Rapid identification of nontuberculous mycobacteria directly from positive primary MGIT coltures by MALDI-TOF MS

Introduction: Over the last years, nontuberculous mycobacteria (NTM) have emerged as important human pathogens. Accurate and rapid mycobacterial species identification is needed for successful diagnosis, treatment, and management of infections caused by NTM. Matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS) was demonstrated to effectively identify mycobacteria isolates subcultured from solid or liquid media rather than new positive cultures. The aim of the present study is to develop a rapid method for direct identification of NTM from primary MGIT cultures by MALDI-TOF MS. Materials and Methods: A total of 20 positive MGIT broths, collected from February to July 2021, were examined by the Bruker Biotyper system with Mycobacteria Library v 2.0 (Bruker Daltonics). Extraction was performed within 24-72 h after automated growth detection by MGIT. Protein extraction was carried out by the manufacturer’s MycoEx protocol. Results were compared with those obtained by the Line probe assay GenoType Mycobacterium CM/AS/NTM-DR (Hain LifeScience). Results: Our results showed concordant identification for all the mycobacteria isolated. In particular, the molecular test identified the mycobacteria as M. avium (n. 5), M.intracellulare (n. 3), M. chimaera (n. 3), M. gordonae (n. 2), M. fortuitum (n. 2), M. tuberculosis complex (n. 2), M. chelonae (n. 1), M. lentiflavum (n. 1) and M. celatum (n. 1). All identifications based on MALDI-TOF MS had scores >1.7 and were concordant with the molecular identifications. MALDI-TOF MS cannot differentiate between M. intracellulare and M. chimaera, two closely related potentially pathogenic species of NTM that are members of the M. avium complex. Discussion and conclusions: Although a small number of strains and a limited diversity of mycobacterial species were analysed, our results indicate that MALDI-TOF MS could represent a useful routine diagnostic tool for identification of mycobacterial species directly from primary liquid culture.