Focus on modified microcins: structural features and mechanisms of action

Microcins are gene-encoded antimicrobial (poly)peptides secreted by Enterobacteriaceae. Produced under conditions of nutrient depletion, they are active against phylogenetically related microbial strains. Therefore, they are considered to play an important role in the microbial competitions within the intestinal flora. Among the limited sample of nine microcins hitherto described, a wide variety of structures and modes of action could be identified. The knowledge on microcins is very uneven, some being extensively studied, and others remaining uncharacterized. In this article, we have focused on a subgroup of highly modified microcins that show very original structures. We present an updated overview on the structures and mechanisms of action of microcins B17, C7 and J25, and on the associated effector proteins, also encoded by the microcin genetic system, which include specific modification enzymes, export proteins, and immunity factors.