Which bacterium is the usual cause of tuberculosis, one of history’s deadliest infectious diseases?

Mycobacterium tuberculosis is the usual cause of tuberculosis, one of history’s deadliest infectious diseases.

M. tuberculosis is a slender, nonmotile, rod-shaped bacterium with a lipid-rich cell envelope. This envelope makes the organism acid-fast: it retains particular laboratory stains even after treatment with acid-alcohol. The same unusual structure contributes to its resistance to drying and helps it persist inside host immune cells.

Tuberculosis most often affects the lungs and spreads through airborne particles released when a person with active pulmonary disease coughs, speaks, or sneezes. Robert Koch announced the discovery of the tubercle bacillus in 1882, a milestone in medical bacteriology. Infection can remain latent without immediate symptoms and later reactivate.

Mycobacterium leprae causes leprosy, not tuberculosis. Tuberculosis is also distinct from ordinary bacterial pneumonia, although several bacteria can infect the lungs. Diagnosis and treatment require specific testing and prolonged combinations of antibiotics.