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The mycobacteria are rod-shaped, aerobic bacteria that do not form spores. The cell wall contains peptidoglycolipids, mycolic acids, and other fatty acids and waxes; many of these compounds are responsible for the various characteristics of mycobacteria (eg, slow growth, acid fastness, resistance to detergents, and common antibiotics). Because of their high lipid content in their cell walls, mycobacteria do not stain well with common aniline dyes, including the regular Gram-stain method. Special staining procedures, using phenol-based, arylmethane dyes (eg, carbolfuchsin), are used instead to stain mycobacteria. Due to the high content of mycolic acids in their cell wall, mycobacteria retain these dyes even after exposure to strong alcohol-acid or mineral-acid solutions. Therefore, mycobacteria are described as “acid-fast” organisms. There are more than 200 Mycobacterium species, including many that are saprophytes. The mycobacteria that infect humans are listed in Table 23-1. By far the most common mycobacterial human pathogens, worldwide, are M. tuberculosis, M. leprae, and M. ulcerans. Mycobacterium tuberculosis causes tuberculosis and is a very important pathogen of humans. Mycobacterium leprae causes leprosy (Hansen’s disease), which is a chronic granulomatous and debilitating disease. Mycobacterium ulcerans causes necrotizing skin and soft tissue infections, which are characterized by formation of ulcers that progressively enlarge over time, if left untreated. In Africa, the disease is known as Buruli ulcer, whereas in Australia, the disease is called Bairnsdale ulcer. Mycobacterium avium-intracellulare (M. avium complex, or MAC) and other nontuberculous mycobacteria (NTM) frequently infect patients with AIDS are opportunistic pathogens in other immunocompromised persons, and occasionally cause disease in patients with normal immune systems.
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