Tooth surface. Coloured scanning electron micrograph (SEM) of the surface of a human tooth (yellow), showing a carpet of spherical bacteria (blue) and red blood cells (red). More than 300 species of bacteria inhabit the human mouth. They feed on the organic film of saliva and food debris that coats the teeth. They produce the enzyme lactate dehydrogenase, which converts sugars into lactic acid. This has a corrosive effect on the tooth enamel, causing tooth decay (dental caries). The red blood cells may have arisen from bleeding gums, an early sign of gum disease. Gum disease is caused by toxins secreted by the bacteria. Magnification unknown.