Failure of VITEK2 to reliably detect vanB- mediated vancomycin resistance in Enterococcus faecium

Abstract

AbstractObjectivesThe increasing prevalence of vancomycin resistant enterococci (VRE) necessitates a reliable detection of VRE especially for low level resistance mediated by vanB in Enterococcus faecium. In this prospective study we analyzed if vanB mediated vancomycin resistance can be reliably detected by Vitek2.Methods1344 enterococcal isolates from routine clinical specimens were tested by Vitek2 (bioMérieux, Nürtingen, Germany). Additionally, a bacterial suspension (0.5 McFarland) was inoculated on a chromID VRE screening agar (bioMérieux) and incubated for 48 hours. If vancomycin was tested susceptible by Vitek2 but growth was detected on the screening agar a PCR for vanA/vanB was performed (GeneXpert vanA/B test kit, Cepheid, Frankfurt, Germany). MICs of vancomycin susceptible by Vitek but vanA/B positive isolates were determined before and after cultivation in a broth with increasing concentration of vancomycin.Results156/492 of E. faecium were VRE, predominantly vanB (87.0%) of which 14 were not identified as VRE by Vitek2 (sensitivity 91.0%). The majority (9/14) demonstrated high-level MICs by broth dilution. Even after exposure to increasing vancomycin concentrations MICs remained nearly identical. Three of the undetected isolates demonstrated initial growth on chromID VRE, after the vancomycin exposure additional 7 isolates demonstrated growth on chromID VRE.ConclusionsVitek2 fails to detect vanB mediated vancomycin resistance consistently, especially but not limited to low-level resistance. As this may lead to treatment failure and further dissemination of vanB VRE, additional methods (e.g. culture on VRE screening agar or PCR) are necessary to reliably identify vanB-positive enterococci in clinical routine.