Virulence Characteristics and Genetic Diversity of <i>Staphylococcus aureus</i> Isolates from Cases of Bovine Mastitis in Eastern Tennessee.

Staphylococcus aureus is an important zoonotic mastitis pathogen that has significant effects on animal and human health. S. aureus is also a foodborne pathogen that causes food poisoning through its diverse enterotoxins. Some studies showed that S. aureus strains that cause infection in a particular host are genetically distinct and are host specific, although most strains are believed to be infective to a wide range of host species. However, there are no clearly defined clonal patterns of S. aureus that possess certain virulence factors responsible for causing a disease. The objectives of this study were: 1) evaluate clonal diversity of S. aureus isolates from cases of bovine mastitis 2) determine staphylococcal enterotoxin production patterns 3) evaluate in vitro adhesion and invasion ability of dominant strains on bovine mammary epithelial cell line (MAC-T cells). Milk samples from bovine mastitis were evaluated at Tennessee Quality Milk Laboratory (TQML) for causative agents. Overall, 111 S. aureus strains were isolated and evaluated for genetic diversity by pulsed field gel electrophoresis (PFGE) and for presence of toxin genes that encode for staphylococcal enterotoxins (sea, seb, sec, see, sej) and toxic shock syndrome toxin 1 (tsst-1) by PCR. The in vitro adhesion and invasion ability of the dominant strains were evaluated on mammary epithelial cell line (MAC-T). We found 16 PFGE types (dominant clones) ranging from A – P. The PFGE type M is the most prevalent of all 16 PFGE types. The PCR results of enterotoxins genes showed that some of these strains were positive for staphylococcal enterotoxin genes including seb (11.7%), sec (2.7%), see (0.9%) and toxic shock syndrome toxin 1 (tsst-1) (7.2%), whereas most strains (75.7%) were negative for enterotoxin genes. In addition, evaluation of association of PFGE types, enterotoxins and other virulence factors, that were evaluated previously, showed that PFGE types O and M tend to cluster with beta-hemolysin, absence of enterotoxins and susceptibility to antimicrobials. Analysis of in vitro adhesion to and invasion into MAC-T cells showed relatively higher number of O strain adhered to and invaded into MAC-T cells followed by M and I strains however were not statistically significant.