Molecular characterization and multilaboratory evaluation of Enterococcus faecalis ATCC 51299 for quality control of screening tests for vancomycin and high-level aminoglycoside resistance in enterococci

Studies were conducted to validate the use of Enterococcus faecalis ATCC 51299 (which is vancomycin resistant and resistant to high levels of gentamicin and streptomycin) and E. faecalis ATCC 29212 (which is susceptible to vancomycin and against which gentamicin or streptomycin and cell wall-active agents have synergistic kill activity) as controls in an agar screening test for vancomycin resistance and high-level streptomycin and gentamicin resistance and a broth microdilution screening test for high-level streptomycin and gentamicin resistance. Both organisms performed as expected in these tests and will serve as appropriate controls. However, E. faecalis ATCC 29212 was occasionally noted to produce light growth on the vancomycin screening plate with certain lots of agar. Quality control ranges for disk diffusion tests with disks with large amounts of streptomycin (300 micrograms) and gentamicin (120 micrograms) were established for E. faecalis ATCC 29212; zone limits are 16 to 22 mm for gentamicin and 14 to 19 mm for streptomycin. No zones for inhibition were seen when E. faecalis ATCC 51299 was tested with these high-content disks.