Microcins are a class of ribosomally synthesized antibacterial peptides produced by Enterobacteriaceae and active against closely related bacterial species. While some microcins are active as unmodified peptides, others are heavily modified by dedicated maturation enzymes. Low-molecular-weight microcins from the post-translationally modified group target essential molecular machines inside the cells. In this review, available structural and functional data about three such microcins--microcin J25, microcin B17 and microcin C7-C51--are discussed. While all three low-molecular-weight post-translationally modified microcins are produced by Escherichia coli, inferences based on sequence and structural similarities with peptides encoded or produced by phylogenetically diverse bacteria are made whenever possible to put these compounds into a larger perspective.