Validity of screening procedures for glycopeptide-resistant enterococci

Screening agars containing different vancomycin concentrations and differen t susceptibility testing procedures were compared to determine their validi ty for the detection of glycopeptide-resistant enterococci (GRE). Direct st reaking of rectal swabs over the surface of a commercially available agar ( Enterococcosel; Becton Dickinson, Germany) containing 4 mu g/ml and 16 mu g /ml vancomycin was followed by incubation for 24 to 48 h. Susceptibility te sts were done by microbroth dilution, disk diffusion, and the E test (AB Bi odisk, Sweden). The microbroth dilution method according to National Commit tee for Clinical Laboratory Standards (NCCLS) was used as the gold standard for detection of GRE. Resistant and intermediately susceptible enterococca l isolates were differentiated to the species level. To detect resistance g enes, the polymerase chain reaction was performed on all intermediately res istant isolates, on all isolates of VanB phenotype, and on 30% of isolates of VanA phenotype. Screening agar containing 4 mu g/ml vancomycin displayed high sensitivity (97.6%) but only low specificity (35%) for the detection of GRE. Screening agar containing 16 mu g/ml vancomycin had a high specific ity of 89.3% and only a slightly lower sensitivity (92.7%) than the screeni ng agar containing 4 mu g/ml vancomycin. For the disk diffusion test? a bre akpoint of <16 mm yielded the optimal combination of sensitivity (98.8%) an d specificity (99.6%). Both sensitivity and specificity of the E test for G RE detection were 100%. However, the E test is too expensive for testing of all enterococci. In conclusion, the combination of an inexpensive screenin g agar with either the E test or the disk diffusion test constitutes a vali d and cost-effective method for the detection of GRE from screening specime ns.