Mycobacterium tuberculosis
Mycobacterium tuberculosis is the single bacterium responsible for tuberculosis. If most bacteria are like fast-living weeds that sprout and spread within hours, this one is a slow, armoured survivor: it divides only about once a day and wears a thick, waxy coat that shields it from drying out, from many disinfectants and from the body's defences.
That waxy wall, rich in fatty molecules called mycolic acids, gives the bacterium its defining laboratory trait — it is acid-fast, meaning it holds onto a red stain even when washed with acid-alcohol. The same coat lets it hide and survive for years inside the very immune cells (macrophages) sent to destroy it, which is why infection can lie dormant and why treatment must be long.
Its slow growth has practical consequences. Cultured in the lab, it may take weeks to form visible colonies, delaying definitive diagnosis and drug-susceptibility results. Because no single antibiotic can reliably clear it, treatment always uses several drugs together over months, both to kill the bacteria and to prevent resistant strains from emerging.
Robert Koch identified M. tuberculosis as the cause of TB in 1882 — a landmark that turned a feared, mysterious illness into an infection with a known, treatable culprit.
The discovery that named the bacterium “Koch's bacillus.”
It belongs to the “Mycobacterium tuberculosis complex,” a group of closely related species (including M. bovis) that cause TB in humans and animals.