Abstract: This study was conducted to explore the universality and homology of the sequence encoding Staphylococcus aureus surface protein A (SasA) in S. aureus strains existing in raw milk. Combined with the investigation into the structure and composition, we can elucidate its role in adhesion to dairy cattle mammary epithelial cells. Seventy-three S. aureus strains were isolated and purified from five dairy cattle farms in the north of China using aseptic techniques. After DNA extraction, we used PCR to amplify and identify SasA gene, then compared the sequence with reference sequences to analyze its conservation. SRR1(serine-rich repeat region 1) and NRR(non-repeat region)were reconstructed using prokaryotic expression system and purified. The differences in adhesion effect of NRR, BSA and SRR1 on dairy cattle mammary epithelial cells (Mac-T) were detected by flow cytometer. Compared with BSA and SRR1, NRR exhibits greater adhesion to cells. To find out the main adhesion domain in NRR, different lengths of NRR fragments were used to inhibit the intact NRR fragment adhesion. PCR combined with the homologous analysis of sequences showed 86.3% of S. aureus carried SasA gene and the gene similarity was over 95% with the adhesion effect of NRR1-2 (230-540 aa) being most obvious. The above results have indicated that SasA gene is ubiquitous and highly conserved in bovine S. aureus. NRR1-2 module plays a major role in adhesion to dairy cattle mammary epithelial cells and is roughly located at the 230-540th residues of the protein domain. It suggests that specific cell receptors, which can bind to this module, might be sites where SasA interacts with the host cells as a kind of adhesion.

Key words: Staphylococcus aureus, Staphylococcus aureus surface protein A, dairy cattle mammary epithelial cells, adhesion