Education Sheets 2001-A

Vancomycin resistant Enterococci (VRE) is a very important group of bacteria in infection control and should be screened for preventing the organism from becoming endemic. Clinical isolates of enterococci should be screened for vancomycin resistance. Agar screening plates (6 mg/mL of vancomycin in brain-heart infusion agar) provide a simple, sensitive test for vancomycin resistance and are recommended by the National Committee for Clinical Laboratory Standard(NCCLS). Resistance to moderate and high concentrations of vancomycin is easily detected by standard susceptibility testing procedures (e.g. disk diffusion test). Modifications have improved the detection of low-level vancomycin resistance (typically VanB enterococci), which had been problematic for some automated systems. Similarly, a 24-h incubation and the use of strong transmitted light to read the plates have improved the accuracy of the disk diffusion method. The E-test method is an accurate alternative for the detection of vancomycin resistance.
Because VanC organisms (e.g. Enterococcus gallinarum and Enterococcus casseliflavus) are intrinsically resistant to vancomycin (low level resistance of vancomycin - MICs 2 - 32 mg/mL) and resistance is nontransferable, isolation of patients found to be colonized or infected with E. gallinarum and E. casseliflavus is not thought to be required. The wrong speciation of vancomycin-resistant enterococcal isolates may lead to wrong implications for treatment and infection control.