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Nocardia, a genus of saprophytic aerobic actinomycetes that are common worldwide, resides in soil, contributing to the decay of organic matter. Nearly 100 species have been identified, mostly on the basis of 16S rRNA gene sequences. Nocardiae are relatively inactive in standard biochemical tests, and speciation is difficult without molecular phylogenetic techniques. Historically, the majority of isolates associated with pneumonia and systemic disease were identified as Nocardia asteroides, but the lineage of the type strain was muddled, and it is now clear that human disease is associated with several species. Most clinical laboratories cannot speciate isolates accurately and may identify them simply as N. asteroides or Nocardia species.

Nine species or species complexes are commonly associated with human disease (Table 199-1). Most systemic disease involves Nocardia cyriacigeorgica, Nocardia farcinica, Nocardia pseudobrasiliensis, and species in the Nocardia transvalensis and Nocardia nova complexes. Nocardia brasiliensis is usually associated with disease limited to the skin. Actinomycetoma—an indolent, slowly progressive disease of skin and underlying tissues with nodular swellings and draining sinuses—is often associated with N. brasiliensis, Nocardia otitidiscaviarum, N. transvalensis complex strains, or other actinomycetes.

TABLE 199-1Nocardia Species Most Commonly Associated with Human Disease and Their In Vitro Susceptibility Patterns


image Nocardiosis occurs worldwide. The annual incidence, estimated on three continents (North America, Europe, and Australia), is ~0.375 cases per 100,000 persons and may be increasing. The disease is more common among adults than among children and more common among males than among females. Nearly all cases are sporadic, but ...

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