An investigational oral microbiome therapy was associated with a lower rate of bloodstream infections in adults receiving allogeneic hematopoietic cell transplants, according to a phase 1b study published in Nature Medicine. The small, partially randomized trial is an early result rather than proof of clinical benefit, but it points to a new way of addressing a serious post-transplant complication: rebuilding protective gut microbial functions.
Allogeneic hematopoietic cell transplantation, often called an allogeneic stem cell transplant, is used to treat a range of blood cancers and other disorders. It replaces a patient’s blood-forming cells with cells from a donor. The procedure can be lifesaving, but patients are highly vulnerable to infection during treatment and recovery. Conditioning therapies, antibiotics, hospitalization and immune suppression can substantially disrupt the gastrointestinal microbiome and damage the intestinal barrier.
That disruption matters because the gut can become a source of invasive infection. When protective microbial communities are depleted and the intestinal barrier is impaired, bacteria can move into the bloodstream. Bloodstream infections are a major clinical concern for transplant recipients, who may have limited immune defenses and can become severely ill quickly.
A cultivated mix of bacterial strains
The product studied, SER-155, is an investigational cultivated, multi-strain oral live biotherapeutic. It was designed to address gastrointestinal microbiome disruption and epithelial barrier integrity in adults undergoing allogeneic transplantation. Unlike a broad transfer of donor stool material, the product contains defined bacterial strains that are cultivated for use as a medicine.
The phase 1b study used two cohorts. The first was open label and enrolled 15 participants. The second was randomized and placebo controlled, with 45 participants. In that second cohort, 34 participants received either SER-155 following microbiome conditioning with oral vancomycin or a placebo, then subsequently underwent transplantation. The reported analysis compared 20 people assigned to SER-155 with 14 assigned to placebo.
Phase 1 studies are primarily intended to examine safety and how a treatment behaves in the body. This trial also included secondary efficacy measures, making the infection findings useful as a signal for future research but insufficient on their own to establish that the treatment prevents infections in routine care.
Safety and engraftment results
The researchers reported that SER-155 was well tolerated and had a safety profile comparable to placebo. There were no treatment-related serious adverse events and no infections caused by species included in the product. Diarrhea and nausea were the most common adverse events reported in the study.
Eight participants died during the trial: four in the open-label cohort and four in the second cohort. In the randomized cohort, two deaths occurred in the SER-155 arm and two occurred in the placebo arm. The investigators reported that none of the deaths was attributable to SER-155.
The study also assessed whether the administered strains could become detectable in recipients’ gastrointestinal microbiomes, a measure described as engraftment. All 16 strains in SER-155 were detected among the 16 participants evaluated for that endpoint. After the first treatment course, the median number of detected strains was 11.5.
That result does not by itself show that the strains caused the observed clinical differences. It does, however, show that a defined multi-strain product could establish detectable presence in this heavily treated patient population, where the microbiome is often profoundly altered.
An infection reduction that needs confirmation
The trial’s key secondary efficacy endpoint was the incidence of bloodstream infection. The reported rate was 10.0% in the SER-155 group and 43% in the placebo group. The authors calculated a 77% relative risk reduction and an odds ratio of 0.15, with a 95% confidence interval of 0.01 to 1.13 and a reported P value of 0.0423.
The result is encouraging, particularly given the clinical severity of bloodstream infections after transplantation. But the randomized comparison was small. With only 34 participants in the relevant treatment and placebo groups, estimates can shift substantially with a small number of events. The broad confidence interval also underscores the uncertainty around the exact size of any effect.
For clinicians and patients, the practical takeaway is not that a new treatment is ready to replace established infection-control measures. Antibiotic stewardship, surveillance, infection prevention practices and prompt treatment remain central to transplant care. Instead, the findings support the case for larger, rigorously designed trials that can test whether microbiome restoration adds a durable and clinically meaningful layer of protection.
A broader shift in microbiome medicine
The study arrives as researchers increasingly examine the microbiome as an active part of medical care rather than a passive consequence of illness and treatment. In transplant medicine, the question is especially pressing because interventions needed to control cancer and infection can also destabilize microbial ecosystems that help support intestinal health.
Defined live biotherapeutics could offer a more standardized route than approaches based on complex donor-derived material. A product with known strains can be manufactured and studied with more consistency, while its individual organisms can be monitored after administration. Those features may be important in patients with serious immune suppression, for whom safety requirements are particularly demanding.
Still, the early-stage evidence leaves important questions. Larger trials will need to confirm the bloodstream-infection finding, characterize which patients benefit most and assess durability across varied transplant settings. Researchers will also need to clarify how the product interacts with antibiotics, conditioning regimens and other factors that shape the microbiome.
For now, SER-155 provides an early clinical demonstration that targeted microbial restoration may be feasible after allogeneic transplantation. Its most consequential finding is not simply that bacterial strains can be delivered orally, but that a carefully defined microbiome intervention may eventually help reduce one of the most dangerous complications facing transplant recipients.
This article is based on reporting by Nature Medicine. Read the original article.
Originally published on nature.com
