Seres Therapeutics has secured up to $3.6 million in additional non-dilutive funding from CARB-X to develop and manufacture an oral liquid formulation of SER-155, its cultivated live biotherapeutic designed to prevent bloodstream infections, including antimicrobial-resistant infections, in medically fragile patients who cannot swallow capsules. The program is advancing toward a well-powered, placebo-controlled Phase 2 in adults undergoing allogeneic hematopoietic stem cell transplantation, following a Phase 1b study that showed a 77% reduction in bacterial bloodstream infections versus placebo, alongside lower systemic antibiotic use and febrile neutropenia. SER-155 holds Breakthrough Therapy and Fast Track designations in this setting.
The funding is modest, but the strategic intent is not. Shifting from capsules to a liquid formulation moves SER-155 squarely into ICU and transplant workflows where nasogastric administration and intubation are common. In hospital prophylaxis, form factor can be the difference between an abstract promise and formulary adoption. The core question for Commercial and Medical leaders is whether a microbiome-based preventative can clear the clinical, operational, and economic thresholds set by antimicrobial stewardship committees and DRG-constrained hospital budgets.
The timing matters. With traditional antibiotics struggling through late-stage attrition and limited commercial returns, stakeholders are increasingly backing prevention-first approaches that can reduce exposure to broad-spectrum agents and downstream resistance. Regulators have warmed to microbiome therapeutics after the first approvals in recurrent C. difficile, but SER-155 targets a higher-acuity population and harder clinical outcomes. Demonstrating a prospective reduction in bloodstream infections, antibiotic days of therapy, and infection-related complications could resonate with hospital decision-makers, particularly if paired with compelling health economic data on ICU length of stay and readmissions.
The move also signals Seres’ post-Vowst pivot from donor-derived products to cultivated consortia manufactured from clonal cell banks, a shift that may improve batch-to-batch consistency, CMC control, and scalability—critical considerations for inpatient supply chains. For Medical Affairs, the safety narrative in profoundly immunocompromised patients will require rigorous education and real-world evidence on colonization dynamics, off-target effects, and interaction with standard prophylaxis regimens. For Market Access, the path likely runs through transplant centers and integrated delivery networks, with value dossiers tuned to institutional budgets, potential pathway inclusion in stewardship protocols, and coding strategies that reflect prophylaxis delivered during inpatient episodes of care.
Competitive implications are nuanced. Microbiome peers have retrenched after high-profile failures and consolidation, yet serious-infection prevention in oncology and transplant settings remains relatively open ground. Adjacent efforts in Europe aiming to restore microbiome diversity in graft-versus-host disease prevention and immune reconstitution may expand the clinical narrative, but direct comparators for bloodstream infection prophylaxis are limited. Success here could create a new category that sits alongside antivirals like letermovir and antibacterial prophylaxis, reframing the standard of care around microbiome resilience rather than pathogen suppression alone.
The next inflection is clinical and operational: can Seres lock in a Phase 2 that regulators and hospitals recognize as practice-changing, while proving that a liquid live biotherapeutic is stable, administrable, and scalable in real-world ICU and transplant settings? If the company can pair definitive outcomes with clear cost offsets, SER-155 could become a beachhead for microbiome-based prevention across autologous transplant, CAR-T, neutropenia, solid organ transplantation, and long-term acute care units, redefining how health systems value prophylaxis in the AMR era.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
