Can I rely on the negativity of my liquid media for mycobacteria (BACTEC™ MGIT™)?

Automated liquid media have represented a major advance in the culture of mycobacteria, limiting the use of solid media and even removing them. In this study, we investigated whether the Bactec MGIT automated system fails to detect mycobacteria and their possible causes. Over a period of three years (2023-2025), a total of 9.111 cultures for mycobacteria were performed using automated liquid media (Bactec MGIT). All negative tubes were inspected visually, and those with suspected growth were stained with Ziehl-Neelsen. Those that were positive were reinoculated, and the grown colonies were identified by molecular detection (Xpert MTB/RIF) and proteomics (MALDI-TOF). A total of 0.2% of all tubes incubated in this system were considered a detection failure, representing 11.1% of all mycobacterial isolates. A total of 22 cases were recorded, of which 8 (36.3%) were identified as M. tuberculosis complex (MTC) and the remaining 14 as NTMs (63.6%): M. lentiflavum, M. gordonae, M. avium, M. abscessus subsp. abscessus, M. chelonae, M. mageritense, M. celatum, and M. paragordonae. In 6 out of 8 patients who were undergoing anti-tuberculosis treatment an MTC species was isolated. When an MNT was isolated, only those isolated in non-respiratory samples were considered clinically significant. A final visual inspection of these negative tubes in the absence of molecular techniques, would be the only way to detect these cases.