Comparison of Antimicrobial Susceptibility Testing Methods to Detect Glycopeptide Resistance in Enterococci: E-test, Vitek, Disk Diffusion and Agar Dilution Method

Lee, Soo Youn; Park, Jin Hi; Park, Hyang Sook; Lee, Mi Ae; Kang, Eun Suk; Hong, Ki Sook

Abstract

BACKGROUNDS: The emergence of resistant strains to glycopeptide in enterococci(GRE) is increasingly serious problem in the worldwide. Automated methods and disk diffusion test have difficulties in detecting vancomycin resistance of some strains of vancomycin-resistant enterococci(VRE), especially having vanC genotypes. And a few studies have been done assessing the ability of antimicrobial susceptibility testing methods to detect teicoplanin resistance in enterococci.
METHODS
We evaluated the abilities of two commercial kits including Vitek GPS-IZ(BioMerieux, Vitek, Inc., USA) and E-test(AB Biodisk, USA), and disk diffusion test to detect glycopeptide resistance using 34 strains of vanA and 15 strains of vanC1/C2 VRE. We compared the results with those of standard agar dilution test.
RESULTS
In detecting vancomycin resistance, no very major or major errors were seen, and minor error rates were observed with disk diffusion(25%), Vitek GPS-IZ(20%) and E-test(8%). Overall sensitivities of all three methods in detecting vancomycin resistance of vanA VRE were 97-100%, but sensitivities in detecting vancomycin resistance of vanC VRE were 20% in disk diffusion, 87% in E-test and 87% in Vitek GPS-IZ. In detecting teicoplanin resistance, very major error rate was high in Vitek GPS-IZ(47%), but no very major or major errors were seen in disk diffusion and E-test; minor error rates of 2% and 6% were seen in Vitek GPS-IZ and E-test, respectively.
CONCLUSION
All three methods detect vancomycin resistance of vanA VRE, but they continue to demonstrate problems in detecting low-level vancomycin resistance and the Vitek GPS-IZ is difficult to detect teicoplanin resistance in enterococci.