Comparison of phenotypic methods for the detection of IMP producing enterobacteriaceae in queensland

Abstract

Background The prevalence of carbapenemase producing enterobacteriaceae (CPE), particularly IMP producers (IMP), in Australia is increasing. Optimal methods for detection of IMP CPE are unknown. We compare six phenotypic methods to determine the most sensitive method for the detection of IMP CPE. Methods We tested 25 molecularly confirmed IMP CPE producers, including 16 Enterobacter cloacae , collected from Queensland Health hospitals. The following methods were performed: disc susceptibility testing (DST), minimum inhibitory concentrations (MICs) by E-test, broth microdilution (BMD) and Vitek 2, modified hodge test (MHT) and Carba NP test. Results Meropenem MIC range (MIC 50/90 ) was 0.25->32 (4/16), 0.25-32(2/16), 1->16(>16/>16) for BMD, E-test and Vitek 2, respectively. Seventeen percent of isolates had a Vitek 2 meropenem MIC between 1-2 |xg/mL. Using EUCAST breakpoints, all isolates tested resistant by Ertapenem E-test and all except for one isolate was resistant by ertapenem DST. CLSI breakpoints increased detection of CPE with meropenem and imipenem. Carba NP test and MHT provided positive results for all isolates. Conclusion: Our results indicate that isolates with a Vitek 2 meropenem MIC of ≥1 μg/mL should undergo further phenotypic testing. Ertapenem DST, ertapenem E-test, Carba NP test and MHT are the most sensitive methods for detection of IMP CPE.