MEDICAL MICROBIOLOGY-MYCOBACTERIUM
The Mycobacteria are named so because of the mold like pellicular growth of these organisms in liquid medium. An important character of the mycobacteria is their ability to resist decolourisation by a weak mineral acid after staining with one of the aryl-methane dyes. This gives the name acid fast bacteria to these organisms.
CLASSIFICATION Mycobacteria have been classified in a variety of ways. However, a clinical classification is more practical and is described
Group 1-Obligate pathogens M. tuberculosis M.leprae M. bovis Group
Group 2. Skin pathogens M. marinum M. ulcerans
Group 3-Opportunistic pathogens M. kansasii M. avium-intracellulare (MAC)
Group 4. Non or rarely pathogenic M. gordonae M. smegmatis Tuberculosis is an infectious disease caused by multiplication of bacilli belonging to the genus Mycobacterium. The principal bacterium is Mycobacte rium tuberculosis (also called Koch’s bacillus) and to a lesser extent M. africanum and M. bovis. These 3 species collectively are called as “tuberculous mycobacteria”. Non-tuberculous mycobacteria can also cause infections simulating tuberculosis.
MYCOBACTERIUM TUBERCULOSIS Morphology: The tubercle bacilli are slender, straight or slightly curved rod shaped organisms measuring 2-4 μm in length and 0.2-0.8 μm in breadth occurring singly, in pairs or in small groups. The bacilli are non-sporing, non-motile and non-capsulated. In suitable liquid culture media, virulent human and bovine tubercle bacilli form characteristic long, tight, serpentine cords in which organisms are alligned in parallel. The bacilli are Gram positive though they do not take the stain readily. These organisms resist decolourisation by 25% sulphuric acid and absolute alcohol for ten minutes and hence these are called acid and alcohol fast.
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