Microcins are gene-encoded antibacterial peptides secreted by enterobacteria. They are believed to play a major role in the microbial competitions within the intestinal tract. We have recently isolated a posttranslationally modified form of the 84-residue microcin E492 named MccE492m. MccE492m posttranslational modification was characterized as a trimer of N-(2,3-dihydroxybenzoyl)-L-serine (DHBS) linked to the Ser84-carboxylate via a beta-D-glucose moiety. MccE492m was shown to bind iron through the trimer of DHBS. This is the first example of a novel type of antibacterial peptide called siderophore-peptide. Recognition of both modified and unmodified MccE492 was shown to be mediated by the FepA, Cir and Fiu iron-siderophore receptors at the outer membrane of E. coli. However, the siderophore-mimicking modification was shown to significantly improve the microcin activity. Thus, we propose that MccE492 and MccE492m parasitize iron-siderophore receptors for uptake into the target bacteria and that improvement of MccE492 antimicrobial activity upon modification results from an increase in the microcin/receptor affinity.