Polymyxin B nonapeptide complexes with lipopolysaccharide

We have recently shown that a polymyxin derivative, polymyxin B nonapeptide (PMBN), which lacks the fatty acid part, the bactericidal activity and the toxicity of polymyxin [1,2] nevertheless causes disorganization of the enterobacterial outer membrane (OM). Thus, PMBN makes the OM of smooth encapsulated Escherichia coil and smooth Salmonella permeable to hydrophobic antibiotics such as actinomycin D, novobiocin, fusidic acid, erythromycin and others, and sensitizes the bacteria to all those agents. Furthermore, PMBN sensitizes the PMBN to the attack of the complement cascade (M. Vaara and T. Vaara, manuscripts submitted for publication). Electron microscopic studies showed that fingerlike projections appear on the OM of bacteria treated with PMBN (M. Vaara and T. Vaara, submitted). These projections appear similar to those caused by polymyxin [3]. Polymyxin interacts with acidic macromolecules, including the lipopolysaccharide (LPS) present in the outer leaflet of the OM [4-8]. We have previously shown that the LPS of polymyxin-resistant pmrA mutants of Salmonella typhimurium binds less polymyxin and is less acidic (due to the nearly stoicheiometric occurrence of 4-amino-4-deoxy-arabinose in the lipid A part of the LPS) than the LPS of the polymyxin sensitive parent strains [8]. In the present communication we show that PMBN does not sensitize the pmrA strain (in contrast to the parent strain) to hydrophobic antibiotic~ Furthermore, we show that PMBN, like polymyxin, inhibits biological activities of isolated LPS (gelation of Limulus-lysate, consumption of complement). These results suggest that PMBN causes a disorganization of the OM by binding to the acidic groups of LPS, as does polymyxin.