Discrepancies between phenotypic and genotypic identification methods of antibiotic resistant genes harboring...

Antimicrobial resistance is a global issue that limits therapeutic options for infections. S. aureus being a member of the ESKAPE group is capable of "escaping" the biocidal action of antimicrobial agents. There are phenotypic and genotypic methods used for the identification of antibiotic resistant genes harboring S. aureus but these methods do not always show concordant results. To address these discrepancies, a total of 335 equine nasal swab samples from four districts of Punjab were collected using a convenient sampling technique. These samples were first subjected to common microbial techniques to identify S. aureus. The disc diffusion assay was performed for the phenotypic identification of antibiotic resistant S. aureus by using discs of oxacillin, penicillin, vancomycin, gentamycin, and tetracycline. After this, PCR was performed by targeting mecA, blaZ, vanB, aaca-aphd, and tetK genes for genotypic identification of respective antibiotic-resistant S. aureus. Phenotypic discrepancies (number of antibiotic resistant isolates found from disc diffusion who appeared to be negative for the resistant gene), and genotypic discrepancies (number of antibiotic sensitive isolates found from disc diffusion who appeared to be positive for the resistant gene) were calculated. The discrepancy ratio for mecA, blaZ, vanB, aaca-aphd, and tetK genes were 3.09, 1.96, 2.67, 1.93, and 1.67 respectively. These discrepant results indicate that the absence or presence of only one gene is not a true marker of resistant or sensitive isolates. There are multiple resistance determinants and resistance mechanisms. This study also highlighted the phenomenon of silencing of antibiotic resistance determinants.