Oral bacteria. Computer-enhanced coloured transmission electron micrograph (TEM) of Porphyromonas gingivalis bacteria (formerly named Bacteroides gingivalis). Two of the bacteria are dividing (upper left and lower right) 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. Magnification: x12,000 when printed 10 centimetres tall.