A STUDY ON EVALUATION OF LABORATORY DETECTION METHODS AND PREVALENCE RATE OF VANCOMYCIN HETERORESISTANCE IN CLINICAL ISOLATES OF STAPHYLOCOCCUS AUREUS IN A TERTIARY CARE HOSPITAL

Abstract

Introduction: Heterogenous vancomycin intermediate Staphylococcus aureus hVISA is characterized by the presence of a resistant subpopulation, usually at a frequency of 1 in10 6, in an otherwise fully susceptible population. Increased cell wall thickness is a consistent feature of these isolates though screening of heteroresistance remains difficult, now It has become necessary for all clinical laboratories to establish and validate methods to detect h-VRSA. To detect vancomycin heteroresistant subpopulation of cells present in methicillin resistant Staphylococcus aureus isolates. This study aims to evaluate the screening methods in detection of vancomycin resistance among 100MRSA isolates and to find the prevalence rate of vancomycin heteroresistance among MRSA isolates.
Materials and Methods: A cross sectional study including 100 clinically significant methicillin resistant staphylococcus aureus{MRSA} isolates, selected by detection of Methicillin resistance by cefoxitin disc diffusion method. The following menthods are compared for detection of hVISA. Vancomycin screen agar (BHIA6V), E test GRD, vancomycin broth microdilution, modified population analysis profile.
Results and Discussion: Among 100 isolates screened by vancomycin screen agar method heteroresistance was detected in 5 isolates out of which 3 were confirmed as hVISA by modified PAP-AUC method. By BMD 3 out of 100 isolates showed MIC4µg/ml that was identified as VISA .All these 3isolates were identified as hVISA by PAP-AUC method. E STRIP GRD shows 2/100 isolates are identified as VISA that is found to be heteroresistant strains. By Modified population analysis profile the PAP-AUC curve cut-off result was ≥ 0.9 for 3 isolates that were confirmed to be hVISA.
Conclusion: This study infers that the combined use of screen agar – BHIA with 6µg/ml and BMD values is effective combination approach to detect heteroresistant strains but modified PAP-AUC is the gold standard method till date to detect hVISA as it is the only reliable method to confirm the heterogeneity of vancomycin susceptibility. From this study the prevalence of hVISA was found to be 3% among MRSA isolates.