Two types of bacteria found in the gut might boost the risk of colon cancer, according to studies in mice and humans.
A dense layer of mucus separates the surface of the colon from the trillions of bacteria that reside in the colon’s interior. Cynthia Sears of Johns Hopkins University in Baltimore, Maryland, and her colleagues studied samples of colon tissue from people with a genetic propensity for developing polyps — tiny growths on the colon walls. The team found that the polyps often contained patches of mucus that had been invaded by bacteria.
Two species dominated the patches: Escherichia coli and Bacteroides fragilis, each carrying a gene that encodes a cancer-promoting toxin. When the two bacteria were introduced into mice predisposed to cancer, the mice developed tumours faster than control animals implanted with only one of the bacterial strains. They were also more likely to die during the study period.