Rapid detection of carbapenemase activity: benefits and weaknesses of MALDI-TOF MS

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) has been introduced as an identification procedure for bacteria and fungi. The MALDI-TOF MS-based analysis of resistance to β-lactam antibiotics has been applied to detect hydrolysis of carbapenems by different bacterial strains. However, the detection of enzymatic carbapenem degradation by MALDI-TOF MS lacks well-standardized protocols and several methods and models of interpretation using different calculations of ratio-of-peak intensities have been described in the literature. Here, we used faropenem and ertapenem hydrolysis as model compounds. In an attempt to propose a universal protocol, the hydrolysis was regularly monitored during 24 h using well-characterized bacterial strains producing different types of carbapenemases (KPC, IMP, NDM, VIM, and OXA-48). Variable responses and different timing for detectable hydrolysis, depending on the enzyme produced, were observed. KPC degrades its template antibiotics very quickly (15 min for some KPC producers) compared to other types of enzymes (more than 90 min for other enzymes). Prior bacterial lysis was shown to be of no interest in the modulation or optimization of the hydrolytic kinetics. The adequate detection of carbapenem hydrolysis would, therefore, require several MALDI-TOF MS readouts for the timely detection of rapid hydrolysis without missing slow hydrolysis. This enzymatic constraint limits the implementation of a standard protocol in routine microbiology laboratories.