Single-concentration broth microdilution test for detection of high-level aminoglycoside resistance in enterococci

Growth in a single broth microdilution well containing gentamicin at a concentration of 500 μg/ml predicted high-level resistance to gentamicin (MIC, ≥ 2,000 μg/ml) in 505 of 508 clinical isolates of enterococci. Failure to achieve synergistic killing with the combination of penicillin and an aminoglycoside was demonstrated with 100% specificity in 20 strains which showed resistance to 500 μg of gentamicin per ml. We recommend this procedure be adopted as a routine screening test to detect high-level aminoglycoside resistance in enterococci.