Comparative evaluation of MIC of Vancomycin among methicillin resistant Staphylococcus aureus (MRSA) isolates in tertiary care hospital

Abstract

Vancomycin, a glycopeptide antibiotic with in-vitro activity against all staphylococci and clinical response to methicillin
resistant staphylococcus aureus (MRSA) infection, became the backbone of treatment due to MRSA’s universal
resistance to the antimicrobial agents belonging to beta-lactams and There are no other viable options. In patients with
staphylococcal infections, this study demonstrated a significant conflict of minimum inhibitory concentrations (MICs) of
vancomycin for MRSA strains between an automated system, Vitek 2, and the method. test E-strip. In the Microbiology
laboratory of JSS hospital which is a tertiary care center situated in Mysuru, 90 isolates were acquired from various
clinical samples to analyse Vancomycin MIC. Out of 90 isolates, 2.2% MRSA isolates showed highest vancomycin MIC
2μg/ml by Vitek-2 method, where no isolates showed MIC up-to 2μg/ml by E-strip method. But the higher vancomycin
MIC 1μg/ml was observed in 82.2% by vancomycin E-strip method compared to 37.7% by Vitek-2 method. Lowest MIC
0.5μg/ml showed by Vitek-2 method in 58.8% compared to 4.4% by E-strip method. The study concludes that all S. aureus
isolates were resistant to methicillin by both Vitek-2 system and cefoxitin disc diffusion method and also identified as
VSSA (vancomycin susceptible Staphylococcus aureus) by Vitek-2 and E-test method. Higher vancomycin MIC ≥1μg/ml
in 93.3% may be due to using of vancomycin improperly and infrequently in MRSA infection with lowest MIC value.
Therefore, this method can also be used as routine laboratory practice or as alternative method where Vitek 2 system or
other methods are not available.