Interbacterial communication can be mediated by soluble secreted factors and direct cell-cell contact. Recently, Aoki et al. identified a new contact-dependent communication pathway by which strains of uropathogenic Escherichia coli can inhibit the growth of other microbes within a mixed population. Two novel gene products--CdiA and CdiB, which seem to be members of a two-partner secretion family with homologs in many pathogens--mediate this contact-dependent inhibition (CDI). A third gene product, CdiI, provides immunity to CDI, as does expression of either P or S pili. The interplay between CDI and immunity factors could directly affect the course of an infection and modulate both the dispersion and the chronic persistence of bacterial pathogens within the host.