The Pathogenesis of Nocardia

Nocardiae are gram-positive, partially acid-fast, filamentous bacteria that grow by apical extension, forming elongated cells with lateral branching. Most species of Nocardia have been recovered from soil, plant material, and water in most regions of the world. Diseases in humans caused by nocardiae may be divided into at least six general categories based on the route of infection, site of disease, and subsequent pathological responses. They are pulmonary nocardiosis, extrapulmonary nocardiosis, systemic nocardiosis, central nervous system (CNS) nocardiosis, cutaneous, subcutaneous, and lymphocutaneous nocardiosis. A mycetoma is a chronic, progressive, pyogranulomatous disease that usually develops at the site of a localized injury such as a thorn prick. Nocardia asteroides, Nocardia farcinica, and Nocardia otitidiscaviarum have caused significant outbreaks worldwide in dairy cattle, usually in the form of mastitis. Nocardiae are facultatively intracellular pathogens that resist the microbicidal activities of polymorphonuclear neutrophils (PMNs). Concentrated culture filtrates from GUH-2 grown in a chemically defined medium also induced apoptosis, as well as dopamine depletion. Cells of N. asteroides GUH-2 in the log phase of growth adhered by way of the filament tip to the surface of both pulmonary epithelial cells (Clara cells) and brain capillary endothelial cells in mice. The differential and selective adherence displayed by nocardiae both in vitro and in vivo suggested distinct multiple ligands for host cells on the nocardial surface.