A new chromogenic agar medium, chromID VRE^®, to screen for vancomycin-resistant Enterococcus faecium and Enterococcus faecalis

Abstract number: 1732_74

Ledeboer N., Tibbetts R., Dunne M.

Objectives: Enterococcus species are members of the normal intestinal flora, and are the most common aerobic Gram-positive cocci found in the large bowel of humans. The organisms have gained more notoriety, however, as nosocomial pathogens, now grouped as the third most common blood borne pathogen in the United States. At least one factor accounting for this pathogenicity is their propensity to carry plasmid-mediated resistance markers to vancomycin. The development of reliable and rapid methods for the identification of patients colonised with vancomycin-resistant enterococci (VRE) is central to the prevention of person-to-person transmission of this agent within the hospital environment. To this end, we conducted clinical trials for a chromogenic agar medium designed by bioMérieux (France) (chromID VRE) to recover VRE from stool and identify the isolated colonies as either Enterococcus faecium (VREfm) or E. faecalis (VREfs) based on different colony colour.

Methods: We compared the performance of this medium with bile esculin azide agar supplemented with vancomycin (BEAV). For this study, 120 stool samples were plated on each test medium and examined after 24 and 48 hours of incubation. Culture positive results were further identified using biochemical reactions (VITEK 2^®), vancomycin susceptibilities, and PCR.

Results: At 24 hours, the sensitivity and specificity were: BEAV: 96%, 73%; chromID VRE: 90%, 89%. The positive predictive values for identification of positive samples by the chromogenic medium and BEAV at 24 hours were: chromID VRE 83%, BEAV 68%. Increased length of incubation (48 hr) did not significantly improve sensitivity, specificity, or positive predictive value for either medium. In addition, chromID VRE identified 2 isolates of VREfs that were not recovered using BEAV. Furthermore, the chromID VRE was capable of identifying patients colonised with both VREfm and VREfs – a feature useful for epidemiology follow-up that is not available with BEAV.

Conclusions: We conclude that this new and innovative chromogenic agar, chromID VRE, provides improved recovery of VRE from stool specimens and the added advantage of differentiation between VREfs and VREfm with an increase of specificity towards enterococci intrinsically resistant to vancomycin. Extended incubation beyond 24 hours did not significantly improve recovery of VRE and resulted in a decreased specificity.