N. asteroides causes nocardiosis.
Important Properties & Pathogenesis
Nocardia species are aerobes and are found in the environment, particularly in the soil. In immunocompromised individuals, they can produce lung infection and may disseminate. In tissues, Nocardia species are thin, branching filaments that are gram-positive on Gram stain.
Many isolates of N. asteroides are weakly acid-fast (i.e., the staining process uses a weaker solution of hydrochloric acid to decolorize than that used in the stain for mycobacteria). If the regular-strength acid is used, N. asteroides will decolorize. N. asteroides is weakly acid-fast because it contains a layer of mycolic acid in its cell wall, similar to, but less than, the mycobacteria.
N. asteroides typically causes pneumonia, lung abscess with cavity formation, lung nodules, or empyema. From the lung, the organism can spread to various organs, notably the brain, where it causes brain abscess. Disease occurs most often in immunocompromised individuals, especially those with reduced cell-mediated immunity. Nocardia brasiliensis, a different species of Nocardia, causes skin infections in the southern regions of the United States and mycetoma, usually in tropical regions.
Diagnosis in the laboratory involves (1) seeing branching rods or filaments that are gram-positive (see Figure 22–1) or weakly acid-fast in an acid-fast stain and (2) seeing aerobic growth on bacteriologic media in a few days.
Treatment is with trimethoprim-sulfamethoxazole. Surgical drainage may also be needed. Occasional drug resistance occurs. No vaccine or prophylactic drug is available.