require the issuance of warrants, which could potentially dilute our stockholders’ ownership interest.

If we raise additional funds through collaborations, strategic alliances or licensing arrangements with third parties, in addition to our existing collaboration agreements, we may have to relinquish valuable rights to our technologies, future revenue streams, research programs, or product candidates or grant licenses on terms that may not be favorable to us. If we are unable to raise additional funds through equity, debt financings, or collaborations when needed, we may be required to delay, limit, reduce or terminate our product development programs or any potential future commercialization efforts or grant rights to develop and market product candidates that we would otherwise prefer to develop and market ourselves.

As discussed in Note 1 of the consolidated financial statements included elsewhere in this Annual Report on Form 10-K, we have the responsibility to evaluate whether conditions or events raise substantial doubt about our ability to meet our future financial obligations as they become due within 12 months after the date the consolidated financial statements are issued. The ability to obtain sufficient proceeds from additional equity offerings, collaborations or other financing with terms favorable or acceptable to us cannot be considered probable, as these events are outside of our control. Accordingly, management has concluded that these circumstances raise substantial doubt about our ability to continue as a going concern. Based on our currently available cash resources, and considering our future operating plans, we anticipate that we will require additional funding following the third quarter of 2026.

Contractual Obligations and Commitments

We have entered into arrangements that contractually obligate us to make payments that will affect our liquidity and cash flows in future periods. Such arrangements include those related to our lease commitments.

Lease Commitments

Our lease commitments reflect payments due under our operating lease agreements for our corporate headquarters, office and laboratory space, that expire between November 2028 and April 2033. As of December 31, 2025, our contractual commitments for our leases were $118.4 million, of which $20.0 million is expected to be paid within one year, and $98.4 million will be paid over the remaining term of such leases. We do not have any commitment for leases that had not yet commenced as of December 31, 2025. For additional information on our leases and timing of future payments, please read Note 6, Leases, to the consolidated financial statements included in this Form 10-K.

Profit Sharing Payments

We and SPN shared 50/50 in the net profit or net loss achieved during the period from the Closing Date until December 31, 2025, or the Profit Sharing Period. During the Profit Sharing Period, we reimbursed SPN for (i) certain payments under the exclusive license agreement between us and Memorial Sloan Kettering Cancer Center, (ii) certain costs incurred in connection with an ongoing post-marketing safety study of VOWST and (iii) 80.1% of all rent and other costs due to the landlord under the lease for our Waltham facility. As of December 31, 2025, the remaining liability associated with all of these obligations was $3.3 million, all of which is expected to be paid within one year.

Other Obligations

We enter into contracts in the normal course of business with CROs for clinical trials, preclinical research studies and testing, manufacturing and other services and products for operating purposes. These contracts generally provide for termination upon notice, and therefore we believe that our non-cancelable obligations under these agreements are not material.

Recently Issued and Adopted Accounting Pronouncements

For a discussion of recent accounting standards see Note 2, Summary of Significant Accounting Policies, to our consolidated financial statements included in this report.

Item 7A. Quantitative and Qualitative Disclosures about Market Risk

Interest Rate Fluctuation Risk

We are exposed to market risk related to changes in interest rates.

As of December 31, 2025, our cash and cash equivalents consisted of cash in operating bank accounts. Our interest income is sensitive to changes in the general level of U.S. interest rates. However, because of the short-term nature of the instruments in our portfolio, an immediate 10% change in market interest rates would not have a material impact on our financial position or results of operations.

Item 8. Financial Statements and Supplementary Data

The financial statements required to be filed pursuant to this Item 8 are appended to this report. An index of those financial statements is found in Item 15.

Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure

None.

Item 9A. Controls and Procedures

Limitations on Effectiveness of Controls and Procedures

In designing and evaluating our disclosure controls and procedures, management recognizes that any controls and procedures, no matter how well designed and operated, can provide only reasonable assurance of achieving the desired control objectives. In addition, the design of disclosure controls and procedures must reflect the fact that there are resource constraints and that management is required to apply judgment in evaluating the benefits of possible controls and procedures relative to their costs.

Evaluation of Disclosure Controls and Procedures

Our management, with the participation of our principal executive officer and our principal financial officer, has evaluated the effectiveness of our disclosure controls and procedures (as defined in Rules 13a-15(e) and 15d-15(e) under the Securities Exchange Act of 1934, as amended), as of the end of the period covered by this Annual Report on Form 10-K. Based on such evaluation, our principal executive officer and principal financial officer concluded that as of December 31, 2025, our disclosure controls and procedures were effective at the reasonable assurance level.

Changes in Internal Control Over Financial Reporting

There were no changes in our internal control over financial reporting that occurred during the three months ended December 31, 2025 that have materially affected, or are reasonably likely to materially affect, our internal control over financial reporting.

Management’s Annual Report on Internal Control Over Financial Reporting

Our management is responsible for establishing and maintaining adequate internal control over our financial reporting, as such term is defined in Rule 13a-15(f) and 15d-15(f) under the Securities Exchange Act of 1934, as amended.

Our management conducted an assessment of the effectiveness of our internal control over financial reporting based on the criteria set forth in “Internal Control – Integrated Framework (2013)” issued by the Committee of Sponsoring Organizations of the Treadway Commission.

Based on this assessment, our management concluded that, as of December 31, 2025, our internal control over financial reporting was effective.

This Annual Report on Form 10-K does not include an attestation report of our independent registered public accounting firm regarding internal control over financial reporting. As we are a non-accelerated filer, management's report is not subject to attestation by our independent registered public accounting firm.

Item 9B. Other Information

Disclosure in lieu of reporting on a Current Report on Form 8-K.

None.

Insider Trading Arrangements and Policies.

On November 25, 2025, Teresa L. Young, Ph.D., our Chief Commercial and Strategy Officer, adopted a Rule 10b5-1 plan (the “Young 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 31,680 shares of common stock and (2) the net number of shares of common stock underlying 1,152

unvested restricted stock units after giving effect to the number of shares to be sold outside of the Young 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and March 12, 2027, unless otherwise terminated according to its terms.

On November 26, 2025, Kelly Brady, M.S., our Executive Vice President, Chief Operating Officer, adopted a Rule 10b5-1 plan (the “Brady 10b5-1 Plan”) intended to satisfy the affirmative defense of Rule 10b5-1(c) of the Exchange Act. The plan provides for the periodic sale of (1) up to 7,611 shares of common stock and (2) the net number of shares of common stock underlying 797 unvested restricted stock units after giving effect to the number of shares to be sold outside of the Brady 10b5-1 Plan to satisfy tax withholding obligations (which net amount is not currently determinable) between March 17, 2026 and November 17, 2026, unless otherwise terminated according to its terms.

Other than the foregoing, no director or officer of the Company adopted or terminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408 of Regulation S-K.

Item 9C. Disclosure Regarding Foreign Jurisdictions That Prevent Inspections

Not applicable.

PART III

Item 10. Directors, Executive Officers and Corporate Governance

Director Biographical Information

Name Age Position

Dennis A. Ausiello, M.D. (3)(4) 80 Director

Stephen Berenson (1)(3) 65 Director

Willard H. Dere, M.D. (1)(4) 72 Director

Claire M. Fraser, Ph.D. (1)(4) 70 Director

Kurt C. Graves (2) 58 Director

Richard N. Kender 70 Executive Chair and Interim Chief Executive Officer

Robert Rosiello (2) 68 Director

Eric D. Shaff 50 Director

Hans-Juergen Woerle, M.D., Ph.D. (4) 60 Director

(1)

Member of the audit committee.

(2)

Member of the compensation and talent committee.

(3)

Member of the nominating and corporate governance committee.

(4)

Member of the science and clinical development committee.

Dennis A. Ausiello, M.D., has served as a member of our board of directors since April 2015. Dr. Ausiello has served as the Jackson Distinguished Professor of Clinical Medicine at Harvard Medical School and Director, Emeritus of Harvard Medical School’s M.D./Ph.D. Program since 1996, Chair of Medicine, Emeritus, and Director of the Center for Assessment Technology and Continuous Health (CATCH) at Massachusetts General Hospital, which he co-founded, since 2012, and Physician-in-Chief Emeritus at Massachusetts General Hospital since 2013. From 1996 to April 2013, Dr. Ausiello served as the Chief of Medicine at Massachusetts General Hospital. Dr. Ausiello is a member of the Institute of Medicine of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences. Dr. Ausiello has served on the board of directors of Alnylam Pharmaceuticals since April 2012, and previously served as Vice Chairman of the board of directors of Spexis AG, a clinical-stage biopharmaceutical company, from December 2021 to December 2025 and on the board of directors of Pfizer Inc. from 2006 to 2020, where he currently serves on the advisory board since 2019. Dr. Ausiello also serves on the boards of directors of numerous privately held companies. Dr. Ausiello received a B.A. in Biochemistry from Harvard College and an M.D. from the University of Pennsylvania. We believe that Dr. Ausiello is qualified to serve on our board of directors because of his extensive experience as a physician and as a director of pharmaceutical companies.

Stephen A. Berenson has served as a member of our board of directors since August 2019 and served as our Chairman between December 2019 and March 2026. Mr. Berenson is an Advisor Partner at Flagship Pioneering, a life sciences innovation firm which conceives, creates, resources and develops first-in-category bioplatform companies, after serving as Managing Partner since June 2017. Prior to Flagship, Mr. Berenson spent 33 years in various roles as an investment banker at J.P. Morgan, most recently serving in the role of Vice Chairman of Investment Banking from 2005 to April 2017, where he focused on providing high-touch strategic advice and complex transaction execution to leading companies across all industries globally. He was co-founder of J.P. Morgan’s Global Strategic Advisory Council and co-founder of the firm’s Board Initiative. Mr. Berenson has served on the board of directors of Inari, a privately held agricultural company, since January 2024. He previously served as chairman of the board of directors of Cellarity, a privately held pharmaceutical company between July 2021and December 2025, as chairman of SAIL Biomedicines, a privately held pharmaceutical company, between August 2024 and December 2025, and on the board of directors of Moderna, Inc., a pharmaceutical and biotechnology company, between October 2017 and August 2024. Mr. Berenson received an S.B. in Mathematics from the Massachusetts Institute of Technology. We believe that Mr. Berenson is qualified to serve on our board of directors because of his extensive experience working with rapidly-growing companies across various industries.

Willard H. Dere, M.D., has served as a member of our board of directors since July 2017. Dr. Dere has served as Chief Advisor to the Chief Executive Officer and Chief Medical Officer of Angita Bio, a biotechnology company, since July 2022. Dr. Dere has also been Professor Emeritus, Department of Internal Medicine, at the University of Utah School of Medicine since July 2022. From November 2014 until June 2022, Dr. Dere held multiple roles at the University of Utah Health Sciences Center, including Associate Vice President for Research, Co-Director of the Utah Clinical and Translational Science Institute, and Co-Director of the Center for Genomic Medicine. Prior to his professorship, from 2003 until 2014, Dr. Dere worked at Amgen, where he was Senior Vice President and head of Global Development, and led development programs in multiple therapeutic areas. From 1989 to 2014, he worked at Eli Lilly and led multiple development programs, and also worked in clinical pharmacology, regulatory affairs and safety. Dr. Dere has served on the boards of directors of BioMarin Pharmaceutical, Inc. since 2016 and Metagenomi, Inc. since August 2021, and

previously served on the boards of directors of Mersana Therapeutics, Ocera Therapeutics and Radius Health. Dr. Dere received his B.A. in History and Zoology and M.D. from the University of California, Davis, completed his internal medicine residency training at the University of Utah, and his postdoctoral training in endocrinology and metabolism at the University of California, San Francisco. We believe Dr. Dere is qualified to serve on our board of directors due to his extensive academic experience and his knowledge of the biotechnology industry.

Claire M. Fraser, Ph.D., has served as a member of our board of directors since January 2023. Dr. Fraser has been a faculty member at the University of Maryland School of Medicine in Baltimore, Maryland for the past 19 years and is the Founding Director of the Institute for Genome Sciences and Professor Emerita of Medicine and Microbiology and Immunology. From 1998 to 2007, she served as President and Director of The Institute for Genomic Research, a not-for-profit research organization engaged in human and microbial genomics studies. Dr. Fraser has served on the Board of Directors of Becton, Dickinson, and Company, a medical technology company, since 2006, and previously served as the Chair of the Board and a director of the American Association for the Advancement of Science. Dr. Fraser received her bachelor’s degree in Biology from Rensselaer Polytechnic Institute, her Ph.D. in Pharmacology from State University of New York-Buffalo and is an elected member of both the National Academy of Sciences and the National Academy of Medicine. We believe Dr. Fraser is qualified to serve on our board of directors due to her extensive academic experience and her knowledge of the microbiome industry.

Kurt C. Graves has served as a member of our board of directors since November 2015. Mr. Graves has served as the Chairman, President and Chief Executive Officer of i20 Therapeutics, Inc., a biotechnology company, since August 2023, and as a director since August 2021. He previously served as the Executive Chairman of i20 Therapeutics' board of directors from August 2021 to August 2023. Mr. Graves was previously the Chairman, President and Chief Executive Officer of Intarcia Therapeutics, Inc., a biotechnology company, from September 2010 to December 2020, and on its board of directors from August 2010 to December 2020. Previously, he served as Executive Vice President, Chief Commercial Officer and Head of Strategic Development at Vertex Pharmaceuticals Inc., or Vertex, from July 2007 to October 2009. Prior to joining Vertex, Mr. Graves held various senior leadership positions at Novartis Pharmaceuticals Corporation, or Novartis Corp., from 1999 to June 2007, including the Global General Medicines Business Unit Head and Global Chief Marketing Officer for the pharmaceuticals division of Novartis Corp. from September 2003 to June 2007. Prior to Novartis Corp., Mr. Graves held senior leadership positions at Merck and Astra-Merck where he led the U.S. Business Unit responsible for Prilosec, Nexium and Prilosec OTC over a 10-year period. He served as Chairman on the board of directors of Radius Health, Inc. from May 2011 to March 2020, and as a director on Achillion Pharmaceuticals, Inc., or Achillion, from June 2012 to January 2020, when Achillion was acquired. Mr. Graves received a B.S. in Biology from Hillsdale College. We believe Mr. Graves is qualified to serve as a member of our board of directors because of his extensive experience in the life sciences industry, membership on various boards of directors and his leadership and management experience.

Richard N. Kender has served as our Executive Chair and Interim Chief Executive Officer since March 2026 and has been a member of our board of directors since October 2014. From October 1978 to September 2013, Mr. Kender held positions in a variety of corporate areas at Merck & Co., Inc., or Merck, a pharmaceutical company, most recently serving as Senior Vice President of Business Development and Corporate Licensing. Mr. Kender has served on the board of directors of Longeveron Inc. since May 2024. He previously served on the boards of directors of Poxel S.A. between March 2015 and July 2025, Bicycle Therapeutics PLC between July 2019 and June 2025, INC Research Holdings, Inc. (now known as Syneos Health) between December 2014 and August 2017, Abide Therapeutics, Inc., a privately held company, between December 2015 and May 2019, Omega Therapeutics between June 2024 and April 2025, and ReViral Ltd., a privately held company, from November 2019 to June 2022. Mr. Kender received a B.S. in Accounting from Villanova University and an M.B.A. from Fairleigh Dickinson University. We believe Mr. Kender is qualified to serve on our board of directors because of his finance experience and knowledge of the biotechnology industry.

Robert Rosiello has served as a member of our board of directors since August 2025. Mr. Rosiello is an Executive Partner at Flagship Pioneering, which he joined in 2018 and where he focuses on building capability to help originate, manage, and grow new Flagship companies. From September 1984 to June 2015, Mr. Rosiello worked at McKinsey & Company advising CEOs and boards of leading health care, technology, and consumer companies. He served as a senior partner for 18 years and was a member of McKinsey’s Senior Partner Review and Compensation Committees. Mr. Rosiello led the work that shifted McKinsey’s recruiting toward non-MBAs and developed innovative leadership training for McKinsey’s senior partners. He also led significant pro bono work for Save the Children, Americares, Carnegie Corporation, and Fairfield University. From July 2015 to August 2016, Mr. Rosiello was both Executive Vice President and Chief Financial Officer at Valeant Pharmaceuticals, where he led the finance, human resources, and IT functions. He led Valeant’s financial restatement and regained timely reporting status with the SEC. Mr. Rosiello has served on the Board of the Marine Biological Laboratory, a privately held company, since July 2024, New England Conservatory of Music, a privately held company, since July 2024 and SANA Biotechnology since June 2025. Mr. Rosiello previously served on the boards of Axcella Health, Inc. between October 2022 and November 2023, Evelo Biosciences between April 2023 and November 2023, Inari Agriculture, a privately held company between September 2015 and December 2021 and Omega Therapeutics, Inc between October 2024 and January 2025. Mr. Rosiello also previously served on the Board and Executive Committee of Catholic Charities of New York, between March 2012 and December 2025. Mr. Rosiello received his B.A. in economics from the University of North Carolina, where he was a Morehead Scholar and graduated Phi Beta Kappa, an M.Sc. in economics from the London School of Economics, and an M.B.A. from Harvard

Business School. We believe that Mr. Rosiello’s extensive business and financial experience qualifies him to serve on our board of directors.

Eric D. Shaff has served as a member of our board of directors since January 2019. Mr. Shaff has been the President and Chief Executive Officer of PsiThera, Inc. since September 2025. Previously, he served as our President and Chief Executive Officer between January 2019 and July 2025 and as our Chief Operating and Financial Officer and Executive Vice President between January 2018 and January 2019 and as our Chief Financial Officer between November 2014 and January 2019. From January 2012 to November 2014, Mr. Shaff was Vice President of Corporate Finance for Momenta Pharmaceuticals, or Momenta, a biotechnology company, where he helped manage Momenta’s accounting, finance, planning, and procurement functions, as well as contributing to Momenta’s investor relations efforts. Prior to Momenta, Mr. Shaff held a number of corporate development and finance positions with Genzyme Corporation, a biotechnology company, most recently as Vice President of Finance/Controller for the Personalized Genetic Health division. Mr. Shaff previously served on the board of directors of Sigilon Therapeutics, Inc. from 2017 to August 2023. Mr. Shaff received his B.A. from the University of Pennsylvania and his M.B.A. from Cornell University. We believe Mr. Shaff is qualified to serve on our board of directors because of his extensive business and finance experience and his knowledge of the biotechnology industry.

Hans-Juergen Woerle, M.D., Ph.D., has served as a member of our board of directors since February 2025. Dr. Woerle has served as Chief Medical Officer and Chief Scientific Officer at Nestlé Health Science S.A. since November 2018, where he is responsible for global research and development strategy. Dr. Woerle served on the boards of directors of Cerecin Inc., a clinical-stage biotechnology company, from June 2020 to September 2024 and Enterome, SA, a clinical-stage biopharmaceutical company from June 2020 to March 2025. Dr. Woerle is a board-certified physician and a specialist in internal medicine and endocrinology, holding an adjunct professorship at University of Ulm. Dr. Woerle earned his bachelor's degree, master's degree and medical degree from Ludwig Maximilian University. We believe Dr. Woerle is qualified to serve on our board of directors because of his extensive experience as a physician and in clinical research and development.

Information about our Executive Officers

Name Age Position

Richard N. Kender 70 Executive Chair and Interim Chief Executive Officer

Matthew Henn, Ph.D. 51 President and Chief Scientific Officer

Kelly Brady, M.S. 42 Executive Vice President, Chief Operating Officer

Thomas J. DesRosier 71 Executive Vice President, Chief Legal Officer

Marella Thorell 58 Executive Vice President, Chief Financial Officer

Kelly Brady, M.S. has served as our Executive Vice President, Chief Operating Officer since March 2026. She served as our Senior Vice President, Clinical Development from January 2023 to March 2026, and, prior to this, served as our Vice President, Clinical Operations, Executive Director, Clinical Operations, and Senior Director, Clinical Operations between June 2018 and January 2023. Prior to joining the Company in 2018, Ms. Brady was Senior Director and Global Clinical Program Lead at Akebia Therapeutics, Inc., where she oversaw pivotal programs in anemia due to chronic kidney disease. She also held roles of increasing responsibility in operations and program management at Acetylon Pharmaceuticals, Inc., with the company’s eventual acquisition by Celgene Corporation. Earlier in her career, she served as the Global Phase 3 Clinical Operations Leader at Millennium Pharmaceuticals, Inc. (later Takeda Oncology), executing pivotal global oncology trials for ADCETRIS®, and as a member of the operations team at Osiris Therapeutics, Inc., managing global graft‐versus‐host disease studies for the world’s first approved stem‐cell therapy Prochymal. Ms. Brady holds a B.S. in Neuroscience from Lafayette College and an M.S. in Biotechnology from Johns Hopkins University.

Thomas J. DesRosier has served as our Chief Legal Officer, Executive Vice President, and Secretary since May 2016. He has also served as our Co-President/Chief Executive Officer from July 2025 to March 2026. From 2015 to 2016, Mr. DesRosier served as Executive Vice President, Chief Legal and Administrative Officer and Secretary of ARIAD Pharmaceuticals, Inc., a biopharmaceutical company, from 2014 to 2015 as Executive Vice President, Chief Legal and Administrative Officer and Secretary of Cubist Pharmaceuticals, Inc., or Cubist, a biopharmaceutical company, and from 2013 to 2014 as Senior Vice President, Chief Legal Officer and Secretary of Cubist. Before that, Mr. DesRosier served as Senior Vice President, General Counsel North America of Sanofi from 2011 to 2013. From 1999 to 2011, Mr. DesRosier held leadership roles of increasing seniority within the legal group of Genzyme Corporation, a biotechnology company, culminating in his role as Senior Vice President, Chief Legal Officer. Mr. DesRosier has served as a member of the board of directors of Avanir Pharmaceuticals, a privately held company and wholly-owned subsidiary of Otsuka Pharmaceutical Company, Ltd., since June 2017. Mr. DesRosier earned a B.A. in Chemistry from the University of Vermont and a J.D. from Wake Forest University School of Law.

Matthew Henn, Ph.D., has served as our President and Chief Scientific Officer since March 2026. He has also served as our Executive Vice President and Chief Scientific Officer from February 2019 to March 2026. Dr. Henn has been involved in the discovery and development of multiple live biotherapeutics, including two breakthrough therapy designated biologics, across infectious, inflammatory, and oncology indications, and has authored over 75 peer-reviewed publications. He has extensive research experience in microbiology, genomics, and computational biology, as well as in drug pharmacology and clinical translation. His research has focused on microbial populations and the functional role of microbes in both environmental and human disease applications, and the development of genomic and functional screening technologies to study these populations. Prior to joining Seres in 2012, he was the Director of Viral Genomics and Assistant Director of the Genome Sequencing Center for Infectious Diseases at the Broad Institute of Massachusetts Institute of Technology and Harvard. He has served on various National Institutes of Health, or NIH, working groups on antimicrobial resistance and microbiome research, as a scientific advisor for NIH’s Viral Pathogen Bioinformatics Resource Center, as an ad-hoc reviewer and editor of various peer-reviewed journals, and as a scientific advisor to non-profit and for-profit organizations. He currently serves on the Microbiome Therapeutics Innovation Group board of directors, the World Microbiome Partnership steering committee, and Life Sciences Cares board of advisors. Dr. Henn is formally trained in ecology and evolutionary biology and earned his Ph.D. in ecosystem sciences from the University of California at Berkeley, where he was a NASA Earth Systems Sciences Fellow, and trained as a National Science Foundation Postdoctoral Fellow in Microbiology at Duke University.

Marella Thorell has served as our Executive Vice President and Chief Financial Officer since March 2024. She has also served as our Co-President/Chief Executive Officer from August 2025 to March 2026. From September 2022 to December 2023, Ms. Thorell served as the Chief Financial Officer and Treasurer of Evelo Biosciences, Inc., a biotechnology company. From January 2021 to July 2022, she served as Chief Accounting Officer and previously as Head of Finance at Centessa Pharmaceuticals PLC, or Centessa, a pharmaceutical company. In that role, she led the establishment of Centessa’s finance operations, led its public company readiness activities in connection with its initial public offering, and oversaw accounting operations. Previously, from October 2019 to December 2020, Ms. Thorell served as Chief Financial Officer at Palladio Biosciences, a biotechnology company, prior to its acquisition by Centessa. Before that, Ms. Thorell spent over ten years at Realm Therapeutics PLC, a biopharmaceutical company,

serving in various roles of increasing responsibility, including Chief Financial Officer and Chief Operating Officer. Ms. Thorell served on the board of directors and as the Audit Committee Chair of Carisma Therapeutics, a biopharmaceutical company, from June 2024 through December 2025 and previously served on the boards of directors of ESSA Pharma Inc., a pharmaceutical company from July 2019 to October 2025, and Vallon Pharmaceuticals, Inc., a pharmaceutical company, from February 2021 until its reverse-merger with GRI Bio in April 2023. Ms. Thorell holds a B.S. in Business from Lehigh University.

Teresa L. Young, Ph.D., has served as our Executive Vice President, Chief Commercial and Strategy Officer since June 2020. Previously, Dr. Young served as Vice President, Global Commercial Strategy at Sage Therapeutics from March 2018 to June 2020, where she led development of Sage’s global commercial capabilities, including global marketing, insights and analytics and new product planning. Prior to that, she held commercial leadership roles of increasing responsibility at Bristol-Myers Squibb from November 2010 to March 2018, culminating in her role as Vice President and General Manager, Cardiovascular, in which she led the global ELIQUIS® business to become the company’s largest product by revenue. Earlier in her career, Dr. Young held marketing and sales roles at GlaxoSmithKline from June 1993 to November 2010, where she catalyzed growth for the company’s Urology, Diabetes and NeuroHealth organizations. Dr. Young is a member of the Women in Bio and Healthcare Businesswomen’s Association and served on the Advisory Board of the Healthcare Businesswomen’s Association. Dr. Young received her B.S. in pharmacy and her Ph.D. in healthcare marketing from the University of South Carolina.

Code of Ethics

Our board of directors has adopted a Code of Business Conduct and Ethics applicable to all officers, directors and employees, which is available on our website at www.serestherapeutics.com in the “Investors and News” section under “Corporate Governance.” We intend to satisfy the disclosure requirement under Item 5.05 of Form 8-K regarding amendment to, or waiver from, a provision of our Code of Business Conduct and Ethics, as well as Nasdaq’s requirement to disclose waivers with respect to directors and executive officers, by posting such information on our website at the address and location specified in the preceding sentence. The information contained on our website is not incorporated by reference into this Annual Report on Form 10-K.

Other

The remainder of the information required to be disclosed by this item will be contained in the Proxy Statement for our Annual Meeting of Stockholders to be held in 2026 and is incorporated herein by reference.

Item 11. Executive Compensation

The information required to be disclosed by this item will be contained in the Proxy Statement for our Annual Meeting of Stockholders to be held in 2026 and is incorporated herein by reference.

Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters

The information required to be disclosed by this item will be contained in the Proxy Statement for our Annual Meeting of Stockholders to be held in 2026 and is incorporated herein by reference.

Item 13. Certain Relationships and Related Transactions and Director Independence

The information required to be disclosed by this item will be contained in the Proxy Statement for our Annual Meeting of Stockholders to be held in 2026 and is incorporated herein by reference.

Item 14. Principal Accountant Fees and Services

The information required to be disclosed by this item will be contained in the Proxy Statement for our Annual Meeting of Stockholders to be held in 2026 and is incorporated herein by reference.

PART IV

Item 15. Exhibits and Financial Statements Schedules

(a)(1) Financial Statements.

See the “Index to Consolidated Financial Statements” on page F-1 below for the list of financial statements filed as part of this report.

(a)(2) Financial Statement Schedules.

All schedules have been omitted because they are not required or because the required information is given in the Consolidated Financial Statements or Notes thereto set forth below beginning on page F-1.

(a)(3) Exhibits.

The following is a list of all exhibits filed as a part of this Annual Report on Form 10-K.

Incorporated by Reference Filed/

4.2 Description of Capital Stock *

100

31.1 Rule 13a-14(a)/15d-14(a) Certification of Interim Chief Executive Officer *

31.2 Rule 13a-14(a)/15d-14(a) Certification of Chief Financial Officer *

101

32.1 Section 1350 Certification of Interim Chief Executive Officer **

32.2 Section 1350 Certification of Chief Financial Officer **

101.SCH Inline XBRL Taxonomy Extension Schema Document *

* Filed herewith.

** Furnished herewith.

# Indicates management contract or compensatory plan.

^ Schedules have been omitted pursuant to Item 601(a)(5) of Regulation S-K. The Company hereby undertakes to furnish supplemental copies of any of the omitted schedules upon request by the SEC.

† Portions of this exhibit (indicated by asterisks) have been omitted pursuant to Regulation S-K, Item 601(b)(10)(iv). Such omitted information is both (i) not material and (ii) the type that the Registrant treats as private or confidential.

Item 16. Form 10-K Summary

None.

102

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

SERES THERAPEUTICS, INC.

Date: March 12, 2026 By: /s/ Richard N. Kender

Richard N. Kender

Executive Chair and Interim Chief Executive Officer

Pursuant to the requirements of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

Signature Title Date

Richard N. Kender

Marella Thorell

/s/ Stephen Berenson Director March 12, 2026

Stephen Berenson

/s/ Dennis A. Ausiello Director March 12, 2026

Dennis A. Ausiello, M.D.

/s/ Rob Rosiello Director March 12, 2026

Rob Rosiello

/s/ Willard H. Dere Director March 12, 2026

Willard H. Dere, M.D.

/s/ Claire M. Fraser Director March 12, 2026

Claire M. Fraser, Ph.D.

/s/ Kurt C. Graves Director March 12, 2026

Kurt C. Graves

/s/ Hans-Juergen Woerle, M.D., Ph.D. Director March 12, 2026

Hans-Juergen Woerle, M.D., Ph.D.

/s/ Eric D. Shaff Director March 12, 2026

Eric D. Shaff

103

INDEX TO CONSOLIDATED FINANCIAL STATEMENTS

Page

Report of Independent Registered Public Accounting Firm (PCAOB ID 238) F-2

Consolidated Balance Sheets as of December 31, 2025 and 2024 F-4

Notes to Consolidated Financial Statements F-9

F-1

Report of Independent Registered Public Accounting Firm

To the Board of Directors and Stockholders of Seres Therapeutics, Inc.

Opinions on the Financial Statements

We have audited the accompanying consolidated balance sheets of Seres Therapeutics, Inc. and its subsidiaries (the “Company”) as of December 31, 2025 and 2024, and the related consolidated statements of operations and comprehensive income (loss), of stockholders' equity (deficit) and of cash flows for each of the three years in the period ended December 31, 2025, including the related notes (collectively referred to as the “consolidated financial statements”).In our opinion, the consolidatedfinancial statements present fairly, in all material respects, the financial position of the Company as of December 31, 2025 and 2024, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2025in conformity with accounting principles generally accepted in the United States of America.

Substantial Doubt About the Company’s Ability to Continue as a Going Concern

The accompanying consolidated financial statements have been prepared assuming that the Company will continue as a going concern. As discussed in Note 1 to the consolidated financial statements, the Company incurred recurring losses from operations and has an accumulated deficit that raise substantial doubt about its ability to continue as a going concern. Management’s plans in regard to these matters are also described in Note 1. The consolidated financial statements do not include any adjustments that might result from the outcome of this uncertainty.

Basis for Opinion

These consolidated financial statements are the responsibility of the Company’s management. Our responsibility is to express an opinion on the Company’s consolidated financial statements based on our audits. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) (PCAOB) and are required to be independent with respect to the Company in accordance with the U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audits of these consolidated financial statements in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audits to obtain reasonable assurance about whether the consolidated financial statements are free of material misstatement, whether due to error or fraud. The Company is not required to have, nor were we engaged to perform, an audit of its internal control over financial reporting. As part of our audits we are required to obtain an understanding of internal control over financial reporting but not for the purpose of expressing an opinion on the effectiveness of the Company's internal control over financial reporting. Accordingly, we express no such opinion.

Our audits included performing procedures to assess the risks of material misstatement of the consolidated financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the consolidated financial statements. Our audits also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the consolidated financial statements. We believe that our audits provide a reasonable basis for our opinion.

Critical Audit Matters

The critical audit matter communicated below is a matter arising from the current period audit of the consolidated financial statements that was communicated or required to be communicated to the audit committee and that (i) relates to accounts or disclosures that are material to the consolidated financial statements and (ii) involved our especially challenging, subjective, or complex judgments. The communication of critical audit matters does not alter in any way our opinion on the consolidated financial statements, taken as a whole, and we are not, by communicating the critical audit matter below, providing a separate opinion on the critical audit matter or on the accounts or disclosures to which it relates.

Research and Development Expenses Associated with Other Operational Costs

As described in Note 2 to the consolidated financial statements, research and development costs are expensed as incurred. Research and development expenses include other operational costs related to the Company’s research and development activities, including allocated facility-related expenses and external costs of outside vendors engaged to conduct both preclinical studies and clinical trials, and certain costs of fulfilling the Company's obligations under the Transition Services Agreement (TSA). The Company's research and development expense for the year ended December 31, 2025 was $49.1 million, a significant portion of which is associated with other operational costs.

F-2

The principal consideration for our determination that performing procedures relating to research and development expenses associated with other operational costs is a critical audit matter is a high degree of auditor effort in performing procedures related to research and development expenses associated with other operational costs.

Addressing the matter involved performing procedures and evaluating audit evidence in connection with forming our overall opinion on the consolidated financial statements. These procedures included, among others, testing research and development expenses associated with other operational costs on a sample basis by obtaining and inspecting source documents, such as underlying agreements with outside vendors, purchase orders and invoices received, where applicable.

/s/ PricewaterhouseCoopers LLP

Boston, Massachusetts

March 12, 2026

We have served as the Company’s auditor since 2014.

F-3

SERES THERAPEUTICS, INC.

CONSOLIDATED BALANCE SHEETS

(In thousands, except share and per share data)

December 31,

Assets

Current assets:

Accounts receivable due from SPN - related party 360 2,068

Accounts receivable 157 —

Prepaid expenses and other current assets (1) 3,093 5,813

Other non-current assets 31 31

Liabilities and Stockholder's Equity

Current liabilities:

Accrued expenses and other current liabilities 3,972 10,719

Accrued liabilities due to SPN - related party 3,278 17,750

Operating lease liabilities, net of current portion 72,576 82,966

Commitments and contingencies (Note 10)

Stockholders’ equity:

Total liabilities and stockholders’ equity $ 138,193 $ 139,810

[1]Includes $0 as of December 31, 2025 and $2,683 as of December 31, 2024 of unbilled receivable from SPN (related party) related to certain costs of the transition services performed by the Company. See Note 3, Discontinued Operations and TSA, for further details.

The accompanying notes are an integral part of these consolidated financial statements.

F-4

SERES THERAPEUTICS, INC.

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS)

(In thousands, except share and per share data)

Year Ended December 31,

Revenue:

Grant revenue 789 — —

Total revenue 789 — —

Operating expenses:

Other income (expense):

Interest expense — — (2,468 )

Net income from discontinued operations, net of tax $ — $ 125,907 $ 76,406

Other comprehensive income:

Unrealized gain on investments, net of tax of $0 — — 10

Currency translation adjustment — — 2

Total other comprehensive income — — 12

[2] Includes $13,311 and $6,292 for the years ended December 31, 2025 and 2024 related to reimbursement received from SPN (related party) for transition services provided by the Company.

The accompanying notes are an integral part of these consolidated financial statements.

F-5

SERES THERAPEUTICS, INC.

CONSOLIDATED STATEMENTS OF STOCKHOLDERS’ EQUITY (DEFICIT)

(In thousands, except share data)

Accumulated

Common Stock Additional Other Total

Par Paid-in Comprehensive Accumulated Stockholders'

Shares Value Capital Income Deficit Equity (Deficit)

Issuance of common stock upon exercise of stock options 13,028 — 877 — — 877

Stock-based compensation expense — — 34,101 — — 34,101

Other comprehensive income — — — 12 — 12

Issuance of common stock under ESPP 29,421 — 487 — — 487

Stock-based compensation expense — — 20,971 — — 20,971

Issuance of common stock upon exercise of stock options 23,900 — 501 — — 501

Issuance of common stock under ESPP 17,839 — 230 — — 230

Stock-based compensation expense — — 10,840 — — 10,840

Adjustment for fractional shares due to reverse stock split 100 — — — — —

The accompanying notes are an integral part of these consolidated financial statements.

F-6

SERES THERAPEUTICS, INC.

CONSOLIDATED STATEMENTS OF CASH FLOWS

(In thousands)

Year Ended December 31,

Cash flows from operating activities:

Adjustments to reconcile net loss to net cash used in operating activities:

Net (accretion) amortization of (discounts) premiums on investments — — (236 )

Gain on sale of VOWST Business, net of transaction costs — (146,707 ) —

Amortization of debt issuance costs — 1,413 1,139

Loss associated with extinguishment of debt — 23,351 1,625

(Gain)/loss on disposal of fixed assets (192 ) 317 —

Impairment of long-lived assets — 3,267 —

Change in fair value of warrant liabilities — (546 ) (1,554 )

Collaboration (profit) loss sharing - related party (1) — — 5,158

Changes in operating assets and liabilities:

Accounts receivable due from SPN - related party 1,708 (2,068 ) —

Prepaid expenses and other current and non-current assets 2,720 987 (29,124 )

Accounts receivable (157 ) — —

Collaboration receivable - related party — 8,674 (8,674 )

Deferred income - related party — (4,124 ) 7,730

Deferred revenue - related party — — (1,325 )

Accrued liabilities due to SPN - related party (14,472 ) (15,708 ) —

Cash flows from investing activities:

Purchases of property and equipment (252 ) (380 ) (7,975 )

Proceeds from sales of property and equipment 210 — —

Purchases of investments — — (4,426 )

Sales and maturities of investments — — 22,983

Sales of restricted investments — 1,401 —

Proceeds from sale of VOWST Business — 141,272 —

Net cash (used in) provided by investing activities (42 ) 142,293 10,582

Cash flows from financing activities:

Proceeds from exercise of stock options 501 — 877

Proceeds from Securities Purchase Agreement - related party — 13,516 —

Proceeds from issuance of debt, net of issuance costs — — 103,378

Supplemental disclosure of cash flow information:

Supplemental disclosure of non-cash investing and financing activities:

Lease liability arising from obtaining right-of-use assets $ — $ — $ 3,046

Prepaid rent reclassified to right-of-use assets $ — $ — $ 4,634

Recognition of warrant liabilities $ — $ — $ 2,100

[1]Includes non-cash collaboration profits and losses related to pre-launch activities; subsequent to the approval of VOWST in April 2023, collaboration (profit) loss sharing - related party is included within changes in operating assets and liabilities.

F-7

The accompanying notes are an integral part of these consolidated financial statements.

F-8

SERES THERAPEUTICS, INC.

NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

(amounts in thousands, except share and per share data)

Nature of the Business and Basis of Presentation

Seres Therapeutics, Inc. (the “Company”) was incorporated under the laws of the State of Delaware in October 2010 under the name Newco LS21, Inc. In October 2011, the Company changed its name to Seres Health, Inc., and in May 2015, the Company changed its name to Seres Therapeutics, Inc. The Company is a clinical-stage company focused on improving patient outcomes in medically vulnerable populations through discovery and development of novel live biotherapeutics. The Company led the successful development and approval of VOWST (previously referred to as SER-109), the first FDA-approved orally administered microbiome therapeutic and a Breakthrough Therapy designated drug, which was sold to Nestlé Health Science (as defined below) in September 2024. The Company's live biotherapeutic product (“LBP”) candidates, including SER-155, SER-603, SER-147, and other potential candidates in earlier development, are consortia of bacteria designed to optimize specific, targeted pharmacological properties, and are formulated for oral delivery. The Company is designing LBP candidates to target the prevention and treatment of a broad swath of infections, and to treat inflammatory and immune (“I&I”) diseases by modulating host function to increase epithelium integrity and to induce immune homeostasis and tolerance, as well as to prevent the colonization and overgrowth of pathogens in the gastrointestinal (“GI”) tract.

SER-155, the Company's most advanced LBP candidate, is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve GI epithelial barrier integrity, and induce immune homeostasis to prevent bacterial bloodstream infections (“BSIs”), including those that can harbor antimicrobial resistance (“AMR”), as well as other pathogen-associated negative clinical outcomes in patients undergoing allogeneic hematopoietic stem cell transplantation (“allo-HSCT”). In the Company's placebo-controlled Phase 1b study of SER-155 in allo-HSCT, SER-155 was associated with a 77% relative risk reduction in bacterial BSIs and a significant reduction in systemic antibiotic exposure as well as a lower incidence of febrile neutropenia, as compared to placebo through day 100 post HSCT. SER-155 was generally well tolerated, with no observed treatment-related serious adverse events. Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT, including the submission of a final protocol to the FDA in January 2026, study site evaluation and qualification with its CRO, and manufacturing of drug substance, the Company has paused additional investment in that program while continuing to seek funding for the Phase 2 study.

The clinical data from the SER-155 Phase 1b study in allo-HSCT, along with the Company's extensive preclinical and translational clinical data compiled over the past decade support and inform the advancement of the Company's earlier stage programs targeting I&I diseases. The Company is evaluating SER-155 in immune checkpoint-related enterocolitis, or irEC, and the Company is developing SER-603, broadly in inflammatory bowel disease, or IBD, including ulcerative colitis, or UC, and Crohn's disease. The Company is currently exploring potential collaborations related to those I&I disease programs.

The Company has built and deploys a reverse translational platform and knowledge base for the discovery and development of live biotherapeutics, and maintains extensive proprietary know-how that may be used to support future research and development efforts. This platform incorporates high-resolution analysis of human clinical data to identify microbiome biomarkers associated with disease and non-disease states; preclinical screening using human cell-based assays and in vitro/ex vivo and in vivo disease models customized for live biotherapeutics; and a strain library and associated microbiological capabilities that spans broad biological and functional breadth. This platform and knowledge base are integrated through a proprietary knowledge graph and agentic artificial intelligence, enabling rapid identification of specific microbes, microbial genes, and microbial metabolites/peptides associated with disease and the design of therapeutic consortia of bacteria for specific pharmacological properties to restructure the gut microbiome and modulate functional pathways associated with disease. In addition, the Company owns a valuable intellectual property estate related to the development and manufacture of live biotherapeutics.

At a special meeting of stockholders held on September 26, 2024, the Company’s stockholders approved, and on September 30, 2024 (the "Closing Date"), the Company completed, the sale (the “Transaction”) of its VOWST microbiome therapeutic business (the “VOWST Business”), including inventory and equipment, certain patents and patent applications, know-how, trade secrets, trademarks, domain names, marketing authorizations and related rights, documents, materials, business records and data and contracts that are used or held for use primarily in the development, commercialization and manufacturing of the microbiome product sold under the brand name VOWST as provided for in accordance with the terms of the Purchase Agreement (the “Product”), to Société des Produits Nestlé S.A. (“SPN”), a wholly-owned subsidiary of Nestlé S.A., and its designated affiliates (collectively, “Nestlé Health Science”) pursuant to the Asset Purchase Agreement, dated as of August 5, 2024 (the “Purchase Agreement”), by and among the Company and SPN, and a wholly-owned subsidiary of Nestlé S.A. As consideration for the Transaction, SPN paid or agreed to pay, as applicable, the following Transaction Consideration:

(i)

a cash payment, which was paid upon completion of the Transaction (“Closing”), of $100,000, less approximately $17,857 owed by the Company to an affiliate of SPN as of March 31, 2024 under the prior license agreement between the Company and the SPN affiliate, less approximately CHF 2,000 in satisfaction of fees due under the Bacthera Manufacturing Agreement (defined below);

(ii)

cash installment payments of $50,000, which was received on January 15, 2025 and $25,000, which was received on July 1, 2025 (offset by $1,421 paid by the Company to Nestlé on July 1, 2025 related to certain employment obligations assumed by SPN, as described below), conditioned on the Company’s material compliance with obligations under the Transition

SERES THERAPEUTICS, INC.

NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

(amounts in thousands, except share and per share data)

Services Agreement (the “TSA”) (as described below) entered into at Closing between the Company and Nestlé Enterprises S.A., an affiliate of SPN, or NESA;

(iii)

prepayment of the $60,000 milestone payment tied to the achievement of worldwide annual net sales of the Product of $150,000 (the “First Sales Milestone”), which was paid in cash at Closing (the “Prepaid Milestone”), which Prepaid Milestone will accrue interest at a fixed rate of 10% per annum until the First Sales Milestone is achieved and 5% per annum thereafter until the earlier of (x) the date on which the Prepaid Milestone, plus accrued interest thereon, has been repaid in full by set-off and (y) the last day of the Milestone Period (as defined below); and

(iv)

future milestone payments of (x) $125,000 tied to the achievement of worldwide annual net sales of the Product of $400,000 and (y) $150,000 tied to the achievement of worldwide annual net sales of the Product of $750,000, during the period from Closing until December 31 of the calendar year in which the tenth anniversary of Closing occurs (the “Milestone Period”) (together, the “Future Milestone Payments” and, together with the Prepaid Milestone, the “Milestone Payments”).

As they are earned, the Milestone Payments will be satisfied as follows: (i) first, by set-off against all accrued interest on the Prepaid Milestone until the amount of such accrued interest has been paid in full, (ii) second, by set-off against the outstanding balance of the Prepaid Milestone until the Prepaid Milestone has been repaid in full and (iii) thereafter, in cash. If any amount of the Prepaid Milestone (and any accrued interest thereon) remains outstanding as of following the last day of the Milestone Period (defined below), the balance thereof (together with any interest accrued thereon) will be forgiven and the right of set-off of SPN with respect thereto will be deemed forfeited. The installment payment received on July 1, 2025 was offset by $1,421 paid by the Company to Nestlé on the same date related to certain employment obligations assumed by SPN with respect to the period ended as of the Closing Date.

The Company and SPN shared 50/50 in the net profit or net loss (the “Profit Sharing Payments”) achieved during the period from the date of Closing until December 31, 2025 (the “Profit Sharing Period”), with the net profit or net loss calculated as (i) the net sales of VOWST in the United States and Canada, plus (ii) other income received in connection with the grant of a license or sublicense with respect to VOWST in the United States and Canada as described in the Purchase Agreement, minus (iii) allowable expenses directly attributable or reasonably allocable to certain development activities, commercialization activities, medical affairs activities, manufacturing activities or other relevant activities, as described in the Purchase Agreement. During the Profit Sharing Period, the Company reimbursed SPN for (i) certain payments under the exclusive license agreement between the Company and Memorial Sloan Kettering Cancer Center, (ii) certain costs incurred in connection with an ongoing post-marketing safety study of VOWST and (iii) 80.1% of all rent and other costs due to the landlord under the lease for the Company’s Waltham facility.

At Closing, in exchange for a payment to be made by SPN to Bacthera AG, the Long Term Manufacturing Agreement, dated November 8, 2021 between the Company and Bacthera AG (the “Bacthera Manufacturing Agreement”) was terminated and each of Bacthera and Seres released one another from any and all losses, liabilities or other obligations arising thereunder with respect to the period ending at the Closing Date, including without limitation any milestone payments required to be paid to Bacthera thereunder.

On the Closing Date, the Company and SPN entered into a securities purchase agreement (the “Securities Purchase Agreement”) pursuant to which SPN purchased 714,285 shares (the “Shares”) of common stock at Closing, at a purchase price per share of $21.00, for an aggregate purchase price of $15,000. Under the terms of the Securities Purchase Agreement, SPN agreed not to sell or transfer the Shares for a period of six months after Closing, subject to certain customary exceptions. The Company agreed to register the resale of the Shares by SPN within 90 days of Closing. On October 1, 2024, the Company filed a registration statement to register the Shares, which became effective on October 11, 2024. In addition, under the terms of the Securities Purchase Agreement, for as long as SPN, together with its affiliates, beneficially owns at least 10% of the Company’s outstanding shares of common stock, the Company has agreed to take such action within its control to include one individual designated by SPN in the slate of nominees recommended by the Company’s board of directors (or the applicable committee of the board of directors) to the Company’s stockholders for election to the board of directors at the applicable stockholder meeting. SPN designated Hans-Juergen Woerle, M.D., Ph.D. and on February 4, 2025, the Company’s board of directors appointed Dr. Woerle to the board as a Class III director, with a term expiring at the Company’s 2027 annual meeting of stockholders. The Securities Purchase Agreement contains customary representations and warranties and closing conditions.

On the Closing Date, the Company entered into a TSA with NESA, which provides for services to be performed by the Company in order to facilitate a transition of the business associated with the VOWST Business to NESA and its affiliates. The scope of the transition services includes the provision of certain manufacturing services and certain administrative functions related to the VOWST Business and operations, including the maintenance of certain manufacturing services and the related facility in which such services are currently conducted. The Company provided the manufacturing services until December 31, 2025 and other services for the duration specified in the schedule to the TSA for each service. NESA paid the Company for certain fixed costs, including a monthly fixed fee and a variable per batch fee for preserved raw material suspension ("PRMS") manufacturing, and reimbursed the Company for certain costs of the transition services performed by the Company under the TSA, including labor. The know-how and other intellectual property

F-10

SERES THERAPEUTICS, INC.

NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

(amounts in thousands, except share and per share data)

generated in connection with the performance of the TSA will be owned by NESA with the Company having a non-exclusive license to such know-how and other intellectual property under the Cross-License Agreement. During the term of the TSA, the Company transferred the specifications for materials and documentation necessary to enable PRMS manufacturing services to a third party service provider designated by NESA. In the event of a material failure by the Company to deliver PRMS under the TSA, NESA had step-in rights to negotiate to enter into a direct lease with the landlord of the manufacturing facility with respect to the portion of such facility used in connection with the VOWST Business or to cause such services to be performed, with any reasonable out-of-pocket costs and expenses incurred in connection therewith reimbursed by the Company.

MCRB US Equity

MCRB · 10-K · period ended 2025-12-31

Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations

Item 7A. Quantitative and Qualitative Disclosures about Market Risk

Item 8. Financial Statements and Supplementary Data

Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure

Item 9A. Controls and Procedures

Item 9B. Other Information

Item 9C. Disclosure Regarding Foreign Jurisdictions That Prevent Inspections

Item 10. Directors, Executive Officers and Corporate Governance

Item 11. Executive Compensation

Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters

Item 13. Certain Relationships and Related Transactions and Director Independence

Item 14. Principal Accountant Fees and Services

Item 15. Exhibits and Financial Statements Schedules

Item 16. Form 10-K Summary

MCRB US Equity☆

MCRB · 10-K · period ended 2025-12-31

Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations

Item 7A. Quantitative and Qualitative Disclosures about Market Risk

Item 8. Financial Statements and Supplementary Data

Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure

Item 9A. Controls and Procedures

Item 9B. Other Information

Item 9C. Disclosure Regarding Foreign Jurisdictions That Prevent Inspections

Item 10. Directors, Executive Officers and Corporate Governance

Item 11. Executive Compensation

Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters

Item 13. Certain Relationships and Related Transactions and Director Independence

Item 14. Principal Accountant Fees and Services

Item 15. Exhibits and Financial Statements Schedules

Item 16. Form 10-K Summary

MCRB US Equity