Growth and Detection of Mycobacteria
Direct microscopic examination ofacid-fast stained smears is the most rapid method for the detection ofacid fast bacilli (AFB) in clinical specimens. For maximum sensitivity,smears are examined using the Auramine-O fluorochrome stain. Eventhough a positive acid-fast smear indicates that AFB are present,culture is essential since negative smears are not diagnostic andpositive smears are not specific.
The digestion, decontamination,concentration, and inoculation of culture media remain the standardapproach for the recovery of AFB from clinical specimens. Conventionalculture techniques can detect 10 to 100 viable organisms per milliliterof sample. TDSHS uses a combination of liquid and solid media toprovide optimal AFB recovery. Positive cultures are identified by highperformance liquid chromatography (HPLC) run on a daily basis, geneticprobes, and/or biochemical tests, and reported immediately. Culturesare examined for a total of 6 weeks before being reported as negative.
TDSHS uses the BACTEC MGIT 960 systemfor liquid culture of AFB. The BACTEC MGIT 960 culture system has beenshown to be both a rapid and sensitive method for the recovery of M. tuberculosisand other mycobacteria from clinical specimens. BACTEC MGIT 960 systemperformance has been demonstrated to be equivalent to that of theradiometric BACTEC 460TB system. The principle benefit of liquidculture 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 allAFB cultures.