Mycobacterium tuberculosis Complex , Mycobacterium tuberculosis, M. bovis
Clinical Manifestations
Tuberculosis primarily affects the lower respiratory system and is characterized by a chronic productive cough, low-grade fever, night sweats, and weight loss.
Structure
Mycobacteria are slender, curved rods that are acid fast and resistant to acids, alkalis, and dehydration. The cell wall contains complex waxes and glycolipids. Multiplication on enriched media is very slow, with doubling times of 18 to 24 hours; clinical isolates may require 4 to 6 weeks to grow.
Classification and Antigenic Types
On the basis of growth rate, catalase and niacin production, and pigmentation in light or dark, mycobacteria are classified into members of the Mycobacterium tuberculosis complex (M tuberculosis, M bovis, M africanum, M microtii) and nontuberculous species. Gene probe technology now facilitates this distinction.
Pathogenesis
Tuberculous mycobacteria enter the alveoli by airborne transmission. They resist destruction by alveolar macrophages and multiply, forming the primary lesion or tubercle; they then spread to regional lymph nodes, enter the circulation, and reseed the lungs. Tissue destruction results from cell-mediated hypersensitivity.
Host Defenses
Susceptibility is influenced by genetic and ethnic factors. Acquired resistance is mediated by T lymphocytes, which lyse infected macrophages directly or activate them via soluble mediators (e.g., gamma interferon) to destroy intracellular bacilli; antibodies play no protective role.
Epidemiology
M tuberculosis is contagious, but only 5–10 percent of infected normal individuals develop active disease. Tuberculosis is most common among the elderly, poor, malnourished, or immunocompromised, especially persons infected with human immunodeficiency virus (HIV). Persistent infection may reactivate after decades owing to deterioration of immune status; exogenous reinfection also occurs.Copyright © 1996
The University of Texas Medical Branch at Galveston