The Pathogen of tuberculosis

Mycobacteria are small, rod-shaped, aerobic, non–spore-forming bacilli. The genus Mycobacterium contains a group of organisms so closely related that they are referred to as the “tuberculosis complex”: M. tuberculosis, M. bovis, M. canetti, M. africanum, and M. microti. However, given the singular epidemiologic, clinical, public health, and therapeutic considerations associated with M. tuberculosis, the term tuberculosis should be reserved exclusively for infection or disease caused by this organism. Disease caused by other organisms of this genus should be referred to as “mycobacteriosis due to M. x” and not as “atypical tuberculosis” or “tuberculosis due to …” .

Organisms of the genus Mycobacterium are found variably in the environment, predominately in soil and water. However, M. tuberculosis has become so adapted to the human body that it has no natural reservoirs in nature other than infected/diseased persons. Although disease caused by strains of M. tuberculosis has been reported rarely in primates, elephants, and other mammals, the presumption is that the infection was originally introduced into the animals by humans.

Mycobacterial cell walls contain high concentrations of lipids or waxes, which makes them resistant to standard staining techniques. They can be induced to take up a dye such as carbol fuchsin by imposing alkalinity or by heating. After dye absorption, they are resistant to the potent decolorizing agent acid-alcohol, a trait that provides the basis of the reference to acid-fast bacilli (AFB).

M. tuberculosis and most of the other mycobacteria grow quite slowly; their replication time in most media is approximately 18 to 24 hours. Readily discernible colonies typically do not appear on solid media for 2.5 to 5 week, so there has been great interest in rapid, novel techniques to indicate the presence, species, and drug susceptibility of mycobacteria, including M. tuberculosis.