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Mycobacteria include a wide range of species pathogenic for humans and animals. Some, such as M. tuberculosis, occur exclusively in humans under natural conditions. Others, such as Mycobacterium intracellulare, can infect various hosts, including humans, but also exist in the free-living state. Most nonpathogenic species are widely distributed in the environment. Diseases caused by mycobacteria usually develop slowly, follow a chronic course, and elicit a granulomatous response. Infectivity of pathogenic species is quite high, but virulence for healthy humans is low. For example, disease following infec-tion with M. tuberculosis is the exception rather than the rule.
Mycobacteria do not produce classic exotoxins or endotoxins. Disease processes are thought to be the result of two related host responses. The first, a delayed-type hypersen-sitivity (DTH) reaction to mycobacterial proteins, results in the destruction of non-activated macrophages containing multiplying organisms. It is detected by intradermal injections of purified proteins from the mycobacteria. The second, cell-mediated immu-nity (CMI) activates macrophages, enabling them to destroy mycobacteria contained within their cytoplasm. The balance between these two responses determines the pathol-ogy and clinical response to a mycobacterial infection.
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