Tuberculosis and Other Mycobacteria

Mycobacteria are a group of microbial pathogens associated with tuberculosis (TB), one of the world's most prevalent human disease and several nontuberculous diseases in humans. Another major human infection caused by this genus is leprosy. TB is predominantly a pulmonary disease infecting lungs but extrapulmonary TB is also prevalent and includes lymphatic, pleural, meningeal, pericardial, skeletal, gastrointestinal, genitourinary, or miliary form. The genus Mycobacterium comprises of about 130 species that are groupable into two major categories: (A) the Mycobacterium tuberculosis complex: it comprises of two obligate pathogenic species, namely M. tuberculosis (the agent of tuberculosis) and Mycobacterium leprae (the agent of leprosy). This complex contains four other species of mycobacteria that also cause TB viz., Mycobacterium bovis, Mycobacterium africanum, Mycobacterium microti, and Mycobacterium canetti. (B) The nontuberculous mycobacteria (also called atypical mycobacteria or environmental mycobacteria): this group comprises of a large number of saprophytic species that live freely in the environment such as in soils, water, and other organic matrices. These organisms may be inhaled via dust particles or ingested via drinking water or food and produce various syndromes. Nontuberculous mycobacteria (NTM) are increasingly being recognized to cause human infections, frequently in immunosuppressed individuals such as those who have organ transplants, individuals being treated for leukemia or cancer, and patients suffering from AIDS. The range of infections caused by NTM species is very broad and includes pulmonary infections (symptoms often indistinguishable from TB), cervical lymphadenitis, skin infections, bone and soft tissue infections, and nosocomial infections. An occupational disease in machinists, designated hypersensitivity pneumonitis (HP) has also been associated with NTM species (Mycobacterium immunogenum and Mycobacterium chelonae) that have the ability to colonize metalworking fluids in occupational environments. Although this chapter focuses primarily on tuberculosis, nontuberculous mycobacteria that are associated with human disease are also discussed. It includes discussions on taxonomy, growth requirements, as well as the morphological characteristics, physiology, pathogenicity, and the metabolic activity of these organisms. Keywords: epidemiology; exposure; Mycobacterium tuberculosis; nontuberculous mycobacteria; prevention; risk factors; treatment; tuberculosis