Genotypes, Virulence Factors and Antimicrobial Resistance Genes of Staphylococcus aureus Isolated in Bovine Subclinical Mastitis from Eastern China

This study was carried out to determine the genotypes, virulence factors and antimicrobial resistance traits of 34 Staphylococcus aureus isolated from subclinical mastitis in Eastern China. Minimal inhibitory concentration (MIC) results showed resistance to erythromycin in all isolates. A high frequency of Methicillin resistant S. aureus (MRSA; 29%) was observed and these isolates were also highly resistant to penicillin, oxacillin, oxytetracycline and chloramphenicol than methicillin sensitive S. aureus (MSSA) isolates. Thirteen pathogenic factors and seven resistance genes including mecA and blaZ gene were checked through PCR. The spaX gene was found in all isolates, whereas cna, spaIg, nuc, clfA, fnbpB, hlA, hlB and seA were present in 35, 79, 85, 59, 35, 85, 71 and 38% isolates, respectively. Nine isolates carried a group of 8 different virulence genes. Moreover, macrolide resistance genes ermB and ermC were present in all isolates. High resistance rate against methicillin was found but no isolate was positive for mecA gene, whereas blaZ and tetK were detected in 82 and 56% isolates, respectively. Genes; fnbpA, seB, seC, seD, dfrK and tetM were not found in any isolate. The statistical association between phenotypic resistance and virulence genes showed, clfA, fnbpB, hlB and seA, were potentially associated with penicillin G, ciprofloxacin, methicillin, chloramphenicol, trimethoprim and oxytetracycline resistance (P≤0.05). REP-PCR based genotyping showed seven distinct genotypes (A-G) prevalent in this region. This study reports the presence of multidrug resistant S. aureus in sub-clinical mastitis which were also highly virulent that could be a major obstacle in the treatment of mastitis in this region of China.