Use of bovine primary mammary epithelial cells for the comparison of adherence and invasion ability of Staphylococcus aureus strains.

Adherence and invasion of epithelial cells are thought to play a role in the pathogenesis of Staphylococcus aureus mastitis. A cell culture model with primary mammary epithelial cells originating from the secretory tissue from the bovine udder was used to study adherence and invasion of S. aureus. The cells were characterized with antibodies against several cell markers that had been validated on histologic cryostat sections of bovine mammary tissue. All cells stained positively with the anticytokeratin antibodies, which are restricted to epithelial cells. The cell cultures contained a small number of alpha-smooth-muscle-actin positive cells (< 1%), probably myoepithelial cells. The use of bovine primary mammary epithelial cells and bovine S. aureus isolates, which were cultured in milk serum, results in a system similar to in vivo. Strain differences for adherence and invasion of S. aureus strains cultured in milk serum were studied. In addition, the correlation between adherence and invasion was evaluated. The number of adhered and invaded bacteria was strain dependent. The percentage of adherence after 5 min of incubation was correlated to the percentage of adherence after 3 h of incubation (r = 0.94; Pearson's correlation test). Fourteen of the 20 strains were able to invade epithelial cells. The percentage of invasion was correlated to the percentage of adherence after 5 min and to the percentage adherence after 3 h (r = 0.95 and 0.90, respectively; Pearson's correlation test). Results indicate that strain differences of adherence and invasion exist for S. aureus and that the invasion is a post adherence event.