[Laboratory media for the cultivation of tubercle bacillus]

A variety of different media for the cultivation of mycobacteria have been described but a few of them are in use today. Those currently used can be characterized by three basic types. The first is egg-based media represented by Ogawa and Löwenstein-Jensen. The second type is agar-based media; the most common one are Middlebrook 7H10 and 7H11. The third type is liquid media such as Middlebrook 7H9. Several weeks of incubation may be required for the isolation of M. tuberculosis on solid media. Substantial improvement in the time to detection and the recovery rate was realized by using broth-based culture system such as the BACTEC 460TB, Septi-Chek AFB, MGIT and BACTEC 9000. In the BACTEC 460TB system, the mycobacteria is detected radiometrically. The processed specimen is added to a modified 7H9 medium (BACTEC 12B) containing 14C-labeled palmitic acid and an antibiotic complex, PANTA. Mycobacterial growth can be ascertained by the liberation of 14CO2 and detected by BACTEC 460TB instrument. The Septi-Chek AFB is a biphasic medium which combines broth and solid media. The liquid medium is a modified Middlebrook 7H9 in a carbon-dioxide-enriched culture bottle. After inoculation of the sample, the bottle is capped with a slide consisting of three solid media; a non-selective Middlebrook 7H11 agar, an egg-based medium, and chocolate agar. A novel system is the MGIT, which is a nonradiometric broth method for the detection of mycobacteria from clinical specimens. The MGIT consists of a modified Middlebrook 7H9 broth and a sensor embedded in silicone on the bottom of a tube. The appearance of orange-colored fluorescence in the sensor when excited indicates the growth of mycobacteria. MB Redox is a modified, serum-supplemented Kirchner medium containing p-indonitrotetrazolium violet (INT) as an indicator of microbial growth. The INT is reduced by the redox system of the mycobacteria to deep violet-colored formazan. This substance is water insoluble and is reduced to the cell surface, by which bacterial clamps can be easily detected by their violet color. At present, the egg-based media are the first choice for the culture of clinical samples. However, there are advantages to each type of medium and not all strains of mycobacteria can be recovered on a single medium. Therefore, it is recommended that one representative of each type of medium be used for primary isolation; one example in Japan may be Ogawa egg medium in combination with Middlebrook 7H11 and MGIT.