The majority of gram-positive, nonspore-forming, anaerobic bacilli isolated from clinical material are most likely which species?

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

Answer: (D)

This item hinges on recognizing the most common gram-positive, nonspore-forming, anaerobic rod recovered from clinical material. Cutibacterium acnes is a regular inhabitant of human skin, especially in sebaceous areas, and fits all three traits: it is a gram-positive, nonspore-forming bacillus that grows under anaerobic conditions. Because skin flora frequently contaminates specimens and because C. acnes can be involved in deeper infections (like prosthetic devices), it is by far the most frequently encountered organism meeting those characteristics in routine clinical isolates. The other organisms listed either are not gram-positive anaerobic bacilli (for example, Capnocytophaga ochracea is gram-negative) or are more specific to particular niches (oral cavity or gut) and thus less likely to be the predominant isolate across a broad range of clinical materials.