Role of phagocytosis in mouse virulence of Salmonella typhimurium recombinants with O antigen 6,7 or 4,12.

The quality of lipopolysaccharide has previously been shown to influence the mouse virulence of Salmonella so that strains with O antigen 4,12 were more virulent than their O-9,12 sister strains. Immunosuppression did not alter this O-antigen-dependent difference in virulence. I have now constructed smooth O-4,12 and O-6,7 sister hybrid strains of Salmonella typhimurium. No other phenotypic differences were found between these strains; they were all "common antigen" positive. In intraperitoneal infection, the O-4,12 strains were more mouse virulent than their O-6,7 sisters. The difference in virulence correlated with a difference in clearance rates; the O-6,7 hybrids were removed from the blood more rapidly than their O-4,12 sisters. No natural bactericidal antibodies were found in the sera of the mice.