Antimicrobial susceptibility of enterococci in vitro

Treating patients with enterococcal infections has become difficult because of the emergence of several resistant traits in the past decade, including high-level resistance to gentamicin, beta-lactamase production, non-beta-lactamase-associated penicillin resistance and vancomycin resistance. To determine the antimicrobial susceptibility and to survey the antibiotic resistance of enterococci, we tested 111 clinical isolates of enterococci from the National Taiwan University Hospital from 1990 to 1991. There were 102 isolates of Enterococcus faecalis, five of Enterococcus faecium, and four of other enterococcal species. All these isolates were tested in vitro by the agar dilution method for determination of the minimal inhibitory concentration (MIC), by the broth microdilution test for screening of high-level resistance to gentamicin and by the rapid chromogenic cephalosporin method for detection of beta-lactamase production. The effect of the combination of two antimicrobial agents against 20 of the 111 isolates, including strains with high-level and low-level resistance to gentamicin, were further checked by the checkerboard titration method. Among 17 antimicrobial agents, the most active agent was ampicillin, followed by vancomycin, ciprofloxacin, ofloxacin and penicillin. Although none included in these were beta-lactamase producers or vancomycin-resistant strains, there was a high proportion (62%) with high-level resistance to gentamicin among our isolates. There were 11 isolates, seven Enterococcus faecalis and four Enterococcus faecium, which had MICs beyond the clinically achievable level of penicillin (MIC > or = 64 micrograms/mL). Because these resistant isolates of enterococci have important therapeutic implications, periodic surveillance and MIC testing are suggested.