What bacterium causes tuberculosis?

The bacterium that causes tuberculosis is Mycobacterium tuberculosis.

Mycobacterium tuberculosis, often abbreviated M. tuberculosis, is a rod-shaped bacterium and the principal human pathogen responsible for tuberculosis, or TB. It most commonly infects the lungs, producing pulmonary TB, but it can also spread to organs such as the lymph nodes, kidneys, bones, and brain.

The bacterium is transmitted through the air when a person with infectious pulmonary or laryngeal TB coughs, speaks, or sneezes. Infection does not always produce immediate illness. In latent TB infection, bacteria remain controlled by the immune system and the person typically has no symptoms or ability to spread the disease. Active TB can develop later if immune defenses weaken.

Robert Koch announced his identification of the tuberculosis bacillus on March 24, 1882. The organism is consequently known as Koch’s bacillus. The tuberculosis complex includes closely related species that can cause TB in humans or animals, but M. tuberculosis is the canonical answer to this question.