Correlation of Phenotypic and Genotypic Traits of Glycopeptide Susceptible and Resistant Enterococcus faecium

Abstract

Identification of Enterococcus faecium (EF) using conventional or commercial ID systems has been problematic due to atypical biochemical tests. These atypical biochemical reactions have been reported in strains with acquired glycopeptide resistance (GR). The purpose of this study was to correlate the phenotypic and genotypic traits of glycopeptide (vancomycin and teicoplanin) resistance and to determine if select biochemical reactions could be used to predict glycopeptide resistance in Enterococcus faecium. A total of 86 EF strains were identified using recommended conventional test methods. Multiplex PCR was used to determine each strain resistant genotypes. Antimicrobial Susceptibility Testing (AST) methods such as disc diffusion, Pasco® and PhoenixTm were used to determine resistant phenotypes. Forty-six strains were vanA-GREF, 22 strains were vanB-GREF and 18 strains were Glycopeptide susceptible EF (GSEF). Phenotypic characteristics were determined through the use of conventional tests and in the Phoenix automated system. Conventional tests for sorbitol (SBT), sucrose (SUC) and raffinose (RAF) and Phoenix tests for tyrosine-AMC (TYR-AMC), tryptophan- AMC (TRY-AMC), pyroglutamic Acid-AMC (PYR-AMC), alanine-AMC (ALA-AMC), phenylalanine-AMC (PHE-AMC), and SBT were shown to provide differentiation between GSEF and GREF. Genotypic analysis was performed on 10 strains of each (GSEF, vanA, vanB) using RAPD-PCR to determine genomic variation. Cluster analysis of the RAPD patterns revealed 3 cluster groups (GSEF, vanA and vanB) and 10 biotypes. The biotypes were then correlated to the conventional and Phoenix substrate results using the classification tree model. The classification tree correlated a GSEF strain to a negative result for conventional SBT and SUC and a negative result for the Phoenix tests of SBT, TYR-AMC and PHE-AMC. A GREF was correlated to a positive result for conventional SUC, SBT and RAF and a positive result for the Phoenix test of SBT. These results indicate that a GREF may potentially be differentiated from GSEF using select biochemical tests for rapid differentiation among E. faecium.