In a study that for the first time establishes a direct link between the microbes inhabiting our bodies and the genetic alterations that drive cancer development, researchers from the Hubrecht Institute, in the Netherlands, have found that mutations linked to colorectal cancer can be caused by common gut bacteria carried by about one out of five people. By exposing cultured human mini-guts to a particular strain of E. coli bacteria, they observed that the bacteria induced a unique pattern of mutations in the DNA of human cells that was also found in the DNA of patients with colorectal cancer, implying that these mutations were induced by the “bad” bacteria.