Which organism is a gram-negative bacillus among the listed choices?

Which organism is a gram-negative bacillus among the listed choices?

• A. Eubacterium lentum
• B. Bifidobacterium dentium
• C. Propionibacterium acnes
• D. Porphyromonas spp

Answer: (D)

Gram-negative bacilli have a thin peptidoglycan layer and an outer membrane, which makes them stain pink with the Gram stain. Porphyromonas species are Gram-negative, anaerobic, non-spore-forming rods that are common in the oral cavity and associated with periodontal disease. This combination—Gram-negative and rod-shaped—fits the description, making Porphyromonas spp the correct choice. The other organisms listed are Gram-positive anaerobic rods (Eubacterium lentum, Propionibacterium acnes, and Bifidobacterium dentium), so they do not match the Gram-negative bacillus criterion.

Gram-negative bacilli have a thin peptidoglycan layer and an outer membrane, which makes them stain pink with the Gram stain. Porphyromonas species are Gram-negative, anaerobic, non-spore-forming rods that are common in the oral cavity and associated with periodontal disease. This combination—Gram-negative and rod-shaped—fits the description, making Porphyromonas spp the correct choice. The other organisms listed are Gram-positive anaerobic rods (Eubacterium lentum, Propionibacterium acnes, and Bifidobacterium dentium), so they do not match the Gram-negative bacillus criterion.