Chemical and electrophoretic changes induced by polymyxin B on outer membrane components from Serratia marcescens

The effects of polymyxin B (PB) on outer membrane (OM) components from resistant (strain 08) and sensitive (strain Bizio) cells of Serratia marcescens were characterized by chemical analysis and polyacrylamide gel electrophoresis (PGE) in sodium dodecylsulfate. Chemical analysis revealed no major differences in the OM fractions after PB treatment of both strains, except for the loss of protein in PB treated OM of the sensitive strain. The yield and composition between the lipopolysaccharides (LPS) of the two strains were different both before and after treatment. PGE revealed that there was a complex formation between the LPS of resistant strain and PB but both dissociation and degradation occurred in the LPS components in the sensitive strain. In addition, it was found that the protein components were destabilized by PB with subsequent loss of some of the components from OM of the sensitive strain. The difference in the amount of LPS and their ability to complex with PB may reflect different degrees of antibiotic susceptibility of these two strains of S. marcescens. A sequential multistep mechanism is proposed for the action of PB on outer membranes of this species.