Phase 1 trial shows SER-155 live biotherapeutic linked to fewer bloodstream infections after allogeneic stem cell transplant

Researchers at City of Hope, Memorial Sloan Kettering, and Seres Therapeutics administered SER-155 to highly immunocompromised patients after allogeneic hematopoietic cell transplantation. The therapy was designed to occupy nutrient niches, produce antimicrobial compounds, and modulate immune tone rather than relying on undefined fecal material. No treatment-related serious adverse events occurred; patients showed lower densities of pathobionts previously linked to translocation and sepsis.

Standard antibiotic prophylaxis disrupts colonization resistance, elevating infection risk. SER-155 was selected for targeted functional properties identified through prior observational cohorts linking specific taxa to protection. Absolute infection rates were lower than historical controls at the same centers, though the single-arm design precludes causal attribution. This approach differs from broad fecal microbiota transplantation by using a reproducible, GMP-manufactured product.

The study remains early-stage. Larger randomized trials will need to confirm clinical endpoints, assess durability of engraftment, and evaluate effects on graft-versus-host disease or relapse. Regulatory precedent for live biotherapeutics in this population is still evolving.