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The bacteria penetrate through the gut barrier and release compounds that damage DNA and inflame colon cells. This can both induce colon cancer and/or allow precancerous cells to grow into cancer.

The strains of bacteria are Bacteroides fragilis and a strain of E. coli. Not everyone has these two types of bacteria but those that do are thought to have gotten them during childhood.

This study analyzed 25 tumor samples taken from people with familial adenomatous polyposis and found the two bacterial species present in large quantities.

Animals were then given a cancer-causing agent to cause mutations in DNA of colon cells. There were few or no tumors until the animals were transplanted with both strains of the gut bacteria…this caused tumor growth.

It is unclear how people acquire these two strains of bacteria early in life or how to get rid of them. But this is the first step in understanding a complex interaction between certain species of gut bacteria and colon cancer.

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    When you find B fragilis in a stool culture, what should your be suspicious for? Answer: colon cancer. This has been a favorite board question on or GI board exam for years. The mechanism, however, was never explained. This is very interesting. May need to be considered in our risk stratification for colon cancer screening.