The potential role of Staphylococcal enterotoxin C and TSST-1 in the infection of bovine mammary epithelial cells

Bovine mastitis is a continuous problem in the dairy industry. The infection has significant impact on health and welfare of animals and causes financial losses for the farmers and the dairy industry. A key bacterium associated with bovine mastitis is Staphylococcus aureus (S. aureus). It produces different exotoxins including more than 24 different staphylococcal enterotoxins and toxic shock syndrome toxin-1 (TSST-1). We have investigated the role of enterotoxin C (SEC) and TSST-1 in the infection process. Five different S. aureus sec or tst-1 deletion mutants, derived from three wild type strains isolated from cases of bovine mastitis, were constructed and characterized regarding growth and enterotoxin formation. These mutant and wild type strains were used to infect bovine mammary epithelial (BME-UV) cells to evaluate their infection ability. The ratio of S. aureus recovered in BME-UV cell lysate after 7 h of infection to the initial S. aureus infection dose was calculated to provide a measure of infection capability of each strain. Deletion of the sec gene overall showed a reduction in infection ratio, suggesting that presence of SEC may play a role during the establishment of infection. In contrast, deletion of tst-1 did not appear to affect the infection capability to the same extent. Proteomic analyses indicated that infection by two out of three S. aureus wild type strains elicited a systematic alteration in the BME-UV cell proteome. The isogenic sec deletion mutants of these two wild type strain also produced differences in the proteome of the BME-UV cells compared to the wild-type infected cells. Altogether the reduced infection ratios and altered protein profiles suggest that SEC can play a role in the S. aureus infection process of BME-UV cells while a role for TSST-1 still remains unclear. Further investigations of their specific functions is important to elucidate if these toxins are potential targets in new preventive strategies or treatments for bovine mastitis.