Escherichia coli Nissle 1917 (O6:K5:H1) is a non-pathogenic commensal faecal isolate forming the basis of the probiotic preparation Mutaflor® used for treatment of various gastrointestinal disorders and diseases. E. coli Nissle 1917 has been shown to produce two bactericidal activities identified as microcins H47 and M (MccM). Microcins are gene-encoded antibacterial peptides with molecular masses inferior to 10 kDa secreted by Enterobacteriaceae. They exert bactericidal activity directed against phylogenetically-related bacteria, using complex mechanisms that involve various bacterial targets. Previously, the genetic system responsible for production and secretion of MccM and for immunity of the producing strain to the microcin has been sequenced, but MccM has never been isolated until now. In the present work, we expressed, isolated and purified MccM. Mass spectrometry of MccM identified a molecular mass of 7282 Da and consequently the cleavage site of the precursor. These data also suggested that MccM could carry a posttranslational modification similar to that characterized for microcin E492. Thus, MccM belongs probably to the recently characterized siderophore-peptide family.