Illustration of a Fusobacterium bacterium. Fusobacterium are Gram-negative anaerobic bacilli (rod-shaped) bacteria, typically long and tapered at both ends (fusiform). Fusobacterium occurs as the main filamentous bacteria in the dental plaque that forms on the surface of teeth. It ferments carbohydrates to lactic acid. Plaque is a cause of periodontal disease and tooth decay in humans. Fusobacterium nucleatum is non-motile and part of the Bacteroides group of bacteria that are obligate animal parasites.