What is the difference between vancomycin-susceptible S. aureus, VISA and VRSA?
Most isolates of S. aureus are susceptible to vancomycin. The concentration of vancomycin required to inhibit these strains (called the minimal inhibitory concentration or MIC) is typically between 0.5 and 2 micrograms/mL (μg/mL). In contrast, S. aureus isolates for which vancomycin MICs are 4-8 μg/mL are classified as vancomycin-intermediate, and isolates for which vancomycin MICs are ≥16 μg/mL are classified as vancomycin-resistant. The revised definitions for classifying isolates of S. aureus are based on the interpretive criteria published in January 2006 by the Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS)*. CLSI lists only susceptible disk diffusion interpretive criteria (in mm) for vancomycin and Staphylococcus spp. There has not been a sufficient number of non-susceptible isolates to develop resistant and intermediate breakpoints. Organisms for which the vancomycin zone diameters are >=15mm are considered susceptible, although several studies show that this
