Porphyromonas bacteria. Coloured transmission electron micrograph (TEM) of Porphyromonas gingivalis bacteria (formerly named Bacteroides gingivalis). These Gram-negative, anaerobic, rod-shaped bacteria are part of the normal flora of the mouth, intestine and urogenital tract. However, they can cause gingivitis (gum disease) and other periodontal infections, including ulcerative gingivitis and mouth abscesses. This can be prevented by using antibacterial toothpastes and mouthwashes. Magnification: x37,000 when printed 10 centimetres wide.