What happened: Researchers at the Japan Advanced Institute of Science and Technology found that Ewingella americana, isolated from a tree frog's gut, eradicated colorectal tumors in 100% of treated mice after a single IV dose. How it works: The bacterium accumulates in hypoxic tumors, rises ~3,000-fold in tumor tissue within 24 hours, kills cancer cells and stimulates strong T- and B-cell responses. Caveat: Clinical reports link the species to sepsis in immunocompromised humans, so extensive safety work and long clinical testing are needed before human use.
Frog Gut Bacterium Eradicated Colorectal Tumors In Mice — Human Use Still Far Off
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Japanese researchers report that a bacterium isolated from a tree frog's gut, Ewingella americana, eliminated colorectal tumors in 100% of treated mice after a single intravenous dose. Published in Gut Microbes, the study is an encouraging preclinical proof of concept but comes with important safety caveats and no immediate path to human treatment.
Mouse Results: Complete Tumor Regression After One Dose
Professor Eijiro Miyako and colleagues at the Japan Advanced Institute of Science and Technology screened 45 bacterial strains collected from amphibians and reptiles and identified E. americana as the most potent anti-tumor candidate. In a standard mouse model of colorectal cancer the single intravenous dose produced complete tumor regression in every treated animal and extended survival by at least 30 days compared with untreated controls.
How The Bacterium Works
The team reports a dual mechanism of action. As a facultative anaerobe, E. americana preferentially accumulates in the low-oxygen (hypoxic) microenvironment of tumors while largely sparing healthy organs. Tumor bacterial burden rose roughly 3,000-fold within 24 hours, and the microbes appear to both secrete factors that directly kill cancer cells and trigger a strong immune response. Treated mice showed increased infiltration of T cells and B cells and heightened inflammatory signals. Notably, mice cured by the treatment rejected attempts to reintroduce the same tumor, suggesting a durable, immune-mediated protection.
Comparison With Standard Therapies
In side-by-side tests using the same colorectal cancer model, anti-PD-L1 immunotherapy and doxorubicin chemotherapy slowed tumor growth but produced complete responses in only a small fraction of animals. The authors note that E. americana "substantially outperformed standard therapies" in this specific preclinical setting, but emphasize these are laboratory results that need careful validation.
Serious Safety Concerns
A major caveat is safety. Although E. americana showed minimal toxicity in healthy mice, clinical case reports tie the bacterium to sepsis in immunocompromised humans — including a documented instance of life-threatening infection after exposure via a contaminated blood transfusion. Cancer patients are often immunocompromised, which raises a clear dilemma for translating this approach into the clinic. Any therapeutic development would require extensive safety engineering, rigorous containment strategies, and carefully phased clinical trials.
Next Steps: More Animal Work and Cautious Development
Miyako’s team plans to test E. americana against additional tumor types, refine dosing to limit inflammatory side effects, and explore combinations with existing immunotherapies. For now, no human trials have been launched: the work remains preclinical and exploratory. The microbiome continues to be a promising source of novel therapies, but safe, effective translation to humans will likely take years of research and clinical testing.
Bottom line: Exceptional results in mice highlight an intriguing anti-cancer mechanism, but safety risks and the gap between animal models and human patients mean this is an early-stage discovery, not a ready-made treatment.
