Tiny viruses that only infect and kill bacteria can help treat deadly antibiotic-resistant bloodstream infections with Staphylococcus aureus, results from a mid-stage trial suggest.

Researchers tested the approach in 42 patients with S. aureus bacteremia that had spread from the blood into the tissues, which they called “one of the most serious and difficult-to-treat bacterial infections.”

Two-thirds were treated intravenously with a “cocktail” of such viruses, known as bacteriophages, being developed by Armata Pharmaceuticals. The others received a placebo. Everyone in the trial also received the best available antibiotic therapy.

At multiple time points, patients receiving the bacteriophage cocktail along with antibiotics had higher clinical success than those receiving only the antibiotics.

At day 12, for example, response rates were 88% with the virus treatment and 58% for the placebo group.

The treatment group also had lower non-response and relapse rates, shorter times to negative blood culture and to resolution of signs and symptoms, and less time in the intensive care unit and in the hospital, the researchers reported at a meeting of infectious disease doctors in Atlanta known as IDWeek 2025.

“These findings provide strong rationale for a Phase 3 study and signal a potential paradigm shift in how we treat antibiotic-resistant infections,” study leader Dr. Loren Miller of Harbor-UCLA Medical Center said in a statement.

“High-purity, phage-based therapeutics like (this one) may one day become a new standard of care for patients facing this life-threatening condition.”