Actinomyces israelii is part of the indigenous flora of which body site?

• A. Skin
• B. Gut
• C. Vagina
• D. Mouth

Answer: (D)

Actinomyces israelii is a Gram-positive, anaerobic filamentous bacterium that normally lives in the mouth as part of the indigenous flora. The oral cavity contains many niches with low oxygen levels, such as dental plaque and periodontal pockets, where this organism thrives as a commensal organism. Because it is a regular resident of the mouth, infections with Actinomyces israelii most often arise when mucosal barriers are breached, leading to actinomycosis in the jaw or surrounding tissues.

While bacteria do inhabit skin, the gut, and the vagina, Actinomyces israelii is classically associated with the oral cavity rather than these sites. The mouth’s ecological environment supports its growth and its role in oral microbial communities, which is why the mouth is the correct indigenous site for this organism.