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Concept
Mycobacterium tuberculosis
Summary
Mycobacterium tuberculosis (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis. First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface primarily due to the presence of mycolic acid. This coating makes the cells impervious to Gram staining, and as a result, M. tuberculosis can appear weakly Gram-positive. Acid-fast stains such as Ziehl–Neelsen, or fluorescent stains such as auramine are used instead to identify M. tuberculosis with a microscope. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, culture, and polymerase chain reaction.
The M. tuberculosis genome was sequenced in 1998.
In 2019, M. tuberculosis was found in a genetically related complex group of Mycobacterium species called Mycobacterium tuberculosis complex that has at least 9 members:
M. tuberculosis sensu stricto
M. africanum
M. canetti
M. bovis
M. caprae
M. microti
M. pinnipedii
M. mungi
M. orygis
It requires oxygen to grow, and is nonmotile. M. tuberculosis divides every 18–24 hours. This is extremely slow compared with other bacteria, which tend to have division times measured in minutes (Escherichia coli can divide roughly every 20 minutes). It is a small bacillus that can withstand weak disinfectants and can survive in a dry state for weeks. Its unusual cell wall is rich in lipids such as mycolic acid and cord factor glycolipid, is likely responsible for its resistance to desiccation and is a key virulence factor.
Other bacteria are commonly identified with a microscope by staining them with Gram stain. However, the mycolic acid in the cell wall of M. tuberculosis does not absorb the stain. Instead, acid-fast stains such as Ziehl–Neelsen stain, or fluorescent stains such as auramine are used.
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