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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Actinomyces and Nocardia

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Actinomycosis

Actinomycosis is a chronic inflammatory condition originating in the tissues adja- cent to mucosal surfaces.

ACTINOMYCOSIS

Actinomycosis is a chronic inflammatory condition originating in the tissues adja- cent to mucosal surfaces. The lesions follow a slow burrowing course with consid- erable induration and draining sinuses eventually opening through the skin. The exact nature depends on the organs and structures involved.

Actinomyces are normal inhabitants of some areas of the gastrointestinal tract of humansand animals from the oropharynx to the lower bowel. These species are highly adapted to mucosal surfaces and do not produce disease unless they transgress the epithelial barrier under conditions that produce a sufficiently low oxygen tension for their multiplication. Such conditions usually involve mechanical disruption of the mucosa with necrosis of deeper, normally sterile tissues (eg, following tooth extraction). Once initiated, growth occurs in microcolonies in the tissues and extends without regard to anatomic boundaries. The lesion is composed of inflammatory sinuses, which ultimately discharge to the sur-face. As the lesion enlarges, it becomes firm and indurated. Sulfur granules are present within the pus but are not numerous. Free Actinomyces or small branching units are rarely seen, although contaminating Gram-negative rods are common. As with other anaerobic infections  most cases are polymicrobial involving other flora from the mucosal site of origin.

Human cases provide little evidence of immunity to Actinomyces. Once established, infections typically become chronic and resolve only with the aid of antimicrobic therapy. Antibodies can be detected in the course of infection but seem to reflect the antigenic stimulation of the ongoing infection rather than immunity. Infections with Actinomyces are endogenous, and case-to-case transmission does not appear to occur.


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