Plasmid-associated resistance of Salmonella typhimurium to complement activated by the classical pathway

The association of the virulence plasmid of Salmonella typhimurium with resistance to the bactericidal activity of human serum was studied in chromosomally isogenic pairs of strains differing in their virulence plasmid status. The presence of the plasmid correlated in three pairs of strains with resistance to serum. The absence of the plasmid correlated with increased sensitivity to serum, whereas the reintroduction of the plasmid to the cell resulted in the restoration of resistance to serum. Complement was activated by the classical and alternative pathways equally well by both strains of a pair, but the differential bactericidal action of serum was apparent only after the activation of complement by the classical pathway. No differences in the chemical compositions or in the molecular weight ranges of the lipopolysaccharides were apparent between paired strains. This work confirms the presence of a virulence plasmid-associated mechanism of resistance to serum and distinguishes it from lipopolysaccharide-mediated resistance.