Detection of enterococcal high-level aminoglycoside resistance with MicroScan freeze-dried panels containing newly modified medium and Vitek Gram-Positive Susceptibility cards

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Abstract
Both conventional and modified MicroScan Type 5 panels and Vitek Gram-Positive Susceptibility cards were compared with agar dilution screen plates for their abilities to detect high-level resistance to gentamicin and streptomycin in 235 enterococcal isolates, including 167 Enterococcus faecalis and 63 E. faecium isolates. The modified Type 5 panels contained dextrose-phosphate broth instead of Mueller-Hinton broth in their high-level-resistance screen wells. The sensitivities for detection of gentamicin and streptomycin high-level resistance were 100 and 100% (E. faecalis) and 100 and 94% (E. faecium) for the modified MicroScan panels, 100 and 89% (E. faecalis) and 100 and 98% (E. faecium) for the conventional MicroScan panels, and 81 and 86% (E. faecalis) and 85 and 94% (E. faecium) for the Vitek cards. All specificities were 100% except for the Vitek cards with streptomycin, where it was 96%. Isolates that showed resistance on the streptomycin agar screen plates were rescreened on plates containing 32,000 micrograms/ml to detect ribosomally mediated resistance. For all three systems, every failure to detect streptomycin high-level resistance occurred in isolates with enzymatic, not ribosomal, resistance. The modified MicroScan Type 5 panels are a suitable method for detecting enterococcal high-level resistance to gentamicin and streptomycin. The Vitek cards are too insensitive for this purpose.