Mycobacterial disease, immunosuppression, and acquired immunodeficiency syndrome.

The mycobacteria are an important group of acid-fast pathogens ranging from obligate intracellular parasites such as Mycobacterium leprae to environmental species such as M. gordonae and M. fortuitum. The latter may behave as opportunistic human pathogens if the host defenses have been depleted in some manner. The number and severity of such infections have increased markedly with the emergence of the acquired immunodeficiency syndrome (AIDS) epidemic. These nontuberculous mycobacteria tend to be less virulent for humans than M. tuberculosis, usually giving rise to self-limiting infections involving the cervical and mesenteric lymph nodes of young children. However, the more virulent serovars of M. avium complex can colonize the bronchial and intestinal mucosal surfaces of healthy individuals, becoming virtual members of the commensal gut microflora and thus giving rise to low levels of skin hypersensitivity to tuberculins prepared from M. avium and M. intracellulare. Systemic disease develops when the normal T-cell-mediated defenses become depleted as a result of old age, cancer chemotherapy, or infection with human immunodeficiency virus. As many as 50% of human immunodeficiency virus antibody-positive individuals develop mycobacterial infections at some time during their disease. Most isolates of M. avium complex from AIDS patients fall into serotypes 4 and 8. The presence of these drug-resistant mycobacteria in the lungs of the AIDS patient makes their effective clinical treatment virtually impossible. More effective chemotherapeutic, prophylactic, and immunotherapeutic reagents are urgently needed to treat this rapidly increasing patient population.