Fusobacterium sp. bacteria, computer illustration. Two main species are F. nucleatum and F. necrophorum. Fusobacteria are gram- negative bacilli (rod-shaped bacteria), typically long and tapered at both ends (fusiform). Fusobacterium sp. are obligate animal parasites, forming part of the intestinal flora of a range of organisms from termites to primates, and including humans. F. nucleatum 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. F. necrophorum is responsible for several diseases, such as necrotising tonsillitis, postanginal septicaemia and Lemierre's disease.