Utilizing the Carba NP test as an indicator of expression level of carbapenemase genes in Enterobacteriaceae.

The Carba NP test was developed to detect carbapenemase-producing Enterobacteriaceae, and uses imipenem as the reaction substrate. In Japan, IMP-6 metallo-β-lactamase (MBL) producers, which are usually resistant to meropenem but susceptible to imipenem, and IMP-1 MBL producers, which are usually resistant to both carbapenems are prevalent. We performed the Carba NP test with IMP-6 and IMP-1 MBL producers, and both types were detected by the Carba NP test with high sensitivity. All IMP-1 MBL producers were detected by the Carba NP test, but the minimum inhibitory concentrations (MICs) of imipenem varied from 0.25 to >32μg/mL, and the time to positivity varied from 0 to 30min. Time to positivity was significantly correlated with expression levels of blaIMP-1, but not with MICs of imipenem. These results suggested that the Carba NP test can be used as a screening assay for carbapenemase gene expression levels among producers of the same type of carbapenemase. Using this approach, it is possible to determine whether the carbapenem resistance of each carbapenemase-producing Enterobacteriaceae isolate is primarily due to carbapenemase production, or to another mechanism such as outer membrane impermeability.