Role of lipopolysaccharide and complement in susceptibility of Escherichia coli and Salmonella typhimurium to non-immune serum.

The role of lipopolysaccharide (LPS) in the susceptibility of Escherichia coli and Salmonella typhimurium to non-immune human serum was investigated using serum-sensitive strains of both enterobacteria. LPS from serum-resistant strains of E. coli and S. typhimurium could activate and completely remove the serum bactericidal activity, and also showed dose-dependent anti-complement activity. These properties were mainly due to the high-molecular-mass LPS: the low-molecular-mass LPS from serum-resistant strains of E. coli and S. typhimurium had only a slight effect on the serum bactericidal activity, and showed only low anti-complement activity, even at high concentration. The results suggest that LPS composition, especially the O-antigen polysaccharide chains, contributes to the susceptibility of E. coli and S. typhimurium strains to complement-mediated serum bactericidal activity.