Today, most cases of tuberculosis are caused by Mycobacterium tuberculosis, although in the years before widespread milk pasteurization, the cattle-infecting species Mycobacterium bovis was a common cause. Mycobacterium tuberculosis, commonly called the tubercle bacillus, is a slender acid-fast, rod-shaped bacterium (figure 23.17). The organismis a strict aerobe that grows very slowly, with a generation time of 12 hours or more. This slow growth makes it difficult to diagnose tuberculosis quickly. Although easily killed by pasteurization, the tubercle bacillus is unusually resistant to drying, disinfectants, and strong acids and alkali. The bacterium has a Gram-positive cell wall, but bound covalently to the cell wall peptidoglycan is a thick layer composed of complex glycolipids, and other lipids compose much of the bacterial surface. Up to 60% of the dry weight of the M. tuberculosis cell wall consists of lipids, a much higher percentage than that in most other bacteria. The lipid-containing cell wall is largely responsible for its acid-fast staining, resistance to drying and disinfectants, and its pathogenicity. ■ pasteurization, p. 114 ■ lipids, p. 33
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