Fusobacterium, illustration. 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. Plaque is a cause of periodontal disease and tooth decay in humans. Fusobacterium ferments carbohydrates to lactic acid. Fusobacterium nucleatum is non-motile, and part of the Bacteroides group of bacteria that are obligate animal parasites.