Mycobacterial Diseases

Caseous necrosis of tuberculosis

This walled-off, friable, cheesy nodule in the subapical region (a Simon focus) develops from organisms spread hematogenously from the initial focus of infection in the lower half of the lung. Reactivation of this lesion would probably destroy the capsule, and the caseous material would be expectorated leaving a cavity.

Tuberculous pleuritis

These non-necrotizing granulomas can be mistaken for granulomas of sarcoidosis. Note the Langhans' giant cells, the epithelioid cells, and the lymphocytes. No organisms were found histologically. At the same time, the patient had caseating granulomas in a cervical lymph node, which showed M. tuberculosis on culture.

Necrotizing granuloma

This necrotizing granuloma shows palisading of epithelioid histiocytes at the margin of the necrosis; i.e., the nuclei are lined up perpendicular to the necrotic center. Giant cells cannot be seen at this magnification. On AFB stains, organisms are found mainly in the zone of necrosis.

Healed granuloma

This healed, fibrotic granuloma shows calcification (blue circle). If active inflammation including giant cells and necrosis are absent, as shown here, and cultures are negative, treatment is usually not indicated.

Mycobacterium tuberculosis

An acid fast stain (Ziehl-Neelsen) shows numerous bacilli. In many cases, only a few organisms can be found after prolonged search in areas of necrosis. Organisms are rarely found in the cellular areas.

See also Case 32

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