Tooth bacteria. Coloured scanning electron micrograph (SEM) of bacteria from the surface of a human tooth. The mixture of bacteria, saliva and carbohydrates is known as plaque It is a cause of tooth decay (caries) and can also lead to the formation of tartar (brown) (calculus). Deposition of tartar in the spaces between the tooth and gum (the gingival crevices) causes gum disease (periodontal disease), which is characterised in its early stages by swollen, inflamed gums which bleed easily. Untreated, the plaque may spread under the tooth, which might loosen and fall out. Magnification: x1000 when printed at 10 centimetres wide.