1.1 Characteristics of Mycobacterium tuberculosis bacillus

Mycobacteria are small rod-shaped bacilli that can cause a variety of diseases in humans. They can be thought of in three main groups:

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. Their treatment is similar (with M. bovis being innately resistant to pyrazinamide and M. africanum being innately resistant to thioacetazone).
This guide only addresses disease caused by Mycobacterium tuberculosis complex.

Mycobacterium leprae causes leprosy.

– 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 and are named so because of the stains used in evaluation of tissue or sputum specimens (i.e. Ziehl-Neelsen stain, Chapter 3).

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.

M. tuberculosis is a strictly aerobic bacterium. It therefore multiplies better in pulmonary tissue (in particular at the apex, where oxygen concentration is higher) than in the deeper organs.