Presence of multiple <i>van</i> genes among glycopeptide non-susceptible <i>Staphylococcus aureus</i> exhibiting <i>in vitro</i> MIC creep phenomenon: A study from north-east India

Background & objectives The global prevalence of vancomycin-resistant Staphylococcus aureus (VRSA) has increased two fold since 2010, accounting for 2.4 per cent of S. aureus infections. The emerging hVISA isolates and their increasing trends pose a serious therapeutic challenge. The present study investigated in vitro vancomycin and teicoplanin minimum inhibitory concentration (MIC) creep in S. aureus and assessed their revertants. Methods A total of 845 isolates were collected for this study, and 246 were confirmed as S. aureus. Molecular characterization of vancomycin resistance was carried out by PCR assay targeting genes types viz: vanA, vanB, vanC, vanC2/C3, vanD, vanE, and vanG. MIC was determined for vancomycin and teicoplanin by agar dilution method. MIC creep and revertant analysis were done by broth dilution method in the presence and absence of antibiotics. Results PCR assay confirmed 12 isolates were harboured vanA, followed by vanD (n=8) and vanB (n=7). The study showed 69 isolates were screened positive for glycopeptide non-susceptibility. While analyzing vancomycin MIC creep, four isolates showed a significant increase in MIC, whereas no creep phenomenon was observed for the rest. In the case of teicoplanin, seven isolates showed the MIC creep phenomenon. Revertant analysis of all the isolates that showed MIC creep phenomenon for vancomycin and teicoplanin reverted to their original MIC when the antibiotic pressure was withdrawn. Interpretation & conclusions In the present study setting, glycopeptide non-susceptibility was found in eight per cent of the isolates, and the present study found the occurrence of multiple van genes from isolates calculated from a single study center will impose a serious challenge in infection control and antibiotic policy. This study also underscores that heterogenic resistant isolates, upon exposure to vancomycin and teicoplanin at a minimum level, exhibited an increase in MIC, which will impact individuals receiving glycopeptide therapy.