Phenotypic Detection of Carbapenemase-Producing Organisms from Clinical Isolates

Abstract

The rapid spread of multidrug-resistant Gram-negative organisms constitutes one of the greatest challenges to global health. While Gram-negative organisms have developed several mechanisms to avert the bactericidal effects of commonly prescribed antibiotic agents, the increasing prevalence of carbapenemase-producing organisms (CPO) is particularly concerning given the rapid spread of mobile genetic elements containing carbapenemase genes, the limited treatment options for infections caused by these organisms, and the high mortality rates associated with CPO infections. Understanding if an organism is carbapenemase producing and, if so, the class of carbapenemase(s) produced has treatment implications, as some agents preferentially have activity against specific carbapenemases. Furthermore, CPO disseminate between patients with greater ease than non-CP-carbapenem-resistant organisms and warrant more intensive infection control measures than would be employed in the absence of carbapenemase production. Phenotypic assays currently used in clinical practice to detect CPO consist of the following: (i) growth-based assays which measure carbapenem resistance based on organism growth in the presence of a carbapenem antibiotic (e.g., modified Hodge test and modified carbapenem inactivation method), (ii) hydrolysis methods which detect carbapenem degradation products (e.g., Carba NP test and matrix-assisted laser desorption-ionization time of flight mass spectrometry), and (iii) lateral flow immunoassays which detect carbapenemase enzymes through the use of specific antibodies. Although there is no single phenotypic test that meets all specifications of the ideal test, as we describe in this review, there are a number of tests that are user-friendly, affordable, accurate, and feasible for implementation in clinical microbiology laboratories of all sizes.

Keywords: Carba NP; Carba NP test; carbapenemase; carbapenemase-producing Enterobacteriaceae; carbapenemase-producing organism; lateral flow antigen; lateral flow assay; modified Hodge test; modified carbapenem inactivation method; phenotypic.

FIG 1

(A) Modified Hodge test. 1, Klebsiella pneumoniae ATCC BAA-1705, positive result; 2, K. pneumoniae ATCC BAA-1706, negative result; 3, clinical isolate, positive result. (B) CLSI Carba NP positive result. Tube A, no imipenem added, red; tube B, imipenem added, yellow. (C) Rapidec Carba NP (bioMérieux, Inc.) positive result. Well d, no imipenem added, red; well e, imipenem added, yellow. (D) Neo-Rapid Carba screen (Rosco Diagnostica) positive result. Tube 1a, no imipenem added, red; tube 1b, imipenem added, yellow (67). (E) Rapid Carb Blue Screen (Rosco Diagnostica) positive result. Tube a, no imipenem added, blue; tube b, imipenem added, yellow. (F) Modified carbapenem inactivation method (mCIM) positive result. (G) mCIM negative result. (H) mCIM and EDTA-mCIM (eCIM) results that are positive for a serine carbapenemase producer, as there is no inhibition of carbapenemase activity in the presence of EDTA. (I) mCIM and eCIM results that are positive for a metallo-beta-lactamase producer, as there is inhibition of carbapenemase activity in the presence of EDTA. (J) NG-Test Carba 5 (NG Biotech) lateral flow immunoassay results for the different carbapenemases detected. Panel A is republished from reference with permission from the publisher. Panels B and F to I are republished from reference with permission from the publisher. The image from panel J was provided by NG Biotech.