Bloodstream infections caused by IMP-8-producing Enterobacteriaceae isolates: the need for clinical laboratory detection of metallo-β-lactamases?

A retrospective study was conducted at a Taiwanese medical center to characterize bloodstream infections caused by IMP-8 metallo-β-lactamase (MBL)-producing Enterobacteriaceae isolates and to assess the need for laboratory detection of IMP producers. We analyzed 37 patients infected with IMP-8 producers (two Escherichia coli, nine Klebsiella pneumoniae, 25 Enterobacter cloacae, and one Citrobacter freundii) and 107 patients infected with non-IMP-8 producers (eight E. coli, 26 K. pneumoniae, 70 E. cloacae, and three C. freundii) that were interpreted as carbapenem-nonsusceptible based on the updated Clinical and Laboratory Standards Institute (CLSI) 2010 guidelines. Only 18 (48.6 %) of the IMP-8 producers were regarded as potential carbapenemase producers based on the CLSI 2012 guidelines. The production of extended-spectrum β-lactamases (ESBLs) was more common in the MBL group (73.0 %) than in the non-MBL group (41.1 %). There were no significant differences in carbapenem susceptibilities, clinical characteristics, carbapenem use for empirical and definitive treatment, and mortality rates between the two groups. Eighteen IMP-8 producers could be deemed as resistant to all carbapenems [minimum inhibitory concentration (MIC) of any carbapenem ≥2 μg/mL]; patients with these isolates had a lower, but non-significant, 28-day mortality rate (27.8 %) than patients infected with non-MBL producers having similar carbapenem MICs (39.0 %) (p = 0.41). A multivariate analysis revealed severity of acute illness as the single independent variable associated with both 7-day and 28-day mortality rates (p < 0.01) for infections caused by Enterobacteriaceae with decreased carbapenem susceptibilities. Our findings suggest that the clinical detection of IMP-producing Enterobacteriaceae is not required even when the "old" CLSI criteria are used.