Comparison of four methods for testing high-level aminoglycoside resistance in enterococci

In a prospective study the prevalence of high-level aminoglycoside resistance (MIC ≥ 2,000 μg/ml) among 62 clinically significant enterococci was investigated. A total of 10⁵ organisms were inoculated a) onto a plate containing 2,000 μg/ml of gentamicin or streptomycin; b) into a microtube for dilution MIC determinations for gentamicin, amikacin, tobramycin and streptomycin; and c) into a single tube containing 500 μg/ml of gentamicin, amikacin, tobramycin or streptomycin in supplemented Mueller-Hinton broth. In addition, tubes containing 500 μg/ml of gentamicin, amikacin, tobramycin or streptomycin were inoculated with five enterococcal colonies ("crude" method). For 45 of the 62 isolates, MICs of gentamicin, amikacin and tobramycin were ≤ 500 μg/ml, while 17 (27 %) showed high-level resistance. The MICs of streptomycin were ≤ 500 μg/ml for 42 of 62 isolates, and ≥ 2,000 μg/ml for 20 (32.3 %). For 8 of the 17 (47 %) isolates showing high-level gentamicin resistance, MICs of streptomycin were ≤ 500 μg/ml. There was complete agreement between the results of the plate method, the microtube dilution MIC and the tube inoculated with 10⁵ CFU, but the crude method gave discordant results for two isolates. It is concluded that a tube containing 500 μg/ml of aminoglycoside is a simple, accurate and inexpensive method for determining high-level aminoglycoside resistance.