Effect of polymyxin on the ultrastructure of the outer membrane of wild type and polymyxin resistant strains of Salmonella

Abstract

The effect of polymyxin on 2 sets of Salmonella mutants was studied by thin section and scanning electron microscopy. Polymyxin (in increasing concentrations, starting just below bactericidal effect) caused the appearance of the previously described rodlike projections on the cell surface of wild type (smooth, polymyxin sensitive) bacteria. These projections seemed to involve the outer membrane of the cell wall. In rough mutants, which are deficient in lipopolysaccharide, the projections were much smaller and flat. Higher concentrations of polymyxin were required to produce morphological effects in polymyxin resistant mutants of both smooth and rough forms. Furthermore, in these mutants, polymyxin caused vesicle like bulging of the total outer membrane quite different in appearance from the rod like projections of the wild type.