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Actinomycetes are a family of bacteria that form long, branching filaments that resemble the hyphae of fungi (Figure 22–1). They are gram-positive, but some (such as Nocardia asteroides) are also weakly acid-fast rods (Table 22–1).


Nocardia asteroides—Gram stain. Arrow points to area of filaments of gram-positive rods. (Source: Dr. Thomas F. Sellers, Public Health Image Library, Centers for Disease Control and Prevention.)

TABLE 22–1Actinomycetes

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on Introduction.



Actinomyces israelii causes actinomycosis.

Important Properties & Pathogenesis

Actinomyces israelii is an anaerobe that forms part of the normal flora of the oral cavity. After local trauma such as a broken jaw or dental extraction, it may invade tissues, forming filaments surrounded by areas of inflammation.

Clinical Findings

The typical lesion of actinomycosis appears as a hard, nontender swelling that develops slowly and eventually drains pus through sinus tracts (Figure 22–2). Hard, yellow granules (sulfur granules) composed of a mass of filaments are formed in pus.


Actinomycosis. Note inflamed lesion with small sinus tract opening anterior to right ear. Yellowish “sulfur granule” can be seen at the opening. (Source: Dr. Thomas F. Sellers, Public Health Image Library, Centers for Disease Control and Prevention.)

In about 50% of cases, the initial lesion involves the face and neck; in the rest, the chest or abdomen is the site. Pelvic actinomycosis can occur in women who have retained an intrauterine device for a long period of time. Actinomyces israelii and Arachnia species are the most common causes of actinomycosis in humans. The disease is not communicable.

Laboratory Diagnosis

Diagnosis in the laboratory is made by (1) seeing gram-positive branching rods, especially in the presence of sulfur granules and (2) seeing growth when pus or tissue specimens are cultured under anaerobic conditions. Organisms can be identified by immunofluorescence. Note that in contrast to N. asteroides (see later), Actinomyces is not acid-fast. There are no serologic ...

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