Biological characteristics and pathogenicity of a Staphylococcus aureus strain with an incomplete hemolytic phenotype isolated from bovine milk

Staphylococcus aureus is a common pathogen capable of infecting both humans and animals and causing various severe diseases. Here, we aimed to determine the biological features and pathogenicity of S. aureus strain Sa9, of the incomplete hemolysis phenotype, isolated from bovine milk. Sa9 was classified as ST97 by multilocus sequence typing, and it showed increased β-hemolysin expression and lower Hla and Hld expression levels compared with that in the S. aureus USA300 strain LAC. RT-PCR and ELISA results showed that the expression levels of inflammatory cytokines were higher in Sa9-induced mouse primary peritoneal macrophages compared with those induced by the LAC strain. However, the Sa9 strain also mediated anti-inflammatory effects by upregulating IL-10 and IFN-β in macrophages, which were not apparently induced by S. aureus culture supernatants. Phagocytosis and whole-blood survival assays were also performed to assess the in vitro survival of bacteria, and the virulence was evaluated in mice. Although the Sa9 strain showed lower ability of intracellular survival in macrophages than LAC, similar multiplication in human whole blood and pathogenicity toward mice were observed. Taken together, we report that the distinctive immune response induced by the S. aureus strain with an incomplete hemolysis phenotype occurs in cattle, and its potential pathogenicity and risk of transmission to humans require attention.