The hydrophobic and anionic 84-residue antimicrobial peptide microcin E492 (MccE492) from Klebsiella pneumoniae was first isolated as an unmodified peptide. We have isolated a MccE492 posttranslationally modified form (MccE492m), which exerts more potent antimicrobial properties than MccE492. Both peptides were submitted to chymotrypsin digestion. A purified 1771.6 u fragment from the MccE492m digest contained the entire 831.2 Da modification. This fragment was submitted to MS fragmentation and 800 MHz NMR analyses. The complete structure of the modification was identified. It carries N-(2,3-dihydroxybenzoyl)-L-serine, which belongs to a group of catechol-type siderophores involved in iron uptake by enterobacteria. Moreover, MSn experiments in presence of FeCl3 showed that the modification was able to capture Fe3+. We show that recognition of both MccE492 and MccE492m is mediated by iron/siderophore receptors at the surface of sensitive bacteria. Therefore, we propose that improvement of MccE492 antimicrobial activity upon modification results from an increase of the microcin/receptor affinity. MccE492m is the first example of a novel type of antibacterial peptides that we call siderophore-peptides.