Growth and Detection of Mycobacteria
Direct microscopic examination of acid-fast stained smears is the most rapid method for the detection of acid-fast bacilli (AFB) in clinical specimens. For maximum sensitivity, smears are examined using the Auramine-O fluorochrome stain. Even though a positive acid-fast smear indicates that AFB are present, culture is essential since negative smears are not diagnostic, and positive smears are not specific.
The digestion, decontamination, concentration, and inoculation of culture media remain the standard approach for the recovery of AFB from clinical specimens. Conventional culture techniques can detect 10 to 100 viable organisms per milliliter of sample. TDSHS uses a combination of liquid and solid media to provide optimal AFB recovery. Positive cultures are identified by high-performance liquid chromatography (HPLC) run on a daily basis, genetic probes, and/or biochemical tests, and reported immediately. Cultures are examined for a total of 6 weeks before being reported as negative.
TDSHS uses the BACTEC MGIT 960 system for the liquid culture of AFB. The BACTEC MGIT 960 culture system has been shown to be both a rapid and sensitive method for the recovery of M. tuberculosis and other mycobacteria from clinical specimens. BACTEC MGIT 960 system performance has been demonstrated to be equivalent to that of the radiometric BACTEC 460TB system. The principal benefit of liquid culture is that some AFB can be detected up to two weeks (on average) prior to the appearance of visible colonies on solid media.
The primary isolation of a small number of M. tuberculosis strains, and other Mycobacterium species with special growth requirements (e.g., M. haemophilum), may require solid media for optimal recovery. TDSHS uses both egg-based (Lowenstein-Jensen) and non-egg-based (Middlebrook 7H11) media for all AFB cultures.