Mycobacterium tuberculosis is the bacterium that causes most cases of tuberculosis. 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. Its genome has been sequenced.
It is a Gram-positive obligate aerobe mycobacterium that divides every 16-20 hours. This 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 rod-like bacillus which can withstand weak disinfectants and can survive in a dry state for weeks but can only grow within a host organism.
MTB is identified microscopically by its staining characteristics: it retains certain stains after being treated with acidic solution, and is thus classified as an "acid-fast bacillus" or "AFB". In the most common staining technique, the Ziehl-Neelsen stain, AFB are stained a bright red which stands out clearly against a blue background. Acid-fast bacilli can also be visualized by fluorescent microscopy, and by auramine-rhodamine stain. The reason for the acid fast staining seen in mycobacteria is because of its thick waxy cell wall.
This waxy cell wall also is responsible for the typical caseous granuloma formation in tuberculosis. The component responsible is called the cord factor.
M. Tubeculosis bacteria are difficult to grow. The typical culture media is the lowenstein-jensen media. Middlebrook media are used these days for faster culture.
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