Protein expression by Streptococcus uberis in co-culture with bovine mammary epithelial cells

Three strains of Streptococcus uberis isolated from dairy cows with mastitis were co-cultured with a bovine mammary epithelial cell line (MAC-T) in Dulbecco's modified Eagle's medium without fetal bovine serum. Protein profiles from culture supernatants and bacterial pellets among different treatments were compared by electrophoresis. There were proteins induced or having increased expression in both supernatant and surface-associated samples from S. uberis co-cultured with MAC-T cells. Some of these proteins were recognized by antibodies in serum obtained from a cow infected by S. uberis . In supernatant samples, there were two distinct protein bands at 35 and 36.8 kDa for all three strains of S. uberis co-cultured with MAC-T cells. These two bands were absent when bacterial protein synthesis was inhibited by chloramphenicol. This study clearly indicates that bacterial protein expression was regulated in response to co-culture with mammary epithelial cells.