This study exploited the possibility to detect Citrobacter freundii-derived CMY-2-like cephalosporinases in Enterobacteriaceae clinical isolates using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Periplasmic proteins were prepared using a modified sucrose method and analyzed by MALDI-TOF MS. A ca. 39,850-m/z peak, confirmed to represent a C. freundii-like -lactamase by in-gel tryptic digestion followed by MALDI-TOF/TOF MS, was observed only in CMY-producing isolates. We have also shown the potential of the assay to detect ACC-and DHA-like AmpC-type -lactamases.
Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is increasingly used as a procedure for identification of pathogenic bacteria and fungi due to its time-and cost-effectiveness (1, 2). Recently, further applications of MALDI-TOF MS focusing on antimicrobial resistance mechanisms, including detection of carbapenemase activity in Enterobacteriaceae, Pseudomonas spp., and Acinetobacter spp., have been described (3-7).In 2007, Camara and Hays described for the first time the use of MALDI-TOF MS for differentiating wild-type Escherichia coli from ampicillin-resistant (Amp r ) plasmid-transformed E. coli strains by the direct visualization of a -lactamase (8). In a recent MALDI-TOF MS study, Schaumann et al. were not able to distinguish Enterobacteriaceae and P. aeruginosa isolates producing extended-spectrum -lactamases (ESBLs) or metallo--lactamases (MBLs) from nonproducers (9). Consequently, so far the attempts to visualize native -lactamases by MALDI-TOF MS in wild-type bacteria have been mostly unsuccessful.We describe here a new assay for the identification of CMY-2-like -lactamases in clinical enterobacterial isolates by MALDI-TOF MS. These enzymes are the most prevalent acquired AmpCtype cephalosporinases in Enterobacteriaceae (10). The method is based on the extraction of periplasmic proteins and the detection of CMY-2-like -lactamases by MALDI-TOF MS according to their molecular weight.Thirty-eight characterized Enterobacteriaceae strains from collections of the Faculty of Medicine and University Hospital in Plzen, Czech Republic, the Hellenic Pasteur Institute in Athens, Greece, and the National Medicines Institute in Warsaw, Poland, were used (Table 1) (11,12). The group included 29 CMY-2-positive clinical isolates, two E. coli transconjugants/transformants with CMY-2-like enzymes (E. coli A15 or DH5␣), and seven non-CMY-producing isolates (13-21). E. coli ATCC 25922 and Klebsiella pneumoniae ATCC 13883 were used as negative controls. Purified CMY-2 enzyme (13) was used as a positive control for MALDI-TOF MS measurements.Isolates were inoculated into 50 ml of Mueller-Hinton broth (Oxoid Ltd.) and incubated at 35°C for 16 h. Cultures were centrifuged at 5,000 ϫ g for 20 min, and the cell pellet was used for the extraction of the periplasmic proteins, performed essentially as described by Naglak and Wang (22). Briefly, the pellet was resuspended in 360 l ...