Opportunistic pneumonia: a clinicopathological study of five cases caused by an unidentified acid-fast bacterium.

Five patients had opportunistic pulmonary infection caused by acid-fast bacilli, unusual clinical presentations and a unique pathological picture. Clinically, these cases mimicked septic pulmonary emboli or bacterial pneumonia. The infection was temporally related to high-dose corticosteroid therapy, given for renal-transplant rejection in four patients and for therapy of lymphocytic lymphoma in one. Histologic sections of lung-biopsy or autopsy material showed an acute suppurative pneumonia with dense alveolar infiltration by neutrophils, without granuloma formation or caseous necrosis. Predominantly intracellular acid-fast bacilli were present. The organism failed to grow in culture on routine bacterial, fungal and mycobacterial mediums. This unusual and possibly new acid-fast organism is a probable cause of suppurative pneumonia in impaired hosts receiving corticosteroid therapy.