Antimicrobial susceptibility and species identification for clinical isolates of enterococci

In order to describe the susceptibility patterns and determine the identities of Enterococcus spp. isolated at a regional teaching hospital in Taichung during the period from June through November 1998, 96 clinical isolates of enterococci were collected for further analysis. Major sources of these isolates included urine, wound, and pus. Species identification was performed using the API 20 Strep system and supplemental tests. The minimum inhibitory concentrations (MICs) of six antimicrobial agents were determined by E-test for each isolate. Disk diffusion tests were also performed and the results were compared with those reported by the clinical laboratory. Because gentamicin susceptibility tests showed inconsistent results in many isolates, MIC determinations by the microbroth dilution method were also performed for these isolates. All isolates were tested for beta-lactamase production using the chromogenic method. The results showed that Enterococcus faecalis was the most frequently encountered species (86.5%), followed by Enterococcus faecium (7.3%), Enterococcus avium (5.2%) and Enterococcus casseliflavus (1.0%). The MIC90 of ampicillin, penicillin, vancomycin, teicoplanin, ciprofloxacin and gentamicin for total isolates were 1, 3, 2, 0.25, 1, and more than 1024 microg/mL, respectively. All isolates were susceptible to vancomycin and teicoplanin. The same rate of resistance (3.1%) was found to penicillin, ampicillin and ciprofloxacin in all isolates. There were 50 (52%) and 48 (50%) isolates with high level streptomycin and gentamicin resistance, respectively. The MIC90 of ampicillin and penicillin for E. faecium were significantly higher than those for other species (96 and > 256 vs. < or = 1 and < or = 3 microg/mL, respectively); in fact, ampicillin or penicillin-resistance was only found in E. faecium. No organism was found to produce beta-lactamase. There were 29 isolates showing discrepant results between the study findings and clinical laboratory report for gentamicin susceptibility. Most isolates (27/ 29) reported as susceptible to gentamicin by the clinical laboratory showed a high level gentamicin resistance by MIC determinations. The inconsistent results may be due to discrepancy in interpretations for heterogeneous resistance, bias in selecting colonies for the disk diffusion test, or variation in the quality of Mueller-Hinton agar used. The results suggest that any pure isolates growing some colonies within the inhibition zone should be considered as gentamicin resistant even if the zone diameter is equal or greater than the susceptible breakpoint. In order to obtain accurate results, the gentamicin MIC should be determined by the dilution method for enterococcal isolates that yield intermediate inhibition zones or zones just above the susceptible limit.