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.