Cancer mutations can be caused by common gut bacteria. By exposing human mini-guts to a particular strain of Escherichia coli, scientist uncovered that these bacteria induce a unique mutational pattern in human DNA. This pattern was also found in colon cancer patients, implying that the bacteria induced these mutations. For the first time the researchers establish a direct link between microbes and mutations driving tumorigenesis. This might lead to new measures in colorectal cancer prevention.