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Assessing pure isolates of gram-negative bacilli for mechanism of carbapenem resistance
Nonsusceptibility to carbapenems in gram-negative bacilli by means of the enzyme KPC (Klebsiella pneumoniae carbapenemase) or NDM (New Dehli metallo-beta-lactamase) is becoming more common. The genes blaKPC and blaNDM encode KPC and NDM enzyme production, respectively. In addition to KPC and NDM production, there are other mechanisms of resistance to carbapenems in gram-negative bacilli, including production of other carbapenemases, or plasmid-encoded AmpC, or extended beta-lactamase production combined with decreased membrane permeability. Detection of carbapenemases by the modified Hodge test may be subjective and is not rapid. Testing for the minimum inhibitory concentration (MIC) determines the level but not the mechanism of resistance. PCR is a sensitive, specific, and rapid means of detecting of a specific portion of the genes encoding KPC and NDM production.
This PCR detects and differentiates both blaKPC and blaNDM. A positive KPC (Klebsiella pneumoniae carbapenemase) PCR indicates that the isolate carries blaKPC. A positive NDM (New Dehli metallo-beta-lactamase) PCR indicates the isolate carries blaNDM.
A negative result indicates the absence of detectable blaKPC or blaNDM DNA; however, false-negative results may occur due to inhibition of PCR, sequence variability underlying primers and, or loss of a plasmid carrying blaKPC and blaNDM.
This assay should be used for testing of isolates of gram-negative bacilli. Request KNSRP / KPC (blaKPC) and NDM (blaNDM) Surveillance PCR, if testing directly from rectal or perirectal swabs is desired.
The assay was validated using 159 gram-negative bacillus isolates, including 135 carbapenemase-producers (105 blaNDM positive and 30 blaKPC positive). The assay had 100% sensitivity and specificity for isolate testing compared with reference methods, including the modified Hodge test, testing for blaKPC using KPC (Klebsiella pneumoniae carbapenemase) PCR and testing for blaNDM by NDM (New Dehli metallo-beta-lactamase) PCR at the Health Protection Agency (HPA), London, UK.
1. Cunningham SA, Noorie T, Meunier D, et al: Rapid and simultaneous detection of genes encoding Klebsiella pneumoniae carbapenemase (blaKPC) and New Delhi metallo-beta-lactamase (blaNDM) in Gram-Negative Bacilli. J Clin Microbiol 2013;51:66-69
2. Multiplex Real-Time PCR Detection of Klebsiella pneumoniae Carbapenemase (KPC) and New Delhi Metallo-beta-lactamase (NDM-1) genes. Centers for Disease Control and Prevention 2011 (unpublished)
3. CLSI Document M100-S23, Vol.33 No.1, 2013. CLSI, Wayne, PA
4. New Carbapenem-Resistant Enterobacteriaceae Warrant Additional Action by Healthcare Providers. Centers for Disease Control and Prevention Health Alert Network, February 14, 2013