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. The M. tuberculosis genome was sequenced ^, in 1999.

M. tuberculosis is an obligate aerobe (Gram positive mycobacterium). 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 classifed as such due to their lack of an outer cell membrane. M. tuberculosis divides every 15 to 20 hours—extremely slowly 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 that can withstand weak disinfectants and can survive in a dry state for weeks but can grow only within a host organism.

M. tuberculosis 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" (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. tuberculosis bacteria are difficult to grow. Lowenstein-Jensen media have traditionally been used for this purpose, but faster results can now be obtained using Middlebrook media.

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