Bloodstream infections are a dangerous complication of allogeneic haematopoietic cell transplantation, partly because antibiotics, chemotherapy and immune suppression disrupt the gut ecosystem and weaken its barrier. A Phase Ib study published in Nature Medicine on 3 November 2026 reports fewer bloodstream infections among recipients of SER-155, a defined 16-strain oral live biotherapeutic, than among placebo recipients.
The result is intriguing but small. Only 34 randomized participants both received their assigned regimen and underwent transplantation: 20 in the SER-155 group and 14 in the placebo group. SER-155 remains investigational and is not an approved infection-prevention treatment.
The product tries to rebuild a damaged ecosystem
SER-155 contains cultivated bacterial strains selected to establish themselves in the gastrointestinal tract and support functions associated with colonization resistance, epithelial-barrier integrity and immune balance. It is a standardized consortium rather than an undefined faecal transplant.
Participants assigned to SER-155 first received four days of oral vancomycin to create an ecological niche, then two treatment courses. Placebo participants received placebo instead of vancomycin and instead of the live biotherapeutic. The comparison therefore tests the combined conditioning-plus-SER-155 regimen; it cannot isolate the contribution of vancomycin from the bacterial product.
The trial prioritized safety and strain engraftment
The two-cohort Phase Ib study began with 15 open-label participants, followed by a randomized, double-blind, placebo-controlled cohort of 45. Some randomized participants did not proceed to transplant, leaving the smaller efficacy set.
SER-155 was reported as generally well tolerated, with no treatment-related serious adverse events and no infections caused by its constituent species. Diarrhoea and nausea were the most common adverse events. Eight participants across the two cohorts died during the study, four in each cohort; investigators did not attribute the deaths to SER-155. All 16 bacterial strains were detected after dosing, with a median of 11.5 strains detected after the first course.
Infections fell, while several other endpoints did not
Bloodstream infections occurred in two of 20 SER-155 recipients, or 10%, and six of 14 placebo recipients, or 42.9%. The reported 77% relative risk reduction sounds large, but the absolute groups were tiny and the odds-ratio confidence interval was wide. The nominal P value was 0.0423.
The company also reported fewer days of systemic antibiotic exposure: a mean of 9.2 days with SER-155 versus 21.1 days with placebo. Other secondary outcomes—including gastrointestinal infection, grade 2 or worse acute graft-versus-host disease, febrile neutropenia and a composite endpoint—were not statistically significant. The prespecified pharmacodynamic endpoint of reduced gut domination by Enterococcus and Enterobacteriaceae was also not met.
Biomarkers support a mechanism, not clinical certainty
Researchers observed lower faecal albumin, a marker associated with intestinal-barrier leakage, and changes in inflammatory and immune-homeostasis biomarkers. Those findings fit the proposed mechanism: a stronger gut barrier may reduce movement of bacteria into the bloodstream. Biomarkers can make a clinical observation more biologically coherent, but they cannot replace replication of the clinical endpoint.
A larger randomized study must confirm infection reduction, define the role of antibiotic conditioning and test consistency across transplant centres. It should also examine antimicrobial-resistance patterns, graft-versus-host disease, survival, longer-term microbiome effects and the consequences of giving live organisms to profoundly immunocompromised patients.
SER-155 illustrates a maturing form of synthetic biology: manufacturing a defined ecological community as a medicine. The Phase Ib result is a credible signal that such a consortium may protect transplant recipients, but the 20-versus-14 comparison is too small to settle efficacy. The next trial will need to show that the effect survives scale.
Featured image: A biomedical laboratory. Contextual image, not the SER-155 study site or manufacturing process. Photo: Trnava University/Unsplash.
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