The microcins

Abstract Microcins are antibiotics of low M _r, constitutively (non-lethally) produced by non-sporulating bacteria, such as Enterobacteriaceae. Their production depends on plasmids and is not inducible by DNA-damaging agents. Hitherto, five types of microcins have been identified by cross-immunity, biochemical and genetic criteria. Microcins have an amino acid or oligopeptide structure and show different mechanisms of action: inhibition of metabolic enzymes (type A) or of DNA replication (type B), or impairment of the cell's energy-generating system (type D). In some cases (microcin B17), a complex genetic system involving up to seven genes may be required for the synthesis. Finally, microcins may play a role in bacterial interactions in natural microbial ecosystems.