Channel-forming bacteriocin. Forms cation-selective channels. Active on enterobacteria, with highest activity against E.coli. Not active on other Gram-negative bacteria, Gram-positive bacteria or fungi. The unmodified protein is active against E.coli and S.enteritidis. When the siderophore ester is present at Ser-99, antibacterial activity against these species is increased and activity is also detected against E.cloacae and K.pneumoniae. Neutralized by its immunity protein MceB (PubMed:9864332). Multimer. Possibly forms a homodimer or a homotrimer. The C-terminal Ser is modified by attachment to a siderophore similar to enterobactin, which can bind one atom of iron. The modification consists of an ester linkage of the serine carboxyl to O6 of a glucose which is linked by a C-glycosidic bond to the 5'-benzoyl of a linear triester of N-(2,3-dihydroxybenzoyl)serine. Presence of the siderophore ester increases the antibacterial activity of the protein. Ser-99 unmodified form. Ser-99 modified form. Belongs to the class IIa microcin family.