Molecular Detection of blaZ and mecA Genes and Study of Antibiotic Resistance Pattern in Clinical Isolates of Staphylococcus aureus from Bovine Mastitis in Coastal Andhra Pradesh

Staphylococcus aureus is known to cause sub clinical and clinical intra mammary infections like mastitis in milch animals. S. aureus demonstrates a distinctive ability to quickly develop a resistance mechanism, starting with penicillin, until against even the most recent, linezolid and daptomycin.

In the present study, 100 milk samples from mastitis of buffaloes in coastal Andhra Pradesh were collected. These isolates were characterized initially by biochemical tests like Mannitol fermention, Catalase test, Coagulase test, Spot Oxidase test, Voges-Proskauer test and the Haemolytic activity on 5% Sheep Blood agar. Provisionally confirmed isolates of S. aureus were further processed for molecular detection of S. aureus with species specific oligonucleotide primers Staur 4 and Staur 6. The sensitivity of confirmed isolates of S. aureus to different antimicrobials was tested against 10 antimicrobials by Kirby-Bauer disc diffusion method. The presence of antibiotic resistance genes like b-lactamase (blaZ) and methicillin resistance (mecA) were screened by PCR using specific primers.

Out of 100 isolates, 69 samples were provisionally positive for S.aureus by biochemical methods and 34 isolates (34.0%) by species specific PCR. High resistance was recorded to Ceftriaxone+Tazobactum, Oxacillin and Amoxycillin (100%), followed by Ceftriaxone+ Sulbactum (93.55%) and Methicillin (83.87%), Penicillin (80.65%), Ampicillin (70.97%) and Ceftriaxone (38.71%). Low resistance was observed for Cefoxitin(6.47%) and Amoxycillin+ Clavulanic acid(0). A high frequency of mecA (61.29%) and blaZ (45.16%) antibiotic resistance genes was found in S. aureus isolates. Both mecA and blaZ genes were present in 22.58% of S. aureus isolates.

The results show that the organisms are acquiring resistance against commonly employed antimicrobials to treat mastitis by acquiring antibiotic resistance genes at an alarmingly fast rate.