Which organism listed is a gram-negative bacillus?

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

Answer: (D)

Understanding how Gram stain differentiates bacteria by cell wall structure and shape is key. Gram-negative organisms have a thin peptidoglycan layer plus an outer membrane, so they don’t retain crystal violet and appear pink after counterstaining; when their shape is rod-like, they’re gram-negative bacilli.

Among these organisms, the ones that are Gram-positive are Eubacterium lentum, Bifidobacterium dentium, and Propionibacterium acnes, all described as Gram-positive rods. Porphyromonas spp., on the other hand, are Gram-negative, anaerobic, rod-shaped bacteria commonly found in the oral cavity. That combination—Gram-negative and bacillus shape—fits Porphyromonas spp. best.