Bactericidal and opsonizing effects of normal serum on mutant strains of Salmonella typhimurium

The bactericidal and opsonizing effects of normal human serum on six strains of Salmonella typhimurium LT-2 having different lipopolysaccharide (LPS) composition were demonstrated through five indices. Complement activity in the presence of antibody was important for the opsonization of all six strains and for the bactericidal effect on rough mutants. Complement activity, either in the presence or absence of antibody, was involved in the ingestion strains of SL 901 (SR) and SL 1032 (Rd1) by human neutrophils. Strain SH 5014 (Rb2) was avidly ingested by neutrophils and totally dependent on complement activity in the presence of antibody. The ingestion of strain SH 2201 (S) was also mediated exclusively by complement activity in the presence of antibody but not as efficiently as were rough mutants. Antibody, as demonstrated by quantitative fluorescence, enhanced the complement activity on the ingestion of the S, SR, and Rb2 strains by neutrophils. The intracellular killing of six strains was enhanced significantly by complement activity in the presence of antibody. The overall survival in the presence of serum and neutrophils decreased as the LPS became shorter. Complement activity in the presence of antibody enhanced extracellular killing only for strains SL 901 (SR) and his 515 (Ra). It was shown that there was no difference between SR and Ra strains in all five indices, suggesting that the one additional O-antigen side chain does not make the SR strain more resistant than the Ra strain. Although resistance by S. typhimurium to host defense mechanisms increases as the LPS chain length increases, the specific LPS structure appears to be of greater importance, especially with respect to opsonization.