Streptococcus mutans. Coloured scanning electron micrograph (SEM). S. mutans is a coccoid shaped, Gram-positive, anaerobic bacteria that is part of the normal bacteria flora of the mouth. It metabolizes sucrose to lactic acid and is a leading cause of tooth enamel decay. The acidic environment created in the mouth by this process is what causes the highly mineralized tooth enamel to decay. S. mutans is one of a few specialized organisms equipped with receptors for adhesion to the surface of teeth. Sucrose is utilized by S. mutans to produce a sticky, extracellular, dextran-based polysaccharide (glucan) that allows them to adhere to each other forming plaque. Other sugars (glucose, fructose, lactose) can be digested by S. mutans to produce lactic acid. It is the combination of plaque and acid that leads to dental decay. Magnification: x8000 when printed at 10cm wide.