Mechanism of bacterial resistance to complement-mediated killing: Inserted C5b-9 correlates with killing for Escherichia coli O111B4 varying in O-antigen capsule and O-polysacchride coverage of lipid A core oligosaccharide

The interaction of C3 and terminal complement component with three isogenic strains of Escherichia coli O111B4 varying in outer membrane and capsule composition was examined. Strains CL99 and 1-1, which possess O-antigen capsule and 74 to 77% coverage of lipid A core oligosaccharide, were sensitive to killing in pooled normal human serum (PNHS) or magnesium ethylene glycoltetraacetic acid PNHS in the presence but not the absence of antibody, although 1-1 contained 35% more lipopolysaccharide than CL99 and was slightly less sensitive to alternative pathway killing. These results indicate that C5b-9 that is bound to the outer membrane of E. coli O111B4 in a form resistant to salt or protease elution correlates with bacterial killing.