Shigella sonnei phase I and phase II: susceptibility to direct serum lysis and opsonic requirements necessary for stimulation of leukocyte redox metabolism and killing.

The synthesis of the lipopolysaccharide O-specific repeat polymer by Shigella sonnei phase I is a clearly defined bacterial virulence factor necessary for penetrating epithelial cells; S. sonnei phase II does not synthesize this antigen and is uniformly avirulent. The serum opsonic requirements, relative to differences in gross lipopolysaccharide structure, were investigated by quantification and comparison of polymorphonuclear leukocyte (PMNL) metabolism and PMNL-mediated microbicidal action to phase I and phase II organisms, using normal and immune serum. The stimulation of PMNL O2-redox metabolism, as required for oxidative killing, was quantified by a chemiluminescent technique, using luminol as a chemilumigenic substrate. Susceptibility to direct serum or serum PMNL-mediated killing was evaluated by serum and serum-phagocytic killing assays. Stimulation of PMNL metabolism and phagocytic killing of S. sonnei phase I required opsonification by specific phase I antibody plus the classical pathway of complement. S. sonnei phase II was susceptible to direct complement-mediated serum killing. Likewise, opsonification of the phase II microbe, as measured by PMNL-associated chemiluminescence, was effected by complement in the absence of immune antibody. These data demonstrate the importance of the O-specific repeat polymer in protecting the microbe from the microbicidal action of PMNL and the bacteriolytic action of serum.