Matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) mass spectrometry (MS) for the identification of mycobacteria from MBBacT ALERT 3D liquid cultures and Lowenstein-Jensen (LJ) solid cultures.

Aims: Conventional methods for the identification of mycobacteria can be demanding and prolonged. Molecular methodologies, although rapid, are expensive and often exclusive to reference laboratories. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) presents a possible alternative method for the identification and differentiation of mycobacteria. This study describes the design and validation of a mycobacteria inactivation protocol and subsequent evaluation of MALDI-TOF MS for the identification of mycobacteria in a clinical microbiology laboratory setting.

Methods: A total of 65 non-tuberculous mycobacteria (NTM) isolates and 86 Mycobacterium tuberculosis complex (MTC) isolates were tested on the Bruker MALDI Biotyper Microflex, V.3.1. NTM solid-culture (Lowenstein-Jensen, LJ slopes) isolates (n=21) and liquid-culture (MBBacT ALERT 3D bottles) isolates (n=44) were tested. MTC solid-culture isolates (n=30) and liquid-culture isolates (n=56) were also tested. Isolates were subjected to the validated inactivation protocol and analysed on the MALDI-TOF MS instrument.

Results: The inactivation protocol designed was successfully validated and applied to all test isolates. MALDI-TOF MS correctly identified 82.8% of all isolates analysed; 96.7% and 96.4% of MTC isolates and 76.2% and 52.3% of NTM isolates were successfully identified from solid and liquid culture, respectively. MALDI-TOF MS failed to identify 35.4% (n=23) of NTM isolates and 3.5% (n=3) of MTC isolates.

Conclusions: MALDI-TOF MS has potential for identifying mycobacteria in the clinical laboratory setting, by reducing identification turnaround time and laboratory costs in isolate referral. Isolates that failed to be identified are explained by limitations of the method.