The most important causative species is Nocardia asteroides, which has a worldwide distribution. N. brasilensis, confined to Central and South America, is another important cause of human pulmonary infections. It is also found in grasses, straw, and grains. In tissue, it is a gram-positive, delicately branching, filamentous organism. It is usually acid-fast, with weak decolorizing agents, which may be useful in distinction from Actinomyces.
Affected tissues contain numerous yellow-white abscesses with a fibrinous exudate.
Polymorphonuclear leukocytes, plasma cells, lymphocytes and histiocytes are found within the abscesses. Inhibition of phagocytic and lysosomal function, reduction in intracellular lysosomal acid phosphatase, and production of superoxide dismutase and catalase by Nocardia growing within macrophages promotes virulence of the organism. Absence of sulfur granules aids in distinction from actinomycosis. Decreased host resistance in this special population relates to impaired phagocytosis and cell-mediated immune response, as well as an altered humoral response, which may result from either the primary disease or from the immunosuppresive therapy. A high index of suspicion in this population with uncommon disease manifestations will be required to identify and appropriately treat these diseases.