Analysis of a persistent outbreak with vancomycin-resistant Enterococcus faecium revealed the need for an adapted diagnostic algorithm.

Objectives: The study institute was challenged with an outbreak of different vancomycin-resistant Enterococcus faecium (VREfm), including vanA- and/or vanB-containing isolates. Remarkably, screening overnight enriched specimens using a vanA and vanB real-time polymerase chain reaction (PCR) gave positive results for vanB with very low cycle threshold values, whereas VREfm-specific enrichment cultures remained negative. This paper describes the analysis of the diagnostic results leading to adaptation of the diagnostic algorithm.

Methods: The results of vanA and vanB screening PCR and VREfm-specific culture (Brilliance VRE) were collected and combined with genotyping data of the identified VREfm isolates. During the outbreak, a second VREfm-specific culture medium (CHROMagar VRE) was introduced, and the results were compared with the results obtained with Brilliance VRE agar.

Results: Thirty-five patients were identified as VREfm carriers, in which four different strains were identified: vanA (STnew-CT7088) and/or vanB (ST80-CT1065, ST117-CT7117 and ST117-CT7118). Complementing the PCR results, culture and genotyping revealed that culture with Brilliance VRE agar was inadequate for detection of the vanB ST117 isolates identified, irrespective of the minimum inhibitory concentration of vancomycin. In contrast, CHROMagar VRE was able to detect the vanB ST117 isolates and other tested isolates correctly.

Conclusions: The vanB ST117 isolates were detected inadequately by the VREfm-specific culture media, possibly contributing to unnoticed spread of VREfm. For this reason, CHROMagar VRE was evaluated during the outbreak and subsequently implemented in routine diagnostics, replacing Brilliance VRE agar.