Pharmacologic data highlight durable GI microbiome modulation, with improvement in epithelial barrier integrity, following SER-155 administration

Additional presentations underscore the role of live biotherapeutics in shaping outcomes in IBD and immunocompromised patients

CAMBRIDGE, Mass., April 20, 2026 (GLOBE NEWSWIRE) -- Seres Therapeutics, Inc. (Nasdaq: MCRB) (Seres or the Company), a leading live biotherapeutics company, today announced three presentations at the 2026 European Society of Clinical Microbiology and Infectious Diseases (ESCMID) global congress, taking place April 17-21 in Munich, Germany.

A poster presentation by Seres highlighted biomarker and clinical pharmacology data from the Company’s Phase 1b study of SER-155 (NCT04995653), evaluating changes in gastrointestinal (GI) microbiome composition following administration in patients undergoing allogeneic hematopoietic cell transplantation (allo-HCT). Data showed that administration of SER-155 induced a significant and durable shift in GI microbiome composition relative to placebo, characterized by high SER-155 species relative abundance. This shift is associated with improved GI epithelial barrier integrity that could reduce the likelihood of bacterial translocation from the GI to the bloodstream. In contrast, placebo-treated participants showed low abundance of SER-155 species and related Firmicutes, high abundance of Bacteroidetes, and a compromised GI epithelial barrier. These findings are consistent with the intended mechanisms of action of SER-155, including protection of the GI epithelial barrier, and align with previously reported clinical observation of reduced incidence of bloodstream infections (BSIs) in SER-155-treated patients following allo-HCT.

Dr. Jonathan (Tsoni) Peled of Memorial Sloan Kettering Cancer Center, an institution with which Seres has a long history of collaboration, will present in a symposium session focused on the role of the microbiome in shaping clinical outcomes in immunocompromised patients. His presentation will highlight emerging strategies to target the gut microbiome in the context of hematopoietic cell transplantation.

“Patients undergoing allogeneic hematopoietic cell transplantation face a significant risk of bloodstream infection, driven in large part by disruption of the gut microbiome,” said Dr. Peled. “Data continue to highlight the potential of targeted live biotherapeutics like SER-155 to restore key microbial functions, preventing bacterial translocation from the GI to reduce infection and improve clinical outcomes in this medically vulnerable population.”

In addition, an oral presentation by Dr. Ines Moura of the University of Leeds describes the use of an in vitro colonic model (MiGut) to recapitulate patient-specific microbiome composition and inflammatory responses in inflammatory bowel disease (IBD). Seres is co-developing with Leeds models to evaluate drug strain engraftment in different patient microbiome backgrounds and measure resulting metabolic and immune functional responses. The collaboration with Leeds on MiGut enables the modeling of patient stratification, conditioning, and live biotherapeutic effects in a pharmacological model, to inform clinical development in IBD and broader opportunities in other diseases.

Presentations Details
Poster Presentation (Abstract 1726)
Title: Effect of investigational, live biotherapeutic, SER-155, on gastrointestinal (GI) microbiome composition in adults receiving allogeneic haematopoietic stem cell transplant (allo-HCT) in a randomised, placebo-controlled phase 1b trial
Session: PS073 – 10c: Infections related to haematopoietic stem cell transplantation
Date and Time: Sunday, February 19 at 12:00 CEST
Presenter: Dr. Elizabeth Halvorsen, Senior Director, Translational Biology at Seres Therapeutics

Oral Presentation (Abstract 8914)
Title: Recapturing individual inflammatory responses in IBD microbiomes using an in vitro colonic model (MiGut)
Session: OS100: Microbiome-host interplay in intestinal disease and under selection pressure
Date and Time: Monday, April 20 at 14:45 CEST
Presenter: Dr. Ines Moura, Lecturer in Medical Microbiology at the University of Leeds

Symposium Presentation
Title: Targeting the gut microbiome: an emerging trend in haematopoietic stem cell transplantation
Session: 10. Immune compromise & transplant ID - The role of the microbiome in shaping clinical outcomes in immunocompromised patients
Date and Time: Tuesday, April 21 at 8:30 CEST
Presenter: Dr. Jonathan (Tsoni) Peled, Bone Marrow Transplant Specialist at MSKCC

About SER-155
SER-155 is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve epithelial barrier integrity, and induce immune tolerance to prevent bacterial bloodstream and antimicrobial resistant (AMR) infections, as well as other pathogen associated negative clinical outcomes, in patients undergoing allo-HSCT for the treatment of hematological malignancies.

SER-155 has been evaluated in a Phase 1b placebo-controlled study in patients undergoing allo-HSCT, which demonstrated a significant reduction in both BSIs and systemic antibiotic exposure, as well as lower incidence of febrile neutropenia. SER-155 has received Breakthrough Therapy designation for the reduction of BSIs and Fast Track designation for reducing the risk of infection and GvHD, in both cases in patients undergoing HSCT.

About Seres Therapeutics
Seres Therapeutics, Inc. (Nasdaq: MCRB) is a clinical-stage biotechnology company developing novel live biotherapeutics, with a focus on inflammatory and immune diseases. The Company led the development and FDA approval of VOWST™, the first orally administered microbiome therapeutic, which was subsequently divested to Nestlé Health Science. SER-155, which has received Breakthrough Therapy and Fast Track designations, is being advanced for patients undergoing allogeneic hematopoietic stem cell transplant (allo-HSCT), and is Phase 2 ready, pending receipt of funding. An investigator-sponsored trial of SER-155 is ongoing in immune checkpoint inhibitor–related enterocolitis (irEC) to further evaluate the potential breadth of the Company’s live biotherapeutic platform. SER-603, in development for irritable bowel disease, is designed to modulate the gastrointestinal microbiome and support mucosal barrier integrity by targeting inflammatory bacteria and associated metabolites. For more information, please visit www.serestherapeutics.com.   

Forward-Looking Statements
This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including statements about: the anticipated content and timing of upcoming presentations and symposiums; the design, timing and results of our clinical studies and data readouts; current or future product candidates and their potential impacts and outcomes; clinical development plans and commercial opportunities; our efforts to create a strategic, R&D, or other partnership; our ability to operationalize a study upon receipt of any financing; our planned strategic focus; the anticipated timing of any of the foregoing; and other statements that are not historical fact.

These forward-looking statements are based on management’s current expectations. These statements are neither promises nor guarantees, but involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements, including, but not limited to, the following: (1) our need for additional funding; (2) our ability to continue as a going concern; (3) we have incurred significant losses, are not currently profitable and may never become profitable; (4) our cost reduction actions may not achieve their intended benefits, including an extended cash runway; (5) our limited operating history; (6) the expected payments from the VOSWT sale are subject to risks and uncertainties; (7) we may not be able to realize the anticipated benefits of the VOWST sale, and may face new challenges as a smaller, less diversified company; (8) we have in the past and may in the future receive notice of the failure to satisfy a continued listing rule from The Nasdaq Stock Market LLC; (9) our novel approach to therapeutic intervention; (10) our reliance on third parties to conduct our clinical trials and manufacture our product candidates; (11) our ability to achieve market acceptance necessary for commercial success; (12) the competition we will face; (13) our ability to protect our intellectual property; (14) impact of our recent management transitions and appointments and our ability, to retain key personnel; and (15) disruptions at the FDA or other government agencies. These and other important factors discussed under the caption “Risk Factors” in our Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on March 12, 2026, as well as our other reports filed with the SEC, could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management’s estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, we disclaim any obligation to do so, even if subsequent events cause our views to change. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date of this press release.

Investor and Media Contacts:
IR@serestherapeutics.com

Carlo Tanzi, Ph.D.
Kendall Investor Relations
ctanzi@kendallir.com

Source: Seres Therapeutics, Inc.