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-polysaccharide coverage of lipid A core oligosaccharide.

The interaction of C3 and terminal complement components 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. In contrast, strain 1-2 lacks O-antigen capsule but contains 84% coverage and resists serum killing in the presence and absence of antibody in both PNHS and magnesium ethylene glycoltetraacetic acid PNHS. All three strains consumed C3 and C9 when incubated in PNHS, but consumption was most rapid with 1-2, which also bound the largest number of C3 molecules per CFU. Between 15 X 10(3) and 24 X 10(3) molecules of C9 per CFU bound to CL99 and 1-1 during incubation in 10% PNHS or 10% magnesium ethylene glycoltetraacetic acid PNHS, and binding was relatively stable. Binding and release of 3 X 10(3) to 8 X 10(3) molecules of C9 per CFU was observed for strain 1-2. The majority of C9 bound to CL99 and 1-1 in the presence of antibody distributed with the outer membrane after lysis of the organisms in a French press, whereas only 16.1 to 20.1% of C9 was deposited on these organisms in the absence of antibody, and 31.5 to 39.8% of C9 on strain 1-2 with or without antibody sedimented with the outer membrane. Between 4.6 X 10(3) and 5.5 X 10(3) molecules of C9 per CFU remained bound in a salt- and trypsin-resistant form to the outer membrane of organisms that were killed, whereas fewer than 1.4 X 10(3) molecules of C9 per CFU were bound to the outer membrane of organisms not killed by serum. 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.