Engineered Probiotic Reduces Colorectal Tumors by 43% in Mouse Trials

Columbia University researchers engineered E. coli Nissle 1917 bacteria to reduce colorectal tumors by 43% in mice, with complete regression observed in three out of seven mice with established tumors

Columbia University researchers engineered E. coli Nissle 1917 bacteria to reduce colorectal tumors by 43% in mice. The treatment involved a single injection of modified bacteria followed by 5-fluorocytosine administration. In trials, complete tumor regression occurred in three of seven mice with established colorectal tumors. The bacteria were engineered to produce cytosine deaminase (converting 5-fluorocytosine to 5-fluorouracil), delete genes that break down 5-fluorouracil, and include immune-stimulating components like an IL-15 superagonist and PD-L1 blocking nanobody. This approach reduced systemic toxicity compared to conventional chemotherapy.

The mechanism targets tumor growth through dual action: the bacteria convert 5-fluorocytosine into the chemotherapy drug 5-fluorouracil within tumors while avoiding systemic breakdown. Initial treatment showed minimal tumor shrinkage before genetic modifications, suggesting the engineered bacteria enhance therapeutic efficacy.

If translated to humans, this approach could lower colorectal cancer treatment costs by reducing reliance on invasive procedures and systemic toxicity. Current therapies often involve high expenses and significant side effects, so a targeted, less toxic method might improve accessibility for resource-limited settings.

The study was published in *Science Translational Medicine* (2026) but remains in early-stage trials. Key caveats: results are from mice only; the 43% figure represents complete regression in three out of seven tumors; and systemic cancer spread was not observed in treated mice. Next steps include human trials to confirm efficacy and safety.

Source: ScienceAlert