Interpretive standards for Enterococcus species against vancomycin
|Disk diffusion||> 17 mm||15-16 mm||< 14 mm|
|Broth dilution||< 4 µg/mL ||8-16 µg/mL ||> 32 µg/mL|
BHI agar with 6 µg/mL vancomycin can be employed as a screen for vancomycin resistance. 1-10 µL of a 0.5 McFarland standard suspension is spotted onto the agar surface and incubated at 35 + 2° C for a full 24 hours. The presence of >1 colony equates to presumptive vancomycin resistance; these isolates would require species identification and a vancomycin MIC.
A motility test and documentation of the presence or absence of a yellow pigment can distinguish between the species with intrinsic (VanC) resistance (E. gallinarum and E. casseliflavus) and those with acquired (VanA and VanB) resistance (E. faecium and E. faecalis).