Mycobacterium tuberculosis

Mycobacterium tuberculosis

Mycobacterium tuberculosis is the bacterium that causes most cases of tuberculosis.

History

M. tuberculosis, then known as the "tubercle bacillus," was first described on March 24, 1882 by Robert Koch, who subsequently received the Nobel Prize in physiology or medicine for this discovery in 1905; the bacterium is also known as Koch's bacillus. The M. tuberculosis genome was sequenced in 1998

Physiology

M. tuberculosis is an obligate aerobe (weakly Gram-positive mycobacterium, hence Ziehl-Neelsen staining, or acid-fast staining, is used). While mycobacteria do not seem to fit the Gram-positive category from an empirical standpoint (i.e., they do not retain the crystal violet stain), they are classified as acid-fast Gram-positive bacteria due to their lack of an outer cell membrane.

M. tuberculosis divides every 15 to 20 hours, which is extremely slow compared to other bacteria, which tend to have division times measured in minutes (for example, E.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, rich in lipids (e.g., mycolic acid), is likely responsible for this resistance and is a key virulence factor.

When in the lungs, M. tuberculosis is taken up by alveolar macrophages, but they are unable to digest the bacterium. Its cell wall prevents the fusion of the phagosome with a lysosome. Specifically, M. tuberculosis blocks the bridging molecule, early endosomal autoantigen 1 (EEA1); however, this blockade does not prevent fusion of vesicles filled with nutrients. Consequently, the bacteria multiply unchecked within the macrophage. The bacteria also carried the UreC gene, which prevents acidification of the phagosome. The bacteria also evade macrophage killing by neutralizing reactive nitrogen intermediates.

Hypervirulent strains

Tuberculosis outbreaks are often caused by hypervirulent strains of M. tuberculosis. In laboratory experiments, these clinical isolates elicit unusual immunopathology and may be either hyperinflammatory or hypoinflammatory. Studies have shown that the majority of hypervirulent mutants have deletions in cell wall modifying enzymes or regulators that respond to environmental stimuli. Studies of these mutants have indicated the mechanisms that enable M. tuberculosis to mask its full pathogenic potential, inducing a granuloma that provides a protective niche and enables the bacilli to sustain a long-term persistent infection.

Strain variation

M. tuberculosis appears to be genetically diverse. This genetic diversity results in significant phenotypic differences between clinical isolates. M. tuberculosis exhibits a biogeographic population structure and different strain lineages are associated with different geographic regions. Phenotypic studies suggest that this strain variation has implications for the development of new diagnostics and vaccines. Micro-evolutionary variation affects the relative fitness and transmission dynamics of antibiotic-resistant strains.

Genome

The genome of the H37Rv strain was published in 1998. Its size is 4 million base pairs, with 3959 genes. 40% of these genes have had their function characterised, with possible function postulated for another 44%. Within the genome are also 6 pseudogenes.

The genome contains 250 genes involved in fatty acid metabolism, with 39 of these involved in the polyketide metabolism generating the waxy coat. Such large numbers of conserved genes shows the evolutionary importance of the waxy coat to pathogen survival.

10% of the coding capacity is taken up by 2 clustered gene families that encode acidic glycine rich proteins. These proteins have a conserved N-terminal motif, deletion of which impairs growth in macrophages and granulomas.

Diagnosis

Sputum is taken in 3 successive mornings as the number of organisms could be low, and the specimen is treated with 3% KOH or NaOH for liquefaction and decontamination. Gram stain should never be performed, as the organism is an "acid-fast bacillus" (AFB), meaning that it retains certain stains after being treated with acidic solution. In the most common staining technique, the Ziehl-Neelsen stain, AFB are stained a bright red, which stands out clearly against a blue background; therefore, the bacteria are sometimes called red snappers. The reason for the acid-fast staining is because of its thick waxy cell wall. The waxy quality of the cell wall is mainly due to the presence of mycolic acids. This waxy cell wall also is responsible for the typical caseous granuloma formation in tuberculosis. The component responsible, trehalose dimycolate, is called the cord factor. A grading system exists for interpretation of the microscopic findings based on the number of organisms observed in each field. It should be noted that the Ziehl- Neelsen stain is positive in only 50% of cases, which means that, even if no organisms are observed, further investigation is still required. Acid-fast bacilli can also be visualized by fluorescent microscopy using auramine-rhodamine stain for screening, which makes them appear somewhat golden in color. Also, M. tuberculosis is grown on a selective medium known as Lowenstein-Jensen medium, which has traditionally been used for this purpose. However, this method is quite slow, as this organism requires 6-8 weeks to grow, which delays reporting of results. A faster results can now be obtained using Middlebrook medium.

It should be taken into consideration that during an advanced stage of tuberculosis, the organism may infect almost any part of the body, which means that a specimen should appropriately be chosen (e.g. intestinal tuberculosis-stool).

See also

References

External links

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