^BStreptococcus mutans. ^b Coloured scanning electron micrograph (SEM) of ^IStreptococcus^i ^Imutans^i bacteria. These are Gram-positive, non- motile, spherical bacteria which typically form chains of cells (as seen here). ^IStreptococcus ^Imutans^i 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 ^IS. ^i ^Imutans^i from multiplying and causing dental caries. Magnification: x8000 at 6x7cm size.