# Seres' SER-155 bacterial capsule linked to fewer bloodstream infections in phase 1b transplant trial

- Published: 2026-11-10 16:57 UTC
- Section: Life & Health › Medicine
- Tags: SER-155, live biotherapeutic product, allogeneic hematopoietic cell transplant, gut microbiome, bloodstream infection, phase 1b trial, oral vancomycin, Nature Medicine, NCT04995653
- URL: https://realtech.media/news/rt-f8f6ab85791006c23b481e3f_seres_ser_155/

**In a phase 1b trial in allogeneic stem cell transplant recipients, oral SER-155 had a safety profile like placebo and was linked to fewer bloodstream infections.**

![Seres' SER-155 bacterial capsule linked to fewer bloodstream infections in phase 1b transplant trial](https://realtech.media/media/2026/11/bf8c66060f5cccb1.jpg)
*Ermell, Wikimedia Commons, CC BY-SA 4.0*

Nature Medicine has published results of a phase 1b study of SER-155, an investigational oral live biotherapeutic made of cultivated bacterial strains. The study enrolled recipients of allogeneic hematopoietic cell transplant in two cohorts: an open-label cohort of 15 participants and a randomized, placebo-controlled cohort of 45. [1] The product is meant to address disruption of the gastrointestinal microbiome and of the intestinal epithelial barrier, which raise the risk of bloodstream infection after transplant. [1]

In the randomized cohort, 34 of 45 participants received treatment or placebo and then underwent transplant. Twenty received SER-155 after microbiome conditioning with oral vancomycin, and 14 received placebo. [1]

## Safety and engraftment

The primary endpoint was safety. According to the authors, SER-155 was well tolerated, with a profile comparable to placebo, no treatment-related serious adverse events and no infections with SER-155 species. The most common adverse events were diarrhea and nausea. [1] Four participants in cohort 1 and four in cohort 2 died during the study, two of them in the SER-155 arm and two in the placebo arm. None of the deaths was attributed to SER-155. [1]

The primary pharmacokinetic endpoint was engraftment of the administered bacteria. All 16 SER-155 strains were detected, with a median of 11.5 strains found after the first treatment course. [1]

## Infection outcomes

The key secondary efficacy endpoint was the incidence of bloodstream infection. It was 10.0% with SER-155 and 43% with placebo (odds ratio 0.15; 95% CI 0.01–1.13; P = 0.0423), a 77% relative risk reduction. [1]

The other secondary efficacy endpoints did not reach statistical significance: gastrointestinal infection, acute graft-versus-host disease of grade 2 or higher, febrile neutropenia, and a composite of bloodstream infection, gastrointestinal infection or acute graft-versus-host disease. [1] The secondary pharmacodynamic endpoint, gastrointestinal domination by Enterococcus and Enterobacteriaceae species, was not achieved. [1]

The randomized cohort was small, and the confidence interval for the odds ratio is wide. The authors conclude that the results warrant further clinical investigation of SER-155. The study is registered on ClinicalTrials.gov as NCT04995653. [1]

## Sources

1. [Nature Medicine](https://doi.org/10.1038/s41591-026-04686-y)
2. [knowridge.com](https://knowridge.com/2026/11/a-capsule-of-live-bacteria-may-prevent-infections-after-stem-cell-transplants)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-f8f6ab85791006c23b481e3f_seres_ser_155/