Mycobacterium tuberculosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

This page is about microbiologic aspects of the organism(s).  For clinical aspects of the disease, see Tuberculosis.

Synonyms and Keywords: M. Tuberculosis

Overview

Mycobacterium tuberculosis is the bacterium responsible for tuberculosis. It is an aerobic, non-encapsulated, non-motile, acid-fast bacillus. M. tuberculosis belongs to the Mycobacterium tuberculosis complex, that also includes bacteria, such as M. bovis and M. africanum. The bacterium has a very slow rate of replication, and its genetic variations account for the different strains and the growing drug resistance. M. tuberculosis has tropism for different kinds of human cells, with preference for cells of the lung. It may infect different species, yet human beings are its frequent natural reservoir.

Taxonomy

Computer-generated image of a cluster of rod-shaped drug-resistant Mycobacterium tuberculosis bacteria. Image provided by the CDC Centers for Disease Control and Prevention [1]
Thin agar culture plates reveal the results of a drug susceptibility test on Mycobacterium tuberculosis bacteria Image provided by the CDC Centers for Disease Control and Prevention [2]

Cellular organisms; bacteria; Actinobacteria; Actinobacteria; Actinobacteridae; Actinomycetales; Corynebacterineae; Mycobacteriaceae; Mycobacterium; Mycobacterium tuberculosis complex; M. tuberculosis[3]

Biology

Mycobacterium tuberculosis belongs to the Mycobacterium tuberculosis complex. This complex includes M. tuberculosis, M. bovis, M. africanum, M. canetti, and M. microti.[4]

M. tuberculosis is an obligate aerobe, non-encapsulated, non-motile, acid-fast bacillus. Slender, straight or slightly curved bacillus with rounded ends, occuring singly, in pairs or in small clumps. It does not form spores and its ideal growing environment includes tissues with high levels of oxygen. It cannot be considered gram-positive or gram-negative due to the high lipid content of the cell wall, which is impermeable to the dyes used until combined with an alcohol. On microscopic examination of sputum samples, the bacteria cannot be distinguished from other acid-fast bacteria, such as Nocardia spp.[4] M. tuberculosis has a very slow rate of replication, taking about 15 to 20 hours to divide. This characteristic, added to its ability to remain in the latent state for long periods of time, account for the long treatment duration required.[4]

Genetic variances in the M. tuberculosis genome lead to important phenotypical changes. There are many different strains of the bacteria, 6 of which have specific geographical distribution. Three strains, the Beijing family, strain W and the W-like strains, were noted to be associated with resistance to treatment drugs.[5][6]

Tropism

M. tuberculosis does not have specific tissue tropism and is capable of infecting almost all human tissues. However, M. tuberculosis strives most in tissues with high oxygen levels, hence the high rate of pulmonary tuberculosis. [4]

Natural Reservoir

Human beings are the main natural reservoir for M. tuberculosis, however, the bacteria may infect other species.[4]

References

  1. "http://phil.cdc.gov/phil/details.asp". 
  2. "http://phil.cdc.gov/phil/details.asp". 
  3. "Mycobacterium tuberculosis". 
  4. 4.0 4.1 4.2 4.3 4.4 Lawn SD, Zumla AI (2011). "Tuberculosis.". Lancet. 378 (9785): 57–72. PMID 21420161. doi:10.1016/S0140-6736(10)62173-3. 
  5. Smith NH, Hewinson RG, Kremer K, Brosch R, Gordon SV (2009). "Myths and misconceptions: the origin and evolution of Mycobacterium tuberculosis.". Nat Rev Microbiol. 7 (7): 537–44. PMID 19483712. doi:10.1038/nrmicro2165. 
  6. Gagneux S, Small PM (2007). "Global phylogeography of Mycobacterium tuberculosis and implications for tuberculosis product development.". Lancet Infect Dis. 7 (5): 328–37. PMID 17448936. doi:10.1016/S1473-3099(07)70108-1. 

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