Streptococcus mutans bacteria. Coloured transmission electron micrograph (TEM) of Streptococcus mutans bacteria. These Gram- positive, non-motile, cocci (spherical) bacteria typically form chains of cells, as seen here. S. mutans is found in the mouth cavity and is a major cause of tooth decay. It is one of the principal bacteria causing plaque formation on teeth. It produces dextran, an adhesive glucose, which sticks to the teeth. Within plaque, bacteria produce lactic acid from dietary carbohydrates, which causes demineralization of the tooth enamel and dentine. Magnification: x8300 when printed at 10 centimetres wide.