Mycobacteria
Mycobacterial infections include tuberculosis, a historic and very important infection found worldwide, and atypical mycobacterial infections. Tuberculosis is the most important infectious disease in the developing countries, transmitted by inhalation of infected droplets. Phagocytosis of the bacteria by alveolar macrophages is followed by development of T-cell mediated immunity. In an immunocompromised host, disseminated infection may develop with primary infection. Residual infection may persist as a Ghon complex. Reactivation of Mycobacterium tuberculosis infection is seen as caseating granulomas in lung apices, and represents delayed hypersensitivity reaction.
Part 1 Tuberculosis
Sajid A. Haque
Abida K. Haque
Pulmonary tuberculosis is infection caused by Mycobacterium tuberculosis and is a classic example of granulomatous inflammation. There has been an increase in tuberculosis, and atypical mycobacterial infections, the latter almost exclusively in patients with acquired immunodeficiency syndrome (AIDS). Primary pulmonary tuberculosis caused by M. tuberculosis is often asymptomatic and results in formation of the Ghon complex in the lower lobes. Reactivation of pulmonary M. tuberculosis may manifest as acute necrotizing pneumonia, cavitation, or fibrosis with bronchiectasis.
Histologic Features
The hallmark of infection with M. tuberculosis is the granuloma with central caseous necrosis surrounded by lymphocytes, macrophages, and Langerhans-type giant cells (nuclei arranged in horseshoe shape); the caseous areas may have microcalcifications.
Mycobacteria tend to be at the periphery of the granuloma and not in the caseous area.
M. tuberculosis is a slender, curved, sometimes beaded rod 1.0 to 4.0 μm in length and 0.5 μm in diameter; the bacilli stain intense red with Ziehl-Nielsen or modified Kinyoun stain.
Fluorescent stain such as auramine or rhodamine may be used (more sensitive than Ziehl-Nielsen) for screening.
The bacilli are gram positive, periodic acid-Schiff (PAS) positive, and Gomori methenamine silver (GMS) positive. GMS stain is useful to demonstrate the bacilli after they have lost their acid fastness due to treatment/degeneration.
Healed tuberculous lesions appear as fibrocalcific nodules.
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