Carba M Test for Rapid Detection and Simultaneous Differentiation of Carbapenemases among Clinical Isolates of Gram Negative Bacteria

Abstract

Introduction: Carbapenemase production is the most common mechanism of carbapenem resistance. The carbapenemases belongs to class A, class B and class D of Ambler's molecular classification. Various tests have been designed for screening and confirmation of these enzymes. The acidimetric based Carba NP (Nordmann Poirel) test is simple and detects carbapenemases within two hours. The test could not differentiate between the serine carbapenemases from Metallo β-Lactamase (MBL). Differentiation of these two classes of enzymes will help in choosing the newer carbapenemase inhibitors like avibactam which selectively act on serine carbapenemases. Aim: To design a test that can simultaneously confirm and differentiate Amblers class A and D from class B carbapenemase enzymes among clinical strains of the Gram-Negative Bacteria (GNB). Materials and Methods: An experimental study was conducted between January-December 2018 on 195 strains of carbapenem resistant and 40 strains of carbapenem sensitive GNB. The carbapenemase genes were detected among all the bacteria by multiplex Polymerase Chain Reaction (PCR). The Carba M test was designed and evaluated for the detection and differentiation of class A and D from class B carbapenemases among the study isolates. Results: The Carba M test had 100% sensitivity and specificity for identification of New Delhi Metallo-β-lactamase (NDM) and Verona Integron-encoded Metallo-β-lactamase (VIM) enzymes. The strains which co-produced two MBL enzymes were detected with 100% sensitivity. The test had 42.85% sensitivity for the detection of Oxacillinase (OXA)-48-like enzymes. Conclusion: The Carba M test is useful in detection and simultaneous differentiation of carbapenemase content of the GNB and will help to choose appropriate carbapenemase inhibitors judiciously