The innate response of mammary epithelial cells to different S. aureus isolates

S. aureus is still the most important contagious mastitis pathogen worldwide. Previous papers showed that S. aureus isolates have different virulence patterns and S. aureus strains are very often herd-specific. The aim of this study was to assess the relationship between S. aureus strains characterized by different virulence patterns and the innate response of a mammary epithelial cell line (BME-UV). Ten S. aureus isolates from subclinical mastitis cases in dairy cows from different herds were considered. Isolates were selected from our strain collection based on their genetic characteristics and antimicrobial susceptibility. BME-UV was exposed to S. aureus cultures for 24 h. Extracellular and intracellular S. aureus counts were performed by standard agar plate dilution method, intracellular and extracellular lysozyme and NAGase were measured by fluorimetric assays. The results at 4 h after challenge showed higher intracellular counts than extracellular ones, suggesting that S. aureus internalization is an early event and it is not depending from the extracellular concentration. During the experimental period, lysozyme concentration was associated to the increase in bacterial counts, what suggests that a minimum concentration of about 102 cfu/ml is needed to elicit a cellular response. Finally, lysozyme and NAGase activities showed to be associated to S. aureus genetic patterns in accordance to previous observation suggesting that the different immune response to S. aureus intramammary infections is influenced both by cow and by S. aureus characteristics.