Investigation of Mutations and Expression Level of GraSR and WalKR Systems Associated with Vancomycin Non-Susceptibility in Methicillin-Resistant Staphylococcus aureus

The present study was conducted to investigate the frequency of antibiotic resistance genes and mutations and expression level of GraSR and WalKR systems associated with vancomycin non-susceptibility in methicillin-resistant Staphylococcus aureus. In 2022, a total of 200 S. aureus isolates were investigated in terms of methicillin resistance and reduced susceptibility to vancomycin. In vitro antibiotic susceptibility testing of MRSA isolates and the frequency of 12 antibiotic resistance genes were assessed using disk diffusion and PCR technique, respectively. Amino acid alteration in two-component regulatory systems including GraSR and WalKR in VISA isolates were determined by PCR-sequencing, the expression of these systems was analysed by RT-qPCR whereas agr- and spa-typing were used for molecular typing of these isolates. Overall, 125 (62.5%) isolates were methicillin-resistant and 75% were multiple-antibiotic-resistant. Vancomycin was the most efficient antibiotic against the isolates and blaZ gene was shown to be the most common (96.66%) gene among MRSA. A highly diverse amino acid substitution was observed in WalKR and GraSR in VISA and VRSA strains. Interestingly, 100% of vancomycin non-susceptible strains had the D148Q substitution in the GraR and I59L was the most common change in GraS. VISA isolates showed higher walR expression levels than the mean level of their progenitor VSSA. Agr types 1 and 3 and spa types t030, t421, t037, t325, t084 and t005 were detected among VISA isolates. The present results indicate high frequency of antibiotic resistance genes in methicillin-resistant S. aureus isolates. Furthermore, our results suggest that GraS, WalK, and WalR are important in vancomycin non- susceptibility and also these findings support the need for future surveillance studies to better elucidate vancomycin resistance in MRSA strains.