Evaluation of modified MicroScan screening tests for high-level aminoglycoside resistance in Enterococcus faecalis

Enterococcus faecalis strains that are refractory to aminoglycoside-penicillin synergistic killing can be predicted by their ability to grow in high concentrations of aminoglycoside (2,000 mg/L) or by disk diffusion tests using high content aminoglycoside disks. Forty-eight well-characterized clinical isolates of E. faecalis were used to evaluate recently reformulated gentamicin and streptomycin synergy screening tests on both overnight and rapid MicroScan (MS) Pos MIC panels (Baxter, West Sacramento, CA). The results obtained with the MS screening tests were compared with those from modified agar disk diffusion results. Discrepancies were resolved by an in-house broth microdilution method. The MS overnight tests detected 25 of 25 (100%) gentamicin high-level resistant strains and 23 of 24 (96%) streptomycin high-level resistant strains. No false high level resistance was found with the MS overnight tests. The MS rapid tests detected 100% of the gentamicin and 100% of the streptomycin high-level resistant strains, but one isolate was falsely high-level resistant to gentamicin (96% specific). The reformulated MS synergy screening tests were acceptable alternatives to disk diffusion for detection of high level aminoglycoside resistance in E. faecalis.