Direct detection of intact Klebsiella pneumoniae carbapenemases produced by Enterobacterales using MALDI-TOF MS

A MALDI-TOF MS-based identification method for KPC-producing Enterobacterales was developed. Methods: The molecular mass of the intact KPC-2 polypeptide was estimated for bla_KPC-2 transformants using MALDI Microflex and the exact mass was confirmed by LC and a high-resolution MS/MS system. A total of 1181 clinical Enterobacterales strains, including 369 KPC producers and 812 KPC non-producers, were used to set up the methodology and the results were compared with those from PCR analyses. For external validation, a total of 458 Enterobacterales clinical isolates from a general hospital between December 2018 and April 2019 were used. Results: The exact molecular mass of the intact KPC-2 protein was 28a»718.13 Da and KPC peaks were observed at m/z 28a»708.87-28a»728.34 using MALDI Microflex. Most of the KPC-2 (99.1%, 335/338) and KPC-3 (100%, 6/6) producers presented a clear peak via this method, while 12.0% (3/25) of the KPC-4 producers had a peak of weak intensity associated with low levels of gene expression. It took less than 20 min for the entire assay to be performed with colonies on an agar plate. External validation showed that the analytical sensitivity and specificity of the method compared with PCR were 100% (59/59) and 99.50% (397/399), respectively. Conclusions: The MALDI-TOF MS-based method for directly detecting the intact KPC protein is applicable to routine tests in clinical microbiology laboratories, supported by its speed, low cost and excellent sensitivity and specificity.