Table 7. Interpretive Standards for Enterococcus species Against Vancomycin.Method
| Susceptible
| Intermediate
| Resistant
|
Disk diffusion
| ≥17 mm
| 15–16 mm
| ≤14 mm
|
Broth dilution
| ≤4 µg/mL | 8–16 µg/mL
| ≥32 µg/mL
|
Screening methodologies
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).
