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Detecting Vancomycin Resistance

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).