Seres Therapeutics Advances SER-428 for Bloodstream Infection Prevention

CAMBRIDGE, Mass., October 15, 2026

Preclinical Data Support SER-428’s Potential

SER-428 is designed to act within the gastrointestinal tract to prevent the colonization and overgrowth of harmful pathogens while supporting mucosal epithelial barrier integrity. The investigational product aims to reduce infection risk in patients who are particularly vulnerable after sepsis and antibiotic exposure. In preclinical in vivo pathogen colonization models, SER-428 reduced gastrointestinal levels of E. faecium and K. pneumoniae by up to 1,000-fold following oral administration. These findings provide preclinical evidence of biological activity against clinically relevant pathogens. Seres said the data support further investigation of SER-428’s potential to reduce gastrointestinal pathogen abundance and, ultimately, bloodstream infections in high-risk patients. The results do not yet demonstrate that the product prevents bloodstream infections in humans, and clinical studies will be needed to assess its safety and effectiveness. The company also reported encouraging formulation characteristics: the liquid product remained stable during the intended in-use period and after simulated delivery through a feeding tube. This feature is intended to facilitate administration in critically ill patients who may be unable to swallow capsules. Seres is developing SER-428 with support from a grant from CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator).

Liquid Formulation Designed for Critically Ill Patients

A key feature of SER-428 is its oral liquid formulation, which may allow administration to patients who cannot take conventional oral capsules. This includes critically ill MICU patients who receive nutrition and medication through feeding tubes. According to Seres, the formulation’s stability during the intended use period and after simulated feeding-tube delivery supports its development for this patient population. The product shares key strains and functions with SER-155, another investigational live biotherapeutic developed by Seres. SER-155 completed a Phase 1b study in patients undergoing allogeneic hematopoietic stem cell transplantation, demonstrating a reported 77% relative reduction in bloodstream infection risk and a well-tolerated safety profile. These results provide clinical context for the broader live biotherapeutic approach but do not establish that SER-428 will produce similar outcomes. Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres Therapeutics, said patients admitted to medical intensive care with sepsis remain vulnerable to subsequent serious infections, particularly when antibiotic exposure disrupts the gastrointestinal microbiome. He noted that SER-428 was designed around the needs of critically ill patients and that the IDWeek findings provide preclinical support for its biological activity and formulation characteristics. The company plans to use these findings to inform its next development steps.

Planned Phase 1b Study and IDWeek Presentation

Seres is designing a Phase 1b open-label clinical trial of SER-428 in MICU patients at high risk of infection. The planned study is being developed in collaboration with Daniel E. Freedberg, M.D., M.S., of Columbia University, and in consultation with CARB-X. Further clinical development will proceed subject to the availability of grant funding or other financial support. The company will present the preclinical findings at IDWeek 2026 in a poster titled “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.” The presentation is scheduled for October 22, from 12:15 to 1:30 p.m. Eastern Time, in Poster Hall B-C, under poster number P-22 in the Novel Agents session. IDWeek brings together professional organizations focused on infectious diseases, healthcare epidemiology, HIV medicine, pediatric infectious diseases and infectious disease pharmacy. Seres Therapeutics is a clinical-stage biotechnology company developing live biotherapeutic products intended to modulate host function, support mucosal barrier integrity and prevent harmful gastrointestinal pathogen overgrowth. Its pipeline includes SER-155, which is being developed for bloodstream infection prevention in transplant recipients, and SER-603, in development for inflammatory bowel disease. The SER-428 program represents another effort to investigate microbiome-based approaches to infection prevention in medically vulnerable populations.

Source::Seres Therapeutics, press release