Mycobacterium tuberculosis

Mycobacterium tuberculosis is the bacterium that causes most cases of tuberculosis.^[1] It 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.^[2][3]

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Physiology

M. tuberculosis is an obligate aerobe ( weakly Gram-positive mycobacterium, hence Ziehl-Neelsen staining (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 an acid-fast Gram-positive bacterium due to their lack of an outer cell membrane.^[1] 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.

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.^[4] The reason for the acid-fast staining is because of its thick waxy cell wall.^[5] 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 choosen (e.g. intestinal tuberculosis-stool).

See also

• Philip D'Arcy Hart

References