Streptococcus bacteria. Light micrograph of Gram- stained Streptococcus mutans bacteria. These Gram- positive, non-motile, spherical bacteria typically form chains of cells, as seen here. Streptococcus 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: x1450 when printed 10cm wide.