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Nocardiosis results from infection with bacteria in the genus Nocardia, saprophytic aerobic actinomycetes that commonly reside in soil worldwide and contribute to the decay of organic matter. Nocardiae are relatively inactive in standard biochemical tests, and speciation with traditional biochemical methods is difficult. Since the year 2000, molecular phylogenetic techniques, primarily based on 16S rRNA gene sequences, have identified more than 100 Nocardia species, many of which are implicated in human disease.

In the past, the majority of isolates associated with pneumonia and systemic disease were identified biochemically as Nocardia asteroides, but the lineage of the type strain was muddled and most human isolates in fact belong to other species. Nine species or species complexes are commonly associated with human disease (Table 169-1). Most systemic disease involves N. cyriacigeorgica, N. farcinica, N. pseudobrasiliensis, and species in the N. transvalensis and N. nova complexes. N. 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, N. otitidiscaviarum, N. transvalensis complex strains, or other actinomycetes. N. asteroides sensu stricto is rarely associated with human disease. However, most clinical laboratories cannot speciate isolates accurately and may identify them simply as N. asteroides or Nocardia species.

TABLE 169-1Nocardia Species Most Commonly Associated with Human Disease and Their In Vitro Susceptibility Patterns

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