Which organism is most commonly associated with acne and is a gram-positive, nonspore-forming anaerobe?

• A. Bifidobacterium dentium
• B. Capnocytophagia ochracea
• C. Eubacterium limosum
• D. Cutibacterium (Propionibacterium) acnes

Answer: (D)

Acne is linked to a gram-positive, nonspore-forming anaerobe that lives in hair follicles and sebaceous glands. The organism most commonly associated with acne is Cutibacterium acnes (formerly Propionibacterium acnes). It resides in the lipid-rich environment of the pilosebaceous unit and thrives without oxygen. Its lipase activity breaks down sebum into free fatty acids, which irritate the follicular walls and provoke an inflammatory response, leading to the pimples, papules, and pustules typical of acne. This bacterium is a normal skin inhabitant, and acne reflects shifts in sebum production and follicular keratinization rather than invasion by a pathogenic organism. The other bacteria listed are primarily associated with oral or gut sites (or are gram-negative), so they don’t fit the same acne-related role as well.