How to detect NDM-1 producers

Abstract

Enterobacterial isolates expressing the carbapenemase NDM-1 are emerging worldwide. Twenty-seven NDM-1-positive isolates of worldwide origin were included in this study to identify these strains as not only pathogens but also colonizers of normal flora for infection control screening. Although susceptibility to carbapenems varied, a combined test (IMP/IMP + EDTA), the Etest MBL, and automated susceptibility testing by Vitek2 (bioMérieux) identified those NDM-1 producers as verified by PCR using specific primers. Screening for carriers of NDM-1 producers may be based on media such as the ChromID ESBL culture medium routinely used to screen for extended-spectrum β-lactamase producers, which gives excellent detection levels with low limits of detection ranging from 8 × 10(0) to 5 × 10(2) CFU/ml. The CHROMagar KPC culture medium had higher limits of detection (1 × 10(1) to 5 × 10(5) CFU/ml) and may be proposed for the follow-up of outbreaks of infections with NDM-1 producers. Colonies growing on these screening media can be verified as NDM-1 producers with molecular methods as described herein.

Fig. 1.

Strategy for identification of NDM-1 producers as a source of clinical infections and for detecting carriers of NDM-1 producers. *, this culture medium can be used for surveillance of outbreaks of infections with NDM-1 producers after validation of its detection sensitivity for the specific strain responsible for an outbreak. **, Etest MBL is reliable when the MIC of imipenem is not too low.