Determination of Vancomycin Minimum Inhibitory Concentration in Methicillinresistant Staphylococcus aureus by Various Phenotypic Methods: A Cross-sectional Study

Introduction: Methicillin-resistant Staphylococcus aureus (MRSA) has become a global threat, causing nosocomial and community-acquired infections, thus significantly contributing to morbidity and mortality. These organisms often exhibit Multidrug Resistance (MDR) to various groups of antimicrobials. The emergence of reduced susceptibility to vancomycin and resistance further exacerbates the problem, leaving clinicians with few therapeutic options to treat serious, life-threatening infections. Aim: To determine the vancomycin susceptibility of MRSA using various phenotypic methods. Materials and Methods: The present cross-sectional study was conducted in the Department of Microbiology, SHKM Government Medical College, Nalhar, Haryana, India, from February 2019 to January 2020. All MRSA isolates were included, totaling 66 MRSA isolates. MRSA isolates were screened using the cefoxitin disc (30 μg) according to the Clinical and Laboratory Standards Institute (CLSI) recommended disc diffusion method. The Minimum Inhibitory Concentration (MIC) of vancomycin was determined using phenotypic methods such as broth dilution, agar dilution and Epsilometer test (E-test). Demographic details such as age, sex, site of the sample and diagnosis were recorded. Statistical analysis was performed using the Pearson’s product-moment correlation. The significance level was set at a probability of <0.05 with a confidence interval of 95%. Results: A total of 66 (30.3%) isolates of Staphylococcus aureus (S. aureus) were identified as MRSA using standard laboratory tests and were studied for vancomycin MIC using phenotypic methods: Broth dilution, agar dilution and E-test. Vancomycin-intermediate S. aureus (VISA) was observed in 54.5%, 42.4% and 51.5% of MRSA by E-test, agar dilution and broth dilution, respectively. The presence of Vancomycin-resistant S. aureus (VRSA) was not detected in the region. When compared with broth dilution, E-test results demonstrated higher accuracy (97%) than agar dilution (90.9%). Conclusion: The present study highlights the emergence of VISA in more than 40% of the MRSA in the region. The higher MIC of vancomycin emphasises the need for strict adherence to antibiotic prescription guidelines to prevent further emergence of vancomycin resistance.