Minimum inhibitory concentration values and problematic disk break points of tigecycline against vancomycin and/or high-level aminoglycoside-resistant enterococci

BackgroundTigecycline is a new, semisynthetic glycylcycline. It is active against important multidrug resistant pathogens.AimThe purpose of this study was to investigate the sensitivity of multidrug-resistant enterococci to tigecycline, and to test the correlation between the minimal inhibitory concentration (MIC) and disk diffusion methods.Materials and methodsThe antimicrobial sensitivity of 108 multidrug-resistant isolates, which included 52 vancomycin-resistant enterococci (VRE) and 56 high-level aminoglycoside-resistant (HLAR) enterococci, was tested by the E test, broth microdilution test and disk diffusion methods.ResultsAll of the isolates were sensitive to tigecycline, as determined by the E test and broth microdilution test. The MIC 90 value (0.19 μg/mL) of tigecycline for HLAR enterococci was higher than that for VRE (0.094 μg/mL). When results were evaluated according to species, the MIC values of tigecycline for Enterococcus faecalis were higher than those for the other species. Eleven (10.1%) isolates produced false resistance results (zone diameter ⩽15 mm) by the disk diffusion method. These cases were classified as major errors. Eight (7.4%) isolates had intermediate sensitivity (sensitivity zone of 16 or 17 mm), which were classified as minor errors. The major and minor error percentages of HLAR enterococci (14.2% major, 10.7% minor error) were higher than those of VRE (5.7% major, 3.8% minor error). These results indicate that tigecycline is effective against multidrug-resistant enterococci. The sensitivity of multidrug-resistant enterococci to tigecycline should be investigated by MIC methods. The disk diffusion method causes major errors, especially for HLAR enterococci.