Objective To explore the value of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS)-based direct-on-target microdroplet growth assay (DOT-MGA) for rapid identification of carbapenem-resistant Enterobacteriaceae (CRE).

Methods Totally, 32 Escherichia coli and 28 Klebsiella pneumoniae isolates were collected in this study. The microorganisms were mixed with and without imipenem, meropenem, or ertapenem in nutrient broth on MALDI-TOF MS target directly as detecting spots and growth controls. These mixtures were incubated at (35 ±1)℃ for 3 h, 4 h, 5 h and 6 h, respectively. Using VITEK MS, CRE were distinguished if the microorganisms could be identified successfully. At the same time, minimal inhibitory concentration (MIC) of these strains was determined by broth microdilution method, and the results were compared with those of DOT-MGA.

Results For Klebsiella pneumoniae, sensitivity, specificity, positive, and negative predictive values of three carbapenems were 100% after 4 h incubation with 92.9% validity of growth controls. For Escherichia coli, validity of growth controls was 100% after 5 h incubation. At this time-point, specificity and positive predictive value of imipenem, meropenem, and ertapenem were 100%, sensitivity was 85.0%, 70.0% and 95.0%, negative predictive value was 80.0%, 66.7% and 92.3%, respectively. The results of Klebsiella pneumoniae DOT-MGA were consistent with the microbroth dilution method (Kappa=1), while the results of Escherichia coli were better (ertapenem 0.93, imipenem 0.81) and normal (meropenem 0.64).

Conclusions Using MALDI-TOF MS-based DOT-MGA to identify CRE is rapid, accurate, and easy to operate, especially for carbapenem-resistant Klebsiella pneumoniae (CRKP), which can assist clinicians to select the most appropriate antibiotics for early treatment.