By comparison with agar dilution results, the E test was investigated for the ability to detect high-level aminoglycoside (gentamicin and streptomycin), ampicillin, and vancomycin resistance among strains representing six enterococcal species. For ampicillin and vancomycin, disk diffusion results also were obtained. No false high-level aminoglycoside resistance occurred, and no false gentamicin susceptibility was noted. With the high-range streptomycin E test (2,048 jig), 24% of the 38 resistant strains were falsely susceptible. However, these discordances could likely be reconciled by adjustments in incubation duration and by using broth microdilution rather than agar screen breakpoint criteria, or by using the lower-range (1,024-p,g) strip. For ampicillin, category results obtained by E test and disk diffusion showed good agreement with agar dilution; E test MICs were generally comparable to agar dilution MICs. The E test was more sensitive than disk diffusion for detecting vancomycin-intermediate strains, but for these strains and those exhibiting low-level vancomycin resistance (MIC, 32 to 128 ,ug/ml), disk diffusion and E test inhibition zones must be interpreted with caution. Given the reliability of E test for detecting resistance to anti-enterococcal agents, the decision to use this method should be based on convenience, cost, testing frequency, and satisfaction with currently used methods.

The ability of six screening methods to detect high-level aminoglycoside resistance in enterococcal species other than Enterococcusfaecalis was investigated. The 85 Enterococcus isolates, which included 55 E. faecium, il E. gallinarum, 9 E. casseliflavus, 5E. raffinosus, 4E. avium, and 1 E. mundtii, were tested by using aminoglycoside-supplemented brain heart infusion agar (BHI), Remel EF Synergy Quad plates, high-content aminoglycoside diffusion disks, standard (prepared in-house) microdilution panels, Pasco MIC Gram Positive microdilution panels, and Vitek GPS-TA cards. When tested on BHI, 32 and 35 strains showed resistance to gentamicin and streptomycin, respectively. Resistance profiles obtained with Remel EF Synergy Quad plates were in complete agreement with those obtained on BHI. However, growth on Mueller-Hinton agar-based plates was not as heavy. Some isolates showed only weak growth and required 48 h for resistance to become evident, especially with swab inoculation of quadrants containing 2,000 ,ug of gentamicin per ml. Profiles obtained by use of the agar-based screens were used as the basis for evaluating the other methods. Disk diffusion showed complete agreement. No false resistance occurred by either microdilution method, but 48 h of incubation was needed for detection of some gentamicin-resistant isolates, and 14% of the streptomycinresistant strains were not detected by standard microdilution. The Vitek GPS-TA card detected 81 and 100% of the gentamicinand streptomycin-resistant isolates, respectively. In general, most methods used to detect high-level aminoglycoside resistance in E. faecalis appear to be reliable for the testing of the other enterococcal species. However, further investigations with a greater number of resistant E. raffinosus, E. avium, and E. mundtii isolates, when they are available, will be useful for establishing the full range of enterococci that can reliably be tested by the various methods.

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