Comparison of phenotypic methods to identify enterococci intrinsically resistant to vancomycin (VanC VRE)

Clinical laboratories must be able to differentiate between enterococci possessing acquired resistance to vancomycin (vanA and vanB genotypes) from those that are inherently resistant (vanC1 and vanC2/C3 genotypes). We compared several routine phenotypic tests to determine the species identity of clinical isolates of enterococci and a PCR assay for the van ligase genes was used to confirm identification of VanC VRE. The Vitek Gram Positive Identification card identified 53/60 (88%) Enterococcus faecalis and E. faecium isolates and 81/141 (57%) VanC VRE without additional testing. Another 32 of the VanC VRE required additional testing (e.g., motility and pigmentation) for correct identification. However, 7 of these 32 VanC VRE were nonmotile. The rapid ID 32 STREP strips identified 50/60 (83%) E. faecalis and E. faecium isolates and 102/141 (72%) VanC VRE. All E. faecalis and E. faecium isolates were nonmotile and did not acidify 1% methyl-alpha-D-glucopyranoside (MGP). Only 93/115 (81%) E. gallinarum and 21/26 (81%) E. casseliflavus/E. flavescens were motile but all 141 VanC VRE acidified MGP. MGP acidification can accurately differentiate VanC VRE from E. faecalis and E. faecium. Because some VanC VRE isolates are nonmotile, MGP acidification is preferred as a simple and less costly test for identification of these isolates.