Nocardia species are soilborne bacteria that are aerobic and slow-growing. In culture, they may require 2 to 4 weeks before colonies appear. Nocardia are gram-positive and weakly acid-fast filaments, 0.5 to 1.0 μm in diameter, that branch at right angles (Figure 143.1). Nine nocardial species pathogenic for humans were described between 1888 and 1996: Nocardia farcinica, Nocardia asteroides, Nocardia carnae, Nocardia brasiliensis, Nocardia otitidiscaviarum (formerly Nocardia caviae), Nocardia transvalensis, Nocardia brevicatena, Nocardia nova, and Nocardia pseudobrasiliensis. Since then, with availability of newer molecular techniques, 24 new nocardial species of human significance have been described, and taxonomy of the genus is in a state of flux. For example, N. asteroides sensu stricto is not currently defined in molecular terms, and reports of isolation of N. asteroides have actually represented several nocardial species. Further studies should provide taxonomic clarification and correlation with disease states. Nocardia are opportunistic pathogens; N. brasiliensis is more virulent, affecting normal hosts, and has a range geographically restricted to areas with warmer climates.

Nocardiosis is typically a suppurative infection with multiple abscesses. It is rarely granulomatous and not fibrotic. Acquisition of infection is by the respiratory tract or by traumatic inoculation. Although nocardia are ubiquitous, they rarely colonize the human respiratory tract. Accordingly, treatment should be initiated when nocardia are repeatedly isolated from pulmonary specimens, particularly from an immunocompromised host.