Which bacterium is the causative agent of tuberculosis?

The bacterium that causes tuberculosis is Mycobacterium tuberculosis.

Often abbreviated M. tuberculosis, this pathogen is also known as Koch's bacillus because Robert Koch identified it in 1882. It is a small, rod-shaped, nonmotile bacterium that primarily infects the lungs, although tuberculosis can spread to organs such as the kidneys, bones, and brain.

Its unusual cell wall contains waxy lipids called mycolic acids. This coating makes the organism difficult to identify with an ordinary Gram stain, so laboratories commonly use acid-fast stains such as Ziehl–Neelsen staining, along with culture or molecular tests. The bacterium grows very slowly: it may divide only once every 18 to 24 hours.

Tuberculosis spreads mainly through airborne particles released when someone with active pulmonary disease coughs, sneezes, speaks, or sings. It is not normally spread by handshakes, toilet seats, or sharing food. Infection may remain latent for years without symptoms before becoming active, which is one reason TB control requires testing, treatment, and contact investigation.