Atypical Mycobacteria

Atypical Mycobacteria
Several species of mycobacteria are characterized as atypical, because they differ in certain respects from the prototype, M. tuberculosis. For example, atypical mycobacteria are widespread in the environment and are not pathogenic for guinea pigs, whereas M. tuberculosis is found only in humans and is highly pathogenic for guinea pigs.
The atypical mycobacteria are classified into four groups according to their rate of growth and whether they produce pigment under certain conditions . Group I organisms produce a yellow-orange–pigmented colony only when exposed to light (photochromogens), whereas group II organisms produce the pigment chiefly in the dark (scotochromogens). Group III mycobacteria produce little or no yellow-orange pigment, irrespective of the presence or absence of light (nonchromogens). In contrast to the organisms in the previous three groups, which grow slowly, group IV organisms grow rapidly, producing colonies in fewer than 7 days.
Group I (Photochromogens)
M. kansasii causes lung disease clinically resembling tuberculosis. Because it is antigenically similar to M. tuberculosis, patients are frequently tuberculin skin test–positive. Its habitat in the environment is unknown, but infections by this organism are localized to the midwestern states and Texas. It is susceptible to the standard antituberculosis drugs.
Group II (Scotochromogens)
M. scrofulaceum causes scrofula, a granulomatous cervical adenitis, usually in children. (M. tuberculosis also causes scrofula.) The organism enters through the oropharynx and infects the draining lymph nodes. Its natural habitat is environmental water sources, but it has also been isolated as a saprophyte from the human respiratory tract. Scrofula can often be cured by surgical excision of the affected lymph nodes.
Group III (Nonchromogens)
M. avium-intracellulare complex (MAI, MAC) is composed of two species, M. avium and M. intracellulare, that are very difficult to distinguish from each other by standard laboratory tests. They cause pulmonary disease clinically indistinguishable from tuberculosis, primarily in immunocompromised patients such as those with AIDS who have CD4 cell counts of less than 200/ L. MAI is the most common bacterial cause of disease in AIDS patients. The organisms are widespread in the environment, including water and soil, particularly in the southeastern United States. They are highly resistant to antituberculosis drugs, and as many as six drugs in combination are frequently required for adequate treatment. Current drugs of choice are clarithromycin plus one or more of the following: ethambutol, rifabutin, or ciprofloxacin. Clarithromycin is currently recommended for preventing disease in AIDS patients.
Group IV (Rapidly Growing Mycobacteria)
Mycobacterium fortuitum-chelonei complex is composed of two similar species, M. fortuitum and M. chelonei. They are saprophytes, found chiefly in soil and water, and rarely cause human disease. Infections occur chiefly in two populations: (1) immunocompromised patients and (2) individuals with prosthetic hip joints and indwelling catheters. Skin and soft tissue infections occur at the site of puncture wounds. They are often resistant to antituberculosis therapy, and therapy with multiple drugs in combination plus surgical excision may be required for effective treatment. Current drugs of choice are amikacin plus doxycycline.