Characterization of Vancomycin Resistance and Associated Genes in Clinical Isolates of Enterococcus Species

Introduction Vancomycin is the drug of choice for multidrug-resistant Enterococcal infections. Increased use of vancomycin has resulted in greater minimum inhibitory concentration (MIC) values and resistance. The objective of this study was to detect and characterize vancomycin resistance among clinical Enterococcus isolates using phenotypic susceptibility testing and correlation with molecular analysis of vancomycin resistance genes (vanA, vanB, vanC1, vanC2/3, vanD, vanE, and vanG). Materials and methods A total of 266 clinically significant, consecutive, nonrepetitive Enterococcus species were identified from clinical isolates collected from 2023 to 2024 in a tertiary care hospital. Sources of the isolates included urine (98), blood (19), and exudative specimens (149). Using the Kirby-Bauer disc diffusion method, antibiotic susceptibility was tested for various antimicrobial agents, including ampicillin, erythromycin, ciprofloxacin, nitrofurantoin, vancomycin, and linezolid. The agar dilution technique was performed as per Clinical and Laboratory Standards Institute (CLSI) 2025 guidelines to ascertain the MIC for vancomycin. Polymerase chain reaction (PCR) was employed to detect the genes encoding vancomycin resistance, namely, vanA, vanB, vanC1, vanC2/3, vanD, vanE, and vanG. Results Among 266 isolates, vancomycin resistance was detected in 74 isolates with MIC ≥ 32 μg/mL (Enterococcus faecium - 57, Enterococcus faecalis - 9, Enterococcus gallinarum - 7, Enterococcus casseliflavus - 1). All the 74 carried van gene. vanA was detected in 70 and vanC1 in 3 isolates of E. gallinarum. One isolate of E. gallinarum harbored both vanA and vanC1. None of the vancomycin-resistant Enterococcus (VRE) isolates in this study carried vanB, vanC2/3, vanD, vanE, or vanG. Conclusion The increase in VRE has alarmed the global community. More recently, the emergence of linezolid-resistant Enterococci is a cause for concern. Hence, there is an imperative need for in-depth research into the mechanisms behind VRE. In this study, the vanA genotype is more widely distributed in clinical isolates.