Molecular Characterization of Antibiotic-Resistant Staphylococcus aureus from Livestock (Bovine and Swine)

Aim: The aim of this study was to figure the prevalence, phenotypic and genotypic antibiotic resistance (AR) pattern of Staphylococcus aureus isolated from bovine and swine nares.
Materials and Methods: Colonies with typical morphology on Baird-Parker agar supplemented with egg-yolk tellurite emulsion were selected and biochemically/genotypically identified as S. aureus. These strains were further subjected to epsilometer test for their sensitivity to various clinically important antibiotics and antibiotic susceptibility testing for amoxicillin/clavulanic acid, and double-disk diffusion testing was performed by the standard disc diffusion method following CLSI guidelines. S. aureus strains were also tested for the presence of AR genes, viz., blaZ, mecA, aacA-aphD, erm (ermA, ermB, ermC), tet (efflux genes tetK and tetL, tetM and tetO of the ribosomal protection family), and vanA.
Results: The nasal cavities of 17 out of 47 randomly selected bovine and 20 out of 28 randomly selected swine were positive for S. aureus, representing the prevalence of 36.2% (95% confidence interval [CI]: 22.5-49.9) and 71.4% (95% CI: 54.7-88.1), respectively. Most of the S. aureus strains showed higher resistance to penicillin (94.6%, minimal inhibitory concentration [MIC]≥1.5 μg/ml) followed by ciprofloxacin (56.7%, MIC≥32 μg/ml) and tetracycline (18.9%, MIC≥32 μg/ml). About 10-15% of the strains were resistant to gentamicin (MIC 16 μg/ml) and oxacillin (MIC 6-8 μg/ml). None of the strains were resistant to vancomycin (MIC 0.25-1.5 μg/ml). In this study, 32.4% strains were resistant to three or more than three antibiotics and prevalence of this multi-drug resistant S. aureus was 45% (95% CI: 26.6-63.4) and 17.6% (95% CI: 6.7-28.5) in swine and bovine nasal samples, respectively. Four strains from pigs were borderline oxacillin-resistant S. aureus MIC 6-8 μg/ml, but none were mecA positive. Two of these strains were β-lactamase hyperproducers. Among the resistance genes blaZ, tetK, tetL, tetM, ermB, and aacA-aphD were found.
Conclusion: Our results demonstrated the absence of mecA and pvl gene, but the presence of multi-drug resistant S. aureus in the nares of healthy animals which has a potential to spread in a community.