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Microbiology, epidemiology, and pathogenesis of nocardiosis

Denis Spelman, MBBS, FRACP, FRCPA, MPH
Section Editor
Daniel J Sexton, MD
Deputy Editor
Anna R Thorner, MD


Nocardiosis is an uncommon gram-positive bacterial infection caused by aerobic actinomycetes in the genus Nocardia. Nocardia spp have the ability to cause localized or systemic suppurative disease in humans and animals [1-5]. Nocardiosis is typically regarded as an opportunistic infection, but approximately one-third of infected patients are immunocompetent [4]. (See 'Immunocompromise' below.)

Two characteristics of nocardiosis are its ability to disseminate to virtually any organ, particularly the central nervous system, and its tendency to relapse or progress despite appropriate therapy.

The microbiology, taxonomy, epidemiology, and pathogenesis of nocardiosis will be reviewed here. The clinical manifestations, complications, diagnosis, and treatment are discussed separately. (See "Clinical manifestations and diagnosis of nocardiosis" and "Treatment of nocardiosis".)


Actinomycetes are a group of aerobic and anaerobic bacteria in the order Actinomycetales. These organisms are phylogenetically diverse but morphologically similar, exhibiting characteristic filamentous branching with fragmentation into bacillary or coccoid forms [6]. Aerobic actinomyces that cause human and veterinary disease include Nocardia, Gordona, Tsukamurella, Streptomyces, Rhodococcus, Mycobacteria, and Corynebacteria. Anaerobic genera of medical importance include Actinomyces, Arachnia, Rothia, and Bifidobacterium.

Nocardia typically appear as delicate filamentous gram-positive branching rods (picture 1) that appear similar to Actinomyces species. Nocardia can usually be differentiated from Actinomyces by acid-fast staining, as Nocardia typically exhibit varying degrees of acid fastness due to the mycolic acid content of the cell wall (picture 2). Another useful clue is that Nocardia grow under aerobic conditions, whereas Actinomyces grow under anaerobic conditions.

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Literature review current through: Nov 2017. | This topic last updated: Jul 27, 2017.
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