The effect of lipopolysaccharides (LPS) on the normal bovine serum (NBS) bactericidal reactions against mixture of S. flexneri 6713 1b strain and its 3b Lac+ recombinant were investigated. The serum killing of S. flexneri strains was inhibited, in different degree, by LPS extracted from either organisms. These properties were mainly due to LPS molecules; the lipid A fraction showed only low anticomplement activity, the polysaccharide fraction inhibited the killing activity of NBS in very low degree even at high concentration. These studies suggest that LPS composition especially the O-antigen polysaccharide chain contributes to the susceptibility of S. flexneri strains to NBS bactericidal activity.
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