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Mycobacterium tuberculosis was first described by Robert Koch in 1882. There are two main species: M. tuberculosis and M. bovis, Bacille Calmette-Guerin (BCG) being a form of the latter. M. tuberculosis is responsible for almost all cases of human tuberculosis. M. bovis is now exceptionally rare in technically advanced nations due to pasteurisation of milk. Tubercle bacilli are long curved rods. They do not stain easily with gram stain because of the high cell lipid content. However, once stained, for example by Ziehl-Nielsen stain, they strongly resist decolourisation with acid or alcohol (and are known as acid alcohol-fast bacilli - AAFB or AFB). The tuberculous mycobacteria multiply slowly (only once every 20 hours or so) in contrast to the rapid multiplication of pyogenic bacteria.
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Bacteriology