Absence of vancomycin-resistance genes in Staphylococcus aureus isolated from potable water

The aim of this study was to investigate the carriage of vancomycin-resistance genes by Staphylococcus aureus found in drinking water supplies. A total of 100 samples (potable water and faucet swabs) were analyzed for the presence of S. aureus and their carriage of vancomycin-resistance genes. Mannitol salt agar was used for the isolation of staphylococci, and confirmation of S. aureus was carried out by means of Gram staining technique, growth on blood agar, production of catalase and coagulase. Antibiotic susceptibility testing was performed by disk diffusion method, and polymerase chain reaction (PCR) was used to investigate the presence of vancomycin-resistance genes (vanA, vanB, vanC, vanS, vanY). Staphylococci were recovered from 25% of water samples (n = 75), of which seven samples were positive for S. aureus, on the other hand 76% (n = 25) of the faucet swabs yielded typical staphylococcal colonies, with 53% being positive for S. aureus. The disk diffusion method showed that all S. aureus resistance to penicillin G, and about 66.6% (n = 51 isolates) were resistance to oxacillin. Of all 51 S. aureus isolates, only 14% were resistant to vancomycin by disk diffusion method, however, the DNA extracted from all confirmed S. aureus did not yield any PCR products with all the primers used for detecting vancomycin-resistance genes. Vancomycin-resistant S. aureus remains rare. The disk diffusion method may give false-resistance with vancomycin, therefore, caution is required with the investigation and interpretation of vancomycin susceptibility testing by agar diffusion method.