In vitro susceptibility of multidrug-resistant Enterobacteriaceae clinical isolates to tigecycline.

OBJECTIVES To assess the in vitro susceptibility of multidrug-resistant Enterobacteriaceae (MDRE) isolates to tigecycline. METHODS Clinical isolates of MDRE tested in this study were obtained from 91 hospitals in Belgium during the period January 2010 to April 2010. MICs of tigecycline were determined by Vitek 2 (VTK) and by the reference broth microdilution (BMD) method, and the results were interpreted based on the 2011 MIC interpretative criteria recommended by EUCAST. RESULTS A total of 501 non-duplicate MDRE isolates were tested. These comprised 284 isolates of Escherichia coli [255 (89.7%) were extended-spectrum β-lactamase (ESBL)-producing isolates], 72 isolates of Klebsiella pneumoniae [53 (73.6%) were ESBL-producing isolates], 72 isolates of Enterobacter aerogenes, 33 isolates of Enterobacter cloacae, 19 isolates of Klebsiella oxytoca and 21 miscellaneous others. The MIC(90) values of tigecycline for E. coli and non-E. coli ESBL-producing Enterobacteriaceae isolates were 0.5 and 2 mg/L by BMD, and 0.5 and 8 mg/L by VTK, respectively. The highest essential and categorical agreement rates between VTK and BMD results using EUCAST breakpoints were observed in E. coli isolates (97.2%), while lower and unacceptable essential and categorical agreement rates were obtained for isolates belonging to species other than E. coli (81.1% and 59.4%, respectively). CONCLUSIONS VTK appears to be a suitable method for routine susceptibility testing of tigecycline only for E. coli isolates, while BMD should be preferred for other Enterobacteriaceae species isolates.