Bacteria multiply by binary fission, but under certain circumstances bacteria within communities respond to their neighbors and change their physiological properties. Aoki et al. now describe a growth inhibitory system in Escherichia coli that requires direct cell-to-cell contact and that may function to regulate the growth of specific cells within a differentiated bacterial population. Two genes were found to be responsible for contact-dependent growth inhibition, CdiA and CdiB, and the authors also identified a DNA region that provides immunity against growth inhibition. CdiA and CdiB belong to the two-partner secretion family of proteins. Functional homologs are present in uropathogenic E. coli, and potential homologs exist in a broad range of bacteria, including many pathogens.