Bacteroides fragilis and Escherichia coli appear to collaborate to promote hereditary and sporadic colon cancers, according to the results of studies published in Cell Host and Microbe and Science.

About 5% of colon cancers are caused by hereditary familial adenomatous polyposis (FAP), where genetic changes from an inherited mutation eventually prompt colon epithelial cells to turn malignant. The study authors found that in patients with FAP, B. fragilis produces a toxin that triggers oncogenetic pathways in those epithelial cells and causes colon inflammation. At the same time, E. coli produces a toxin that causes DNA mutations. 

Of importance, both bacteria need to be present to affect the incidence of cancer. The researchers tested a mouse model of colon cancer and found that animals whose colons were colonized with just one of the species developed few or no tumors. However, when their colons were colonized with both simultaneously, they developed many tumors, suggesting a synergistic effect between the bacteria.

The authors hope the findings can be used to prevent or treat colon cancer, perhaps by preventing the colon from being colonized by one of the bacteria or developing drugs or vaccines that target their toxins. Future studies are planned to better understand why the immune system permits the ongoing inflammation.

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