Studies over the past two decades indicate that genotoxic gut bacteria, which damage or mutate DNA, are critical drivers of colorectal cancer (CRC) pathogenesis (1). Moreover, the notable increase in early-onset CRC incidence highlights the urgency to discover responsible exogenous factors that are amenable to therapeutic intervention. On page 369 of this issue, Cao et al. (2) report that individuals with inflammatory bowel disease (IBD), a common CRC precursor state, display diverse gut bacterial strains that potentially exert genotoxic activity. Pursuing Morganella morganii, a Gram-negative bacterium enriched in the gut microbiota of people with IBD and CRC, the authors identify a class of bacterial genotoxins called indolimines. They find evidence that indolimines promote tumor development in mice, thereby expanding the role of bacterial genotoxic metabolites in colorectal carcinogenesis.