Seres' Liquid Biotherapeutic Offers New Hope in Superbug Fight

CAMBRIDGE, MA – April 09, 2026 – As the global health community braces for a future where common infections could once again become deadly, Cambridge-based Seres Therapeutics is preparing to showcase a novel weapon in the war against antimicrobial resistance (AMR). The company announced it will present its work on a promising liquid biotherapeutic at the upcoming CARB-X Investor Day, a key gathering of innovators and investors dedicated to confronting the world's growing "superbug" crisis.

The presentation will spotlight SER-428, a therapeutic designed not as a pill, but as an oral liquid. This formulation aims to reach some of the most vulnerable patients—those in intensive care units who are unable to swallow capsules—providing a potential new line of defense against drug-resistant infections in a setting where they are most devastating.

The Rising Tide of Antimicrobial Resistance

The battle against AMR is one of the most critical public health challenges of the 21st century. The World Health Organization (WHO) has declared it a top ten global public health threat, with grim projections estimating that annual deaths from drug-resistant infections could soar to 10 million by 2050. This staggering figure would place mortality due to AMR on par with deaths caused by cancer, signaling a catastrophic rollback of modern medical progress.

For decades, antibiotics have been the cornerstone of medicine, making complex surgeries, organ transplants, and chemotherapy possible. But their overuse and misuse have accelerated the evolution of bacteria that can withstand these life-saving drugs. The pipeline for new antibiotics has run dangerously dry, with many large pharmaceutical companies exiting the field due to high development costs and low returns. This innovation gap has created an urgent need for new approaches, moving beyond traditional antibiotics to find creative ways to prevent and treat infections.

This is the landscape where organizations like CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator) have become indispensable. A global nonprofit partnership, CARB-X provides critical non-dilutive funding and expert support to accelerate the development of new antibacterial products, bridging the perilous gap between early-stage research and clinical viability. Since 2016, it has supported 92 projects, playing a role in the development of a significant portion of the world's non-traditional antibacterial clinical pipeline.

Beyond the Pill: A Liquid Lifeline for Critical Care

Seres Therapeutics' SER-428 represents a significant step in this new frontier. It is not a traditional antibiotic but a live biotherapeutic—a consortium of beneficial bacterial strains designed to restore a healthy gut microbiome. A robust microbiome is a patient's first line of defense, capable of outcompeting and preventing the colonization of dangerous, drug-resistant pathogens.

The true innovation of SER-428, however, lies in its delivery. The development of an oral liquid formulation directly addresses a major unmet need in critical care medicine. Patients in the medical ICU are often intubated, sedated, or suffer from dysphagia (difficulty swallowing), making administration of solid oral medications impossible. This leaves them highly susceptible to hospital-acquired infections, including those caused by antibiotic-resistant bacteria. SER-428 is engineered to be administered to these patients, potentially protecting them when they are most at risk.

SER-428 is based on the bacterial strains used in SER-155, the company’s Phase 2-ready candidate for patients undergoing allogeneic hematopoietic stem cell transplants (allo-HSCT). SER-155 has already shown considerable promise. In a Phase 1b study, it demonstrated a significant reduction in bacterial bloodstream infections and was granted both Breakthrough Therapy and Fast Track designations by the U.S. Food and Drug Administration (FDA). This prior clinical validation provides a strong scientific foundation for the development of SER-428.

Seres is now designing a Phase 1b open-label trial for SER-428 in collaboration with Dr. Dan Freedberg at Columbia University, specifically targeting medical ICU patients at high risk of infection.

A Strategic Partnership in a Challenging Financial Climate

The advancement of SER-428 is bolstered by a productive, long-term collaboration with CARB-X. In 2025, Seres received its second award from the accelerator, a non-dilutive grant of up to $3.6 million, to specifically support the development and manufacturing of the SER-428 liquid formulation.

“We are pleased to continue our collaboration with CARB-X, whose support has been instrumental in advancing our efforts to address antimicrobial resistance,” said Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres. “We believe SER-428 can play an important role in preventing and treating antimicrobial resistance and other serious infections in ICU patients, and, with CARB-X support, are progressing toward clinical studies in high-risk populations.”

This type of funding is more critical than ever for clinical-stage biotechnology firms. The announcement comes at a pivotal moment for Seres Therapeutics. After divesting its first FDA-approved microbiome therapeutic, VOWST, to Nestlé Health Science in 2024, the company has been navigating a strategic pivot to focus its resources on its pipeline of therapies for inflammatory and immune diseases.

However, the financial realities of biotech R&D are stark. According to its latest financial disclosures, Seres' current cash reserves are projected to fund operations only through the third quarter of 2026, and the company recently reduced its workforce by 30% to extend its cash runway. This context underscores the immense value of non-dilutive funding from partners like CARB-X, which allows high-impact, high-risk projects like SER-428 to move forward without forcing the company to give up equity at a vulnerable stage. While the broader SER-155 program is ready for Phase 2 trials, its progression remains contingent on securing additional funding or a strategic partnership, highlighting the precarious path of innovation in the current economic climate.

Seres continues to seek collaborators to support its broader pipeline, which includes programs for inflammatory bowel disease (IBD) and for reducing life-threatening infections in transplant patients, demonstrating a focused effort to leverage its live biotherapeutic platform to address significant unmet medical needs.