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

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Abstract
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.