Seres Therapeutics to Present Preclinical Data for SER-428, an Investigational Live Biotherapeutic Designed to Reduce Risk of Bloodstream Infections in Critically Ill Patients, at IDWeek 2026

Further clinical development depends on grant or other funding as Seres designs a trial for intensive-care patients at high infection risk.

Rhea-AI Summary

Seres Therapeutics (Nasdaq: MCRB) will present preclinical data for its investigational live biotherapeutic SER-428 at IDWeek on October 22, 2026. In animal pathogen-colonization models, oral SER-428 produced up to a 1,000-fold reduction in Enterococcus faecium and Klebsiella pneumoniae. These are preclinical findings, not results in patients.

SER-428 is designed to reduce bloodstream infection risk in medically vulnerable patients after sepsis and antibiotic exposure. Its liquid formulation allows dosing in patients unable to take capsules, including through feeding tubes, and remained stable during intended use and after simulated feeding-tube delivery. Seres is designing a Phase 1b open-label trial, in which treatment is known to participants and investigators, for intensive-care patients at high infection risk. Further clinical development is pending funding from grants or other sources.

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Positive

• Moderate pointSeres is designing a Phase 1b open-label trial for intensive-care patients at high infection risk.
• Moderate pointRelated candidate SER-155 completed Phase 1b with 77% relative bloodstream-infection risk reduction and a well-tolerated safety profile.
• Minor pointSER-428 reduced two pathogens by up to 1,000-fold in preclinical animal colonization models.
• Minor pointSER-428 liquid formulation remained stable during intended use and after simulated feeding-tube delivery.
• Minor pointSER-428 development received CARB-X grant support.

Negative

• Moderate pointFurther SER-428 clinical development is pending funding from grants or other sources.

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Market move: MCRB +7.43% vs previous close. SER-428 preclinical data

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On Oct 15, the day this news came out, the latest delayed price for MCRB is 7.43% above the previous close. Our momentum scanner has recorded 3 alerts for this stock so far that day. The latest delayed price is $2.60.

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Key Figures

Pathogen reduction: Up to 1,000-fold Relative risk reduction: 77%

Pathogen reduction

Up to 1,000-fold

SER-428; preclinical in vivo colonization models

Relative risk reduction

77%

SER-155 Phase 1b study; prior BSI result cited in the article

Key Terms

live biotherapeutic product, in vivo, open-label trial, mucosal epithelial barrier integrity

4 terms

live biotherapeutic product medical

"investigational oral liquid live biotherapeutic product (LBP)"

A live biotherapeutic product is a medical therapy made from live microorganisms—such as bacteria or yeasts—developed to prevent, treat or cure disease by altering the body’s microbiome or immune response. For investors it matters because these products face unique scientific, manufacturing and regulatory hurdles (think of them as precision probiotics that require sterile production, cold chains, and rigorous clinical proof), which can affect development timelines, costs and commercial risk.

in vivo technical

"preclinical in vivo pathogen colonization models"

In vivo describes tests or experiments performed inside a living organism, such as an animal or human, to observe how a drug, device or biological process behaves in a real, functioning body. Investors care because in vivo results reveal safety, effectiveness and possible side effects that lab tests cannot, much like road-testing a prototype car in traffic rather than only on a bench — outcomes can strongly influence regulatory approval, clinical success and a company’s valuation.

open-label trial medical

"a Phase 1b open-label trial"

An open-label trial is a clinical study in which doctors, participants and often the researchers all know which treatment is being given, unlike a blinded study where that information is hidden. Think of it like an open rehearsal where everyone can see what’s being used; it can provide early, practical information about safety and how a treatment works, but results may be more influenced by expectations, so investors treat findings as informative but less definitive than blinded trial results.

mucosal epithelial barrier integrity medical

"protect and increase mucosal epithelial barrier integrity"

The structural and functional state of the mucosal epithelial barrier, meaning the layer of tightly connected epithelial cells and their secreted mucus that lines organs such as the gut, airways and reproductive tract; integrity refers to how well those cells, their tight junctions, and the mucus layer prevent unwanted substances, microbes or fluids from crossing into underlying tissue. When integrity is intact, the barrier selectively allows nutrients and gases while blocking pathogens and toxins; loss of integrity increases permeability, which can permit microbial invasion, trigger local inflammation and alter normal tissue function.

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Novel oral liquid formulation of SER-428 is designed for patients who cannot take oral capsules, including critically ill patients receiving nutrition via feeding tube

SER-428 significantly reduced gastrointestinal abundance of clinically relevant pathogens in preclinical in vivo colonization models

CAMBRIDGE, Mass., Oct. 15, 2026 (GLOBE NEWSWIRE) -- Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading live biotherapeutics company, today announced that preclinical data supporting the development of SER-428, an investigational oral liquid live biotherapeutic product (LBP) designed to reduce the risk of bloodstream infections (BSIs) in medically vulnerable patients following sepsis and antibiotic exposure, will be presented at IDWeek 2026, taking place October 21–24 in Washington, D.C.

Poster Details
Number: P-22
Title: SER-428, an Investigational Live Biotherapeutic Product (LBP) with a Novel Liquid Formulation, for Reduction of Bloodstream Infections in High-Risk Patients after Sepsis and Antibiotic Exposure
Session: Novel Agents
Date and Time: October 22, 12:15-1:30 pm EST
Location: Poster Hall B-C

SER-428 shares key strains and functions with SER-155, Seres’ investigational LBP that successfully completed a Phase 1b study in patients undergoing allogeneic hematopoietic stem cell transplantation, demonstrating a 77% relative risk reduction in BSIs and a well-tolerated safety profile. SER-428, developed with support from a grant from CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator), is designed to act in the gastrointestinal (GI) tract to prevent the colonization and overgrowth of pathogens, and to protect and increase mucosal epithelial barrier integrity, with the goal of reducing the risk of infection. Notably, SER-428 is a liquid formulation for the LBP modality that enables dosing in patients who cannot take oral capsules, including critically ill patients who receive nutrition and medications via feeding tube.

“Patients admitted to the medical intensive care unit with sepsis remain at high risk for subsequent serious infections, particularly after antibiotic exposure disrupts the gastrointestinal microbiome and allows pathogenic bacteria to proliferate,” said Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres Therapeutics. “SER-428 builds on our positive clinical experience with SER-155 and is designed specifically with the needs of critically ill patients in mind. The findings presented at IDWeek provide important preclinical support for both the biological activity and the formulation characteristics of SER-428 as we advance the clinical development plans for the program. We appreciate the long-standing support from CARB-X, which has allowed this work to progress.”

In preclinical in vivo pathogen colonization models, oral administration of SER-428 led to up to a 1,000- fold reduction of Enterococcus faecium and Klebsiella pneumoniae. These findings support the potential of SER-428 to reduce GI pathogen abundance, bloodstream infections, and antimicrobial resistance in high-risk patients. The SER-428 liquid formulation was stable, including during the intended in-use period and following simulated feeding tube delivery.

Seres is currently designing a Phase 1b open-label trial, in collaboration with Daniel E. Freedberg, M.D., M.S., of Columbia University, and in consultation with CARB-X, to evaluate SER-428 in Medical Intensive Care Unit (MICU) patients at high risk of infection. Further clinical development of SER-428 will be progressed pending funding from grants or other sources.

IDWeek is an annual meeting organized by a collaboration of professional organizations, including the Infectious Diseases Society of America (IDSA), the Society for Healthcare Epidemiology of America (SHEA), the HIV Medicine Association (HIVMA), the Pediatric Infectious Diseases Society (PIDS), and the Society of Infectious Diseases Pharmacists (SIDP).

About Seres Therapeutics
Seres Therapeutics, Inc. (Nasdaq: MCRB) is a clinical-stage biotechnology company developing novel live biotherapeutics products (LBP), designed to address unmet needs in oncology that can lead to interruption to patients' cancer care and/or mortality, and to treat inflammatory and immune (I&I) diseases, by modulating host function to protect and improve mucosal epithelial barrier integrity, induce immune homeostasis and tolerance, and prevent the colonization and overgrowth of pathogens in the gastrointestinal (GI) tract. The Company previously led the development and FDA approval of VOWST™, the first orally administered microbiome therapeutic, which was subsequently divested to Nestlé Health Science. SER-155, an investigational cultivated multi-strain biotherapeutic, which has received Breakthrough Therapy and Fast Track designations, is being advanced for the prevention of bloodstream infections in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HCT), and is Phase 2 ready, pending receipt of funding. SER-155 is also being developed to address immune checkpoint inhibitor-related enterocolitis (irEC) to provide an immunosuppressive-free alternative and enable patients to continue their ICI cancer therapy. Having recently reported promising results from an investigator sponsored trial, Seres is evaluating the design of a Phase 2 study in irEC. The Company is advancing IND-enabling studies for SER-603, which is in development for inflammatory bowel disease. Mechanistically, Seres' biotherapeutics target the mucosal epithelial barrier-immune interface and are optimized to modulate host function to increase mucosal epithelium integrity, induce immune homeostasis, and prevent the colonization and overgrowth of harmful bacteria in the GI tract. 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 conferences; SER-155, SER-428, and their intended uses and benefits; our clinical development plans for SER-155 and SER-428; the timing and results of clinical studies and data readouts; current or future product candidates and their potential impacts and outcomes; current or future grants or funding from CARB-X or other sources; 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) 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; (7) 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; (8) our novel approach to therapeutic intervention; (9) our reliance on third parties to conduct our clinical trials and manufacture our product candidates; (10) our ability to achieve market acceptance necessary for commercial success; (11) the competition we will face; (12) our ability to protect our intellectual property; (13) impact of our recent management transitions and appointments and our ability to retain key personnel; and (14) disruptions at the FDA or other government agencies. These and other important factors discussed under the caption “Risk Factors” in our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2026, filed with the Securities and Exchange Commission (SEC) on August 5, 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, except as required by law, 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 Contact:  
IR@serestherapeutics.com

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

FAQ

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What did Seres Therapeutics report in the SER-428 preclinical studies?

Oral SER-428 produced up to a 1,000-fold reduction in Enterococcus faecium and Klebsiella pneumoniae in preclinical animal colonization models. The liquid formulation also remained stable during its intended in-use period and following simulated feeding-tube delivery. These findings are not clinical results in patients.

What is the next planned trial for Seres Therapeutics' SER-428?

Seres is designing a Phase 1b open-label trial to evaluate SER-428 in medical intensive-care patients at high risk of infection. Trial design involves collaboration with Daniel E. Freedberg of Columbia University and consultation with CARB-X. Further clinical development is pending funding from grants or other sources.