Effects of polymyxin B nonapeptide on Aeromonas salmonicida.

In contrast to polymyxin B-susceptible gram-negative bacteria of human origin, the fish pathogen Aeromonas salmonicida was resistant to sensitization by polymyxin B nonapeptide (PMBN) to hydrophobic antibiotics. Similarly, sensitization of A. salmonicida strains by PMBN to the bactericidal action of brook trout (Salvelinus fontinalis) serum complement was less pronounced than the similar effect of PMBN against other gram-negative bacteria in certain mammalian sera. The surface array protein (A layer), overlying the outer membrane of virulent A. salmonicida strains, did not appear to cause resistance to PMBN sensitization since an A-layer-deficient mutant showed similar responses to PMBN as its parent strain. Electron micrographs of PMBN-treated A. salmonicida cells revealed very little visible outer membrane disruption when compared with the extensive blebbing on the outer membrane surface caused by PMBN in certain enteric bacteria, including Escherichia coli K-37. However, small extracellular vesiclelike components, which may have been derived from the outer membrane, were numerous around PMBN-treated A. salmonicida cells. In this connection, PMBN caused disruption of the A layer in the form of bulges, breaks, and detached fragments which appeared to be associated with the accumulation of these vesicles underneath the A layer of wild-type A. salmonicida strains.