Nitro-Carba test, a novel and simple chromogenic phenotypic method for rapid screening of carbapeneamase-producing Enterobacteriaceae.

OBJECTIVES: The present study developed Nitro-Carba Test (NCT), a rapid and simple chromogenic method for screening of Carbapenemase-producing Enterobacteriaceae (CPE).

METHODS: The NCT was validated with a total of 31 carbapenemase-producing isolates (9 KPCs, 11 MBLs and 11 OXA-48s) and with 56 non-carbapenemase-producing strains. The assay relies on the hydrolysis of nitrocefin in the presence of carbapenems. The carbapenemases were extracted with lysis buffer prior to addition to wells with and without imipenem (IPM), meropenem (MEM) and ertapenem (ETP). Following the addition of nitrocefin, a change in colour from yellow to red, indicating carbapenemase production, was observed within 20min. The susceptibility profiles of each bacterial strain were also investigated.

RESULTS: A NCT detected all 31 CPEs within a timeframe of only 10seconds to 12min. All carbapenemase producers hydrolyzed nitrocefin in all wells. No colour change in the wells with carbapenems was observed in non-carbapenemase producers. The sensitivity for all three carbapenems was 100%, while specificity of IPM, MEM and ETP were 64.29%, 91.07% and 100%, respectively. IPM, MEM and ETP against all carbapenemase-producing strains had MIC values ranging from 0.5 to ≥256μg/mL, 0.25 to ≥256μg/mL and 1 to ≥256μg/mL, respectively. OXA-48-producing isolates showed lower MIC values compared with producers of MBL and KPC.

CONCLUSION: This assay is a promising method detecting CPE rapidly. The NCT is a simple and reliable method, capable of detecting CPE in even carbapenem-susceptible strains.