BACKGROUND: Antibiotic resistant strains of Enterococcus faecium (EF), especially strains with acquired glycopeptide (GP) resistance, has been reported to show atypical biochemical reactions in both conventional tests and in commercial identification systems. The purpose of this study was to correlate the phenotypic and genotypic traits of GP susceptible and resistant EF strains. MATERIALS/METHODS: Phenotypic characteristics were determined using conventional biochemical tests and in the Phoenix™ automated ID system. The genotypic analysis of the test strains was performed using random amplification polymorphism DNA PCR technique (RAPD) with genomic DNA. The susceptibility of EF to vancomycin and teicoplanin was determined using microbroth dilution and disk diffusion test methods per NCCLS. The resistance genotypes (vanA/vanB) were identified using a multiplex PCR procedure. A total of 46 vanA, 22 vanB and 18 strains of GP susceptible EF (GS-EF) were included in the study.
RESULTS: The UPGMA cluster analysis of RAPD patterns revealed 3 distinct clusters of GSEF, vanA and vanB strains with overlapping between the vanA and vanB clusters but not the GSEF cluster. The correlation between the RAPD results and substrates reactivity either in conventional tests or in Phoenix ID systems was examined using the classification tree statistical method. As revealed by the classification tree, a GS-EF strain would be correlated to negative reactions with sucrose (SUC) and sorbitol (SBT) using conventional tests while vanB would be correlated to positive reactions with SUC, SBT and raffinose. In the Phoenix system, negative reactions SBT, tryptophan-AMC and phenylalanine-AMC would predict a GS-EF strain while a positive reaction in SBT would predict a resistant EF strain (vanA or vanB).
CONCLUSION: These results indicate that selected biochemical reactions could be potentially useful in the rapid differentiation of GS-EF from resistant strains among EF.