Comparison of the Screening Media and Inoculation Methods in Detection of Vancomycin-Resistant Enterococci

Abstract

Background : Vancomycin-resistant enterococci (VRE) became one of the clinically important causative organisms of the nosocomial infection lately, however, detection of VRE is still troublesome. This study was conducted to find the most effective screening media and inoculation method for detection of VRE. Method : Twenty-nine strains of VRE (vanA : 5, vanB : 13, vanC : 11) that had been isolated in our laboratory and 19 strains of vancomycin-susceptible enterococci were used for this study. Vancomycin was added in concentrations of 4 μg/mL, 6 μ g/mL, 8 μg/mL each to brain heart infusion (BHI) agar and Mueller-Hinton (MH) agar. Each strain of organisms was inoculated as a drop using a cotton swab to each medium with the turbidity of McFarland 0.5 standard. Other inoculations were applied by micropipet, with the volume of 1 μL (105 CFU) and 10 μL (106 CFU), respectively. The growth was checked after 18, 24, and 48 hours of incubation at 35℃ and the sensitivity and specificity were calculated. Result : BHI agar appeared to be the better screening medium than MH agar. Because, the sensitivity of detecting vancomycin resistance on BHI and MH agar were 79. 3% to 1o0%, and 62.1% to 96.5%, respectively. In case 6 μg/mL vancomycin was tested, use of micropipet inoculation with either 1 μL or 10 μL inoculum revealed 100% In both sensitivity and specificity. However, when using a cotton swab, the sensitivity fell to 96.5% In case of 4 μg/mL vancomycin, the sensitivity was 100%, the specificity was 94.7%. Conclusion : This study indicated that the effective laboratory method for detecting VRE could be the inoculation with micropipet using BHI agar added 6 μg of vancomycin per ㎖.