Persistence of Streptococcus uberis in bovine mammary epithelial cells

Streptococcus uberis is an important mastitis pathogen that has been associated with chronic persistent intramammary infections (IMI) in dairy cows. To define intracellular persistence, studies on time dependent internalization into and intracellular survival of S. uberis in bovine mammary epithelial cells were conducted. Two strains of S. uberis (UT366 and UT888) and a strain of Staphylococcus aureus (UT955) isolated from cows with clinical mastitis were cocultured with bovine mammary epithelial cells. Survival of bacteria in host cells for extended periods was studied. Streptococcus uberis UT366 showed highest internalization after 60 min of incubation. However, after 8 h of incubation, internalization values for UT888 were the highest. These results suggest that during the early stages of the internalization process, S. uberis UT366 appeared to internalize into bovine mammary cells more efficiently than UT888, but after 8 h following internalization, UT888 survived better than UT366. Nevertheless, both S. uberis strains survived intracellularly for up to 120 h without affecting host cell viability. Staphylococcus aureus internalized more efficiently than both strains of S. uberis evaluated causing death of mammary epithelial cells after 72 h of incubation. These results indicate that S. uberis can survive within mammary epithelial cells for extended periods of time without affecting host cell viability. Intracellular persistence of S. uberis may be associated with spread of the infection to deeper tissues and subsequent development of persistent IMI.