Composition of the outer membrane of Proteus mirabilis in relation to serum sensitivity in progressive stages of cell form defectiveness

Abstract

A serum-resistant strain of Proteus mirabilis was used to determine whether changes in the composition of surface components could be detected following induction of progressive stages of cell form defectiveness by beta-lactam antibiotics. The critical stage was the conversion from filaments to the spheroplast form, which was accompanied by increased susceptibility to the bactericidal action of human serum. Inner and outer membranes of the bacterium, its filament form and its spheroplast form were separated by sucrose density-gradient centrifugation after digestion of peptidoglycan, followed by osmotic lysis of the cells. Outer membranes of the bacterial and the filament forms sedimented at the same density, whilst the outer membrane fraction of the spheroplast form sedimented in a region of lesser density. In addition, the amounts of two major outer-membrane proteins as well as the O-polysaccharide content of the lipopolysaccharide were reduced in the spheroplast form. These results indicate a general disorganization in structure and assembly of components in regard to their interactions with one another in the outer membrane of the spheroplast form.