Virulence of Staphylococcus aureus in a mouse mastitis model: studies of alpha hemolysin, coagulase, and protein A as possible virulence determinants with protoplast fusion and gene cloning.

Mutants of a genetically well-characterized strain of Staphylococcus aureus [SA113(83A)] were isolated after mutagenization. Alpha-hemolysin- (hla), coagulase- (coa), and protein A- (spa) negative mutants were characterized by more than 90 biochemical tests for production of extracellular proteins and biochemical profile to exclude pleiotropy. Protoplast fusion was then used to isolate double-defective (hla and coa) recombinants and recombinants with regained properties, i.e., production of alpha-hemolysin and coagulase. Studies of such mutants and recombinants in the mouse mastitis model showed that one alpha-hemolysin [SA113(83A) hla-5] and one coagulase-negative [SA113(83A) coa-147] mutant were lower in virulence compared with the wild-type strain SA113(83A). The double-negative mutant SA113(83A) hla-5 coa-147 showed a drastic decline in virulence and only induced very mild changes, as determined by microscopic examinations of infected mammary gland tissue. The recombinant with regained properties, however, was as virulent as the wild-type strain. This suggests that alpha-hemolysin and coagulase are virulence determinants of S. aureus. A high-level protein A-producing mutant (U300) showed the same virulence as the parent strain SA113(83A) in this model. One low virulence protein A-negative mutant (U320) did not markedly increase in virulence when a plasmid containing the cloned gene for protein A (pSPA15) was introduced into this mutant. By these and earlier observations, it seems likely that protein A is not an important virulence determinant in mastitis of mice. The reduced virulence of the protein A-negative mutant U320 compared with the wild-type SA113(83A) may be due to pleiotropic loss of some other unknown virulence determinant(s). Our data confirm earlier findings that pleiotropic changes are common in protein A-negative mutants.