Bacteriocins are protein or protein-complex antibiotics produced by a wide variety of bacterial species. By conventional definition, bacteriocins differ from most other antibiotics in that the producer strain is immune to the action of its own bacteriocin and the inhibitory activity of individual bacteriocins is directed only to bacteria which are closely related to the strains which produce them. Bacteriocin production is regulated by plasmid or chromosomal elements and bacteriocin activity is initiated by adsorption of bacteriocin to specific outer membrane receptors on susceptible cells. In Darwinian terms, production of bacteriocin by a bacterial strain, within a particular ecological niche, could be considered advantageous by ensuring elimination of other closely related, and thus competitive, bacteria. In contrast, conservation of bacteriocin receptors appears suicidal if their only function is to initiate cell death. The paper will illustrate the ubiquity of bacteriocins and discuss evidence for their in vivo function in terms of bacterial survival. Evidence will also be presented to indicate that bacteriocin receptors in Escherichia coli and Pseudomonas aeruginosa have important alternative physiological functions in outer-membrane mediated nutrient uptake, particularly with respect to bacterial iron metabolism.