Abstract: Objective  To investigate the properties and the forming reasons of granules in the BACTEC^TM MGIT960-negative culture from Mycobacterium growth indicator tube (MGIT). Methods  The granules were collected from 31 BACTEC^TM MGIT960-negative culture tubes, and then performed acid-fast bacilli (AFB) examination, BACTEC^TM-MGIT960 culture and Lowenstein-Jensen (LJ) medium culture, respectively. The positive cultures were performed species identification and drug susceptibility tests. M. tuberculosis strain H37Rv (ATCC strain 27294) and 8 clinical isolates acquired from the granules subculture were selected for the inoculation tests. The 8 strains included 2 M. tuberculosis complex (MTC) strains, 3 M. xenopi strains and 3 M. intracellulare strains. The isolates were adjusted to the McFarland no. 1 turbidity standard, which were diluted to 10^-5, 10^-6, 10^-7, 10^-8 suspensions. An aliquot of each preparation was inoculated into the MGIT vials in duplicate. Results  Of 31 granules from BACTEC^TM MGIT960-negative culture tubes, 29 were subculture positive by either MGIT 960 or/and LJ medium, in which 24 granules were AFB smear positive, and 18, 4 and 7 isolates were identified as MTC, M. intracellulare and M. xenopi, respectively. A limited amount of 8 clinical isolates and H37Rv strain were inoculated, granule formation was observed in the MGIT-negative culture inoculated respectively with 10^-6 (granule in 4/6 tube), 10^-7(granule in 5/6 tube), 10^-8 (granule in 6/6 tube) of M. xenopi strains after incubation for 6 weeks. Conclusion  The granules in BACTEC^TM MGIT960-negative culture tubes are more likely to be live mycobacteria. The granule formation in the MGIT960 may be correlated with a low bacterial load in the tested specimens, might also be a species-specific feature of M. xenopi and M. intracellulare.

Key words: Mycobacterium tuberculosis, Mycobacterium xenopi, Culture media, Bacteriological techniques