In a striking reminder that medical breakthroughs can emerge from the most unlikely places, scientists have identified a bacterium living in the intestines of Japanese tree frogs that shows exceptional potential as a cancer-fighting agent. The bacterium, Ewingella americana, described in popular reports as a possible “cancer assassin,” completely wiped out colorectal tumors in laboratory mice after a single intravenous dose, recording a 100 percent complete response rate in preclinical experiments.
The discovery was made by a research team led by Professor Eijiro Miyako of the Japan Advanced Institute of Science and Technology. Their findings were published in December 2025 in the peer-reviewed journal Gut Microbes and are the result of an unconventional investigation into the gut microbiota of amphibians and reptiles.
Why scientists looked to frogs and reptiles
Amphibians and reptiles are known to rarely develop spontaneous tumors in the wild, despite exposure to harsh environmental conditions, repeated tissue regeneration, complex life cycles, and pathogen-rich habitats that would normally increase cancer risk. The researchers suspected that this unusual resistance might be linked to protective microbes living in their digestive systems.
To test this idea, the team collected and analyzed 45 bacterial strains from the intestines of Japanese tree frogs, Japanese fire-bellied newts, and Japanese grass lizards. Laboratory screening revealed that nine of these strains showed measurable antitumor activity. Among them, Ewingella americana, isolated from the Japanese tree frog Dryophytes japonicus, emerged as the most powerful.
How the “cancer assassin” works
Ewingella americana is a facultative anaerobic bacterium, meaning it can survive and multiply in both oxygen-rich and oxygen-poor environments. This trait is critical because many solid tumors develop hypoxic cores that are difficult for conventional drugs to reach.
After a single intravenous injection, the bacterium rapidly multiplied inside tumors, increasing by as much as 3,000 times within 24 hours, while largely avoiding healthy tissues. Researchers found that it attacks cancer through two complementary pathways. First, it directly kills tumor cells by releasing toxic substances that destroy them from within. Second, it strongly activates the host immune system, drawing in T cells, neutrophils, and B cells that coordinate a sustained immune attack on the cancer.
This dual action not only eliminated existing tumors but also appeared to create lasting immune protection. In follow-up experiments, mice that had been cured rejected new cancer cells when they were reintroduced, suggesting the development of immune memory similar to a vaccine-like effect.
Remarkable results in animal studies
In mouse models of colorectal cancer, a single dose of Ewingella americana led to complete tumor eradication in every treated animal. The mice showed no tumor recurrence, lived significantly longer, and displayed no evidence of serious damage to healthy organs.
When compared directly with standard cancer treatments in the same models, the bacterium performed better than modern immune checkpoint inhibitors targeting PD-L1 and also outmatched liposomal doxorubicin, a commonly used chemotherapy drug. While these therapies slowed tumor growth, they rarely produced complete responses.
Safety concerns and what comes next
Early safety evaluations in mice were encouraging, with no lasting toxicity detected. However, Ewingella americana has been linked to rare infections in humans, particularly among people with weakened immune systems. Because of this, safety will be a central issue as researchers move forward.
The work remains firmly in the preclinical stage. Before any human use is possible, the bacterium will need to undergo extensive toxicity testing, dose refinement, and carefully controlled clinical trials. This process is expected to take several years.
A new frontier in cancer treatment?
Bacteria-based cancer therapies are gaining attention worldwide, especially for their ability to target hypoxic tumors that resist conventional treatments. The discovery of a naturally occurring strain with such dramatic effects adds a new and intriguing option to this growing field.
While it is far too early to describe this approach as a cure for human cancer, the combination of total tumor elimination in mice and strong immune activation has generated serious scientific interest. The finding suggests that nature’s microbial ecosystems, even those hidden in the gut of a small tree frog, may still hold untapped answers to some of medicine’s most difficult challenges.
For now, the so-called cancer assassin from the amphibian world stands as a powerful reminder that the next major breakthrough in cancer research may come from places we least expect.
