Bovine intramammary infection associated immunogenic surface proteins of Streptococcus uberis

In spite of the increasing prevalence of Streptococcus uberis mastitis, its pathogenesis and associated virulence factors are not clearly defined. The aim of this study was to identify virulence associated genes and their products that can be used to develop effective vaccine to control bovine S. uberis mastitis. S. uberis was co-cultured with primary bovine mammary epithelial cells (PBMEC) or infused into mammary gland of dairy cows. The messenger RNA (mRNA) from S. uberis associated with PBMEC after 2 h or 4 h of co-culture was purified and sequenced. Results showed that virulence-associated genes such as surface lipoprotein (slp), infection induced histidine kinase (iihK), infection induced response regulator (iirR) and extracellular sugar binding protein 1 and 2 (exsbP1 and exsbP2) were among the top-up-regulated genes. To verify this observation in vivo, quantitative real time PCR (qRT - PCR) was conducted on mRNA of S. uberis recovered from milk of infected mammary glands 24 h post infection. Results revealed that in vitro up-regulated virulence-associated genes were also significantly up regulated under in vivo conditions. The iihK and iirR are flanked by exsbP1 and exsbP2 genes and this entire operon seems to be involved in adaptation to glands micro-environment, survival and colonization of the bovine mammary glands. Based on immunogenic epitope prediction of proteins encoded by these up-regulated genes during early stages of host-bacterial interactions slp, exsbP1 and exsbP2 genes were selected, cloned and expressed in E. coli. The purified recombinant proteins (rSlP, rExsbP1 & rExsbP2) reacted strongly with convalescent serum from cows experimentally infected with S. uberis confirming that they are immunogenic. These proteins may serve as potential targets to develop an effective vaccine against S. uberis mastitis.