A study of virulence factors with induced mutants of Staphylococcus aureus.

The contribution of a number of extracellular products of Staphylococcus aureus to the virulence of the organism for mice was studied by comparing a wild-type strain with various mutants derived from it in three virulence tests: (1) subcutaneous and (2) intravenous challenge in normal mice, and (3) subcutaneous challenge in mice after total body X-irradiation. Mutants with lower production or non-production of coagulase, staphylokinase and leucocidin were just as virulent for mice as the wild type in all three tests. Unlike the wild type, mutants with low production or nonproduction of alpha-lysin never gave necrosis after subcutaneous injection in normal mice. One mutant with loss of delta-lysin and unaltered alpha-lysin production gave necrosis only when injected in high doses. Dermonecrosis seems to be caused by a combination of alpha- and delta-lysin. Intravenous injection of each of the two types of mutant in normal mice gave a lower mortality rate than that obtained with the wild type. Mutants with deficient alpha-lysin production, but not delta-lysin-deficient mutants, multiplied more slowly in the kidneys than the wild type under these conditions. alpha-Lysin appears to have a growth-enhancing effect for the organism in vivo, but delta-lysin does not. Differences in virulence between the wild type and mutants could not be demonstrated in irradiated mice.