All colicins require a receptor to localize the toxin to the cell surface and a translocator that is involved in presenting signalling epitopes to the periplasm. Colicins are categorized as group A colicins if they parasitize the Tol system and group B colicins if they parasitize the Ton system (see main text, Table 1). Colicins imported by both systems generally kill cells, either by depolarizing the inner membrane or by cleaving nucleic acids in the cytoplasm. The figure does not depict all colicin cytotoxic activities or all of the proteins that have been implicated in colicin import. pmf, proton motive force.