Which organism exhibits a curved appearance on a fresh Gram stain?

• A. Fusobacterium nucleatum
• B. Actinomyces israelii
• C. Clostridium septicum
• D. Propionibacterium acnes

Answer: (A)

Curved, slender Gram-negative rods are a hallmark of Fusobacterium species, so Fusobacterium nucleatum often appears as curved (fusiform) rods on a fresh Gram stain. This shape comes from the organism’s natural cell form and its thin, elongated cells, which stain Gram-negative and tend to bend rather than remain straight. In the same context, Actinomyces israelii shows Gram-positive, branching filamentous forms; Clostridium septicum appears as Gram-positive straight rods; and Propionibacterium acnes presents as small Gram-positive rods that are often pleomorphic. The distinctive curved morphology on the Gram stain is most characteristic of Fusobacterium, guiding identification in mixed or anaerobic infections.