Mycobacterium tuberculosis
Mycobacterium tuberculosis is the bacteria that causes most cases of tuberculosis. Its genome has been sequenced.It is a Gram-positive aerobic 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.