Streptococcus mutans. Coloured scanning electron micrograph (SEM) of Streptococcus mutans bacteria. These are Gram-positive, non- motile, spherical bacteria which typically form chains of cells (as seen here). Streptococcus mutans is found in the mouth cavity. It is one of the principal bacteria causing plaque formation on teeth, and a major cause of tooth decay. Use of xylitol as a sweetener in foods prevents S. mutans from multiplying and causing dental caries. Magnification: x8000 at 6x7cm size.