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TAXONOMY: Actinobacteria -> Actinobacteria -> Corynebacteriales -> Mycobacteriaceae -> Mycobacterium -> Mycobacterium tuberculosis complex -> Mycobacterium tuberculosis
Mycobacterium tuberculosis is an obligate pathogenic bacterial species in the family Mycobacteriaceae and the causative agent of most cases of tuberculosis. First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on its cell surface (primarily due to the presence of mycolic acid), which makes the cells impervious to Gram staining; M. tuberculosis can appear gram-negative and gram-positive in clinical settings. The Ziehl-Neelsen stain, or acid-fast stain, is used instead. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. The most frequently used diagnostic methods for tuberculosis are the tuberculin skin test, acid-fast stain, and chest radiographs. Humans are the only known reservoirs of M. tuberculosis. When in the lungs, M. tuberculosis is taken up by alveolar macrophages, but they are unable to digest and eradicate the bacterium. Its cell wall prevents the fusion of the phagosome with the lysosome, which contains a host of antimycobacterial factors. Specifically, M. tuberculosis blocks the bridging molecule, early endosomal autoantigen 1 (EEA1); however, this blockade does not prevent fusion of vesicles filled with nutrients. Consequently, the bacteria multiply unchecked within the macrophage. The bacteria also carry the UreC gene, which prevents acidification of the phagosome. In addition, production of the diterpene isotuberculosinol prevents maturation of the phagosome. The bacteria also evade macrophage-killing by neutralizing reactive nitrogen intermediates.