The Actinomyces group includes several species that cause actinomycosis, of which Actinomyces israelii and Actinomyces gerencseriae are the ones most commonly encountered. Several new, recently described species that are not associated with actinomycosis have been associated with infections of the groin, urogenital area, breast, and axilla and postoperative infections of the mandible, eye, and head and neck. Some species have also been implicated in cases of endocarditis, particularly among substance abusers. These newly described species are aerotolerant and form small, nondescript colonies that are probably frequently overlooked as contaminants. On Gram stain, they vary considerably in length; they may be short and club shaped or long, thin, beaded filaments. They may be branched or unbranched. Because they often grow slowly, prolonged incubation of the culture may be necessary before laboratory confirmation of the clinical diagnosis of actinomycosis can be made.

Some strains produce colonies on agar that resemble molar teeth. Some Actinomyces species are oxygen tolerant (aerotolerant) and grow in the presence of air; these strains may be confused with Corynebacterium species (diphtheroids). Actinomycosis is a chronic suppurative and granulomatous infection that produces pyogenic lesions with interconnecting sinus tracts that contain granules composed of microcolonies of the bacteria embedded in tissue elements (Figure 1). Infection is initiated by trauma that introduces these endogenous bacteria into the mucosa. The organisms grow in an anaerobic niche, induce a mixed inflammatory response, and spread with the formation of sinuses, which contain the granules and may drain to the surface. The infection causes swelling and may spread to neighboring organs, including the bones.

Fig1. Actinomyces species. A: Colony of Actinomyces species after 72 hours growth on brain–heart infusion agar, which usually yields colonies about 2 mm in diameter; they are often termed “molar tooth” colonies. (Courtesy of CDC Public Health Image Library, L Georg.) B: Granule of Actinomyces species in tissue with Brown and Breen stain. Original magnification ×400. Filaments of the branching bacilli are visible at the periphery of the granule. Such granules are commonly called “sulfur granules” because of their unstained gross yellow color. (Courtesy of CDC Public Health Image Library.) C: Actinomyces naeslundii in a brain abscess stained with methylamine silver stain. Branching bacilli are visible. Original magnification ×1000. (Courtesy of CDC Public Health Image Library, L Georg.)

Based on the site of involvement, the three common forms are cervicofacial, thoracic, and abdominal actinomycosis. Cervicofacial disease presents as a swollen, erythematosus process in the jaw area (known as “lumpy jaw”). With progression, the mass becomes fluctuant, producing draining fistulas. The disease will extend to contiguous tissue, bone, and lymph nodes of the head and neck. The symptoms of thoracic actinomycosis resemble those of a subacute pulmonary infection and include a mild fever, cough, and purulent sputum. Eventually, lung tissue is destroyed, sinus tracts may erupt through to the chest wall, and invasion of the ribs may occur. Abdominal actinomycosis often follows a ruptured appendix or an ulcer. In the peritoneal cavity, the pathology is the same, but any of several organs may be involved. Genital actinomycosis is a rare occurrence in women that results from colonization of an intrauterine device with subsequent invasion.

Diagnosis can be made by examining pus from draining sinuses, sputum, or specimens of tissue for the presence of sulfur granules. The granules are hard, lobulated, and composed of tissue and bacterial filaments, which are club shaped at the periphery. Specimens should be cultured anaerobically on appropriate media. Treatment requires prolonged administration of penicillin (6–12 months). Clindamycin or erythromycin is effective in penicillin-allergic patients. Surgical excision and drainage may be required.

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