Gram-positive bacilli

Small, Gram-positive bacilli that are diptheroid in appearance - i.e. arranged in clusters, "picket fence" or "Chinese characters" - can be presumptively identified as Propionibacterium acnes if the colonies are indole and catalase positive.

This micrograph depicts the Gram-positive bacterium Propionibacterium acnes grown in thioglycollate medium at 48 hours.