Evaluation of genotypic and phenotypic methods to detect carbapenemase production in gram-negative bacilli

Background: Carbapenemase-producing gram-negative bacteria (CP-GNB) are an urgent and expanding public health threat. Rapid and accurate identification of these organisms facilitates infection prevention efforts in healthcare facilities. The objective of our study was to evaluate methods to detect and identify CP-GNB. Methods: We examined 189 carbapenem-resistant GNB (CR-GNB), including Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii complex, using 3 different methods: 2 methods to screen isolates of GNB for carbapenemase production [the carbapenem inactivation method (CIM) and 2 chromogenic agars] and a molecular method (Cepheid GeneXpert Carba-R) to identify the mechanism of carbapenem resistance and the associated resistance genes (bla_KPC, bla_NDM, bla_IMP, bla_OXA-48-like, and bla_VIM). Results: The CIM was a simple and inexpensive phenotypic screen to differentiate between CR-GNB and CPGNB, with improved analytical performance characteristics and inter-reader correlation compared to the modified Hodge test. Both chromogenic agars evaluated (HardyCHROM CRE and chromID CARBA) were able to support growth of most of the organisms tested, including isolates possessing the bla_OXA-48-like gene. However, these media had a low analytical specificity for carbapenemase production, with breakthrough of CRGNB that did not produce a carbapenemase. The Xpert Carba-R assay was rapid and easy to perform, and demonstrated 100% positive and negative agreement for characterization of genetic determinants of carbapenem resistance. Conclusions: Screening by CIM followed by the Xpert Carba-R PCR is an accurate method for detecting and characterizing CP-GNB, including Enterobacteriaceae, P. aeruginosa, and A. baumannii complex.