E. faecium is the most frequently isolated species of VRE in hospitals and typically produces high vancomycin (>128 µg/ml) and teicoplanin (>16 µg/ml) minimum inhibitory concentrations (MICs). These isolates typically contain vanA genes. A vanB-containing isolate typically produces lower level resistance to vancomycin (MICs 16 to 64 µg/ml) and is susceptible to teicoplanin (MICs <1 µg/ml). Recently, a few vanD–containing isolates of E. faecium with a moderate level of resistance to vancomycin (MICs 64 to 128 µg/ml) and teicoplanin (MICs 4-8 µg/ml) have been reported, as has a novel vanE-containing E. faecalis. E. gallinarum and E. casseliflavus/E. flavescens isolates are intrinsically resistant to vancomycin. These isolates contain vanC genes that typically produce vancomycin MICs of 2 to 16 µg/ml.

Identification of VRE to species level aids in confirming whether an isolate has intrinsic (vanC) or acquired resistance (vanA or vanB). Knowledge of the type of resistance is critical for infection control purposes. vanA and vanB genes are transferable and can spread from organism to organism. In contrast, vanC genes are not transferable, have been associated less commonly with serious infections, and have not been associated with outbreaks.

For species differentiation, motility and pigment tests are easily performed and are important tests to distinguish among species phenotypically. E. faecium and E. faecalis are non-motile, whereas E. gallinarum and E. casseliflavus/E. flavescens generally are motile. Most isolates of E. casseliflavus/E. flavescens have a distinct yellow pigment, which can be observed by collecting growth from an agar plate on a swab. In addition to motility and pigment tests, an organism’s susceptibility profile also helps differentiate vanA and vanB isolates from vanC isolates.