MALDI-TOF MS, a useful instrument for differentiating metallo-β-lactamases in Enterobacteriaceae and Pseudomonas spp

We have evaluated a matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) method for the identification of carbapenemases and for distinguishing metallo-β-lactamases (MBLs). A total of 49 noncarbapenemase-producing and 14 carbapenemase-producing Enterobacteriaceae and Pseudomonas aeruginosa clinical strains, previously characterized by PCR, were included in the study. With MALDI-TOF MS, the presence of carbapenemases was confirmed by the detection of ertapenem hydrolysis (lost of molecular peaks: 476·5 Da, 498·5 Da, 520·5 Da and presence of degradation products) in the mixture of the bacteria with the antibiotic, and classification was achieved by selective inhibition of carbapenemase activity (the ertapenem molecular peak was maintained) with ethylenediaminetetraacetic acid (EDTA). We obtained a good concordance among the results of PCR and MALDI-TOF MS. This method appears to be simple, fast and reliable for distinguishing in few hours different classes of carbapenemases, which can be very useful for epidemiological studies or to establish a specific antimicrobial therapy.

MALDI-TOF mass spectrometry is increasingly present in microbiology laboratories due to its increasing use for bacterial identification. This study describes a method for detection of carbapenemase activity using MALDI-TOF, which is similar to the reference method: the detection of imipenem hydrolysis using UV spectrometry.