MICROBIOLOGY. Actinomycosis is an uncommon, indolent, progressively invasive bacterial infection caused by organisms of the Actinomyces species, which are non-motile, filamentous, branching, gram-positive and predominantly anaerobic bacteria.

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Actinomycosis definition is - infection with or disease caused by actinomyces; especially : a chronic disease of cattle, swine, and humans characterized by hard granulomatous masses usually in the mouth and jaw.