2.2 Characteristics of Mycobacterium tuberculosis bacillus

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  • Mycobacterium tuberculosis are fairly large, non-motile & rod-shaped bacterium that are 2-4 microns (micrometers) in length and 0.2 - 0.5 microns in width.
  • It is an obligate aerobe and hence, MTB complexes are usually always found in the well-aerated upper lobes of the lungs.
  • The bacterium is a facultative intracellular parasite, usually of macrophages, and has a slow generation time of 15-20 hours.
  • They can be classified in to three main groups:
  1. Mycobacterium tuberculosis complex: this group includes M. tuberculosis, M. bovis, M. africanum, M. microti, and M. Canetti. They all can cause “tuberculosis” in humans. The vast majority of tuberculosis is caused by M. tuberculosis, with the other organisms being relatively rare.
  2. Mycobacterium leprae: causes leprosy.
  3. Non tuberculous mycobacteria (NTM): this group includes all the other mycobacteria that can cause diseases in humans. NTM sometimes can cause clinical manifestations (in the lungs, skin, bones, or lymph nodes) similar to those of tuberculosis. Most NTM exist in the environment, are not usually spread from person to person and are often non-pathogenic in persons with intact immune system or healthy lung tissue.
  • All mycobacteria are classical acid-fast organisms. These are named so because they have a waxy coating on its cell surface, primarily due to the presence of mycolic acid, and are able to retain the colour of the stain (i.e. Ziehl-Neelsen stain) after being washed by an acid
  • M. tuberculosis multiplies more slowly than the majority of bacteria; this is why tuberculosis has a slower evolution (causes disease weeks or even months to years after infection) than most other bacterial infections.