A high percentage of Enterococcus faecium strains demonstrate the acquired glycopeptide resistance phenotypes VanA or VanB and are highly resistant to vancomycin. Species identifications are performed in some laboratories where MIC susceptibility testing may not be available. Methods for the phenotypic separation of E. faecium from E. faecalis are available. Most utilize molecular testing methods.
Illustrated in this image are positive reactions for acid production from arabinose and melibiose (yellow color), characteristic of E. faecium. E. faecalis are negative for these reactions. A few preformed substrates, such as beta-galactosidase (E. faecium positive, E. faecalis negative), also separate these two species, accomplished by certain commercial systems that include these substrates.
E. faecium is not motile, an additional characteristic helpful to separate vancomycin-resistant Enterococcus (VRE) species from E. cassiloflavus and E. gallinarum, both of which are motile, carry low-level intrinsic resistance, and present as phenotype VanC.
