Scientists from City of Hope, along with partners, have developed a new microbiome therapy aimed at reducing bloodstream infections in individuals undergoing donor stem cell transplants for hematologic malignancies.
The multicenter phase 1 trial, conducted with Memorial Sloan Kettering Cancer Center and Seres Therapeutics, evaluated a pill-based product called SER-155. The formulation contains 16 carefully selected bacterial strains intended to repopulate the gut and suppress harmful microbes.
According to the study published in Nature Medicine, the therapy was well tolerated. No treatment-related adverse events were reported, and investigators described the safety profile as “excellent.”
The trial is formally cited as Ponce et al., A live biotherapeutic product for adult allogeneic hematopoietic cell transplantation recipients: a partially randomized, placebo-controlled phase 1b trial, Nature Medicine (2026), DOI 10.1038/s41591-026-04686-y.
Patients who received SER-155 experienced fewer bloodstream infections and showed lower levels of pathogenic bacteria linked to sepsis, indicating an early signal of clinical benefit.
A November 8, 2026 report noted that this microbiome therapy shows promise for preventing deadly bloodstream infections, emphasizing its early clinical impact.
Trial focuses on high-risk transplant recipients
The study enrolled adults undergoing allogeneic hematopoietic cell transplantation, a procedure that can cure leukemia but leaves recipients profoundly immunocompromised while their immune systems recover.
Standard care relies on broad-spectrum antibiotics to prevent infection, yet these drugs do not address the underlying disruption of the human microbiome that predisposes patients to invasive pathogens.
Targeted bacterial cocktail differs from fecal transplants
Unlike fecal microbiota transplants, which deliver an undefined mix of microbes, SER-155 uses a defined consortium. Each strain was chosen for a specific function—such as consuming nutrients needed by pathogens, producing antimicrobial compounds, or modulating immune responses.
Trial participants first received a short course of antibiotics to clear existing gut flora, creating space for the introduced bacteria. They then took SER-155 before transplantation and received at least one additional dose after the procedure, aligning with the typical antibiotic regimen used in transplant protocols.
Analysis of stool samples showed successful colonization of the administered strains, especially after the second dosing round, confirming that the live biotherapeutic can establish itself even in a heavily medicated environment.
Although the study was not powered to assess definitive clinical endpoints, early data revealed that treated patients harbored fewer of the most dangerous pathogens and faced a markedly reduced risk of developing bloodstream infections.
Implications for microbiome-based medicine
The findings illustrate that a precisely engineered bacterial consortium can be administered safely to patients with severely compromised immunity. This demonstration expands the therapeutic toolbox beyond conventional antibiotics and suggests a new avenue for infection prevention.
Beyond infection control, the study adds to growing data linking the gut microbiome to cancer treatment outcomes. Investigators note that microbial composition can affect how patients respond to therapy, recover from intensive regimens, and experience treatment-related side effects.
Practical steps for patients
Patients are advised to avoid over-the-counter probiotic supplements unless specifically prescribed. Some commercial formulations contain strains that have been associated with adverse outcomes in immunocompromised individuals.
City of Hope’s microbiome program plans additional studies to evaluate similar live biotherapeutic approaches. Ongoing research aims to create therapies that complement immunotherapy and further improve recovery after stem cell transplantation.