E-Mail IMAGE: Immunofluorescence staining shows that genotoxic colibactin-producing E. coli (green) cause DNA damage (indicated by the presence of the DNA repair protein γH2AX, white) and megalocytosis (abnormal enlargement of cells) (right).... view more Credit: Max Planck Institute for Infection Biology / Amina Iftekhar Escherichia coli bacteria are constitutive members of the human gut microbiota. However, some strains produce a genotoxin called colibactin, which is implicated in the development of colorectal cancer. While it has been shown that colibactin leaves very specific changes in the DNA of host cells that can be detected in colorectal cancer cells, such cancers take many years to develop, leaving the actual process by which a normal cell becomes cancerous obscure. The group of Thomas F. Meyer at the Max Planck Institute for Infection Biology in Berlin together with their collaborators have now been able to "catch colibactin in the act" of inducing genetic changes that are characteristic of colorectal cancer cells and cause a transformed phenotype - after only a few hours of infection.