Bacteriophages killing dental bacteria, illustration. Dental bacteria (brown spheres) can cause plaque, a bacterial biofilm that can form on teeth and can cause tooth decay. An example of a bacterium that metabolises sugar in the mouth in this way is Streptococcus mutans. The bacteria can be killed using viruses called bacteriophages (red) that inject genetic material (lower right) that kills the bacteria and produces new bacteriophages (lower left). For this illustration without labels, see image C047/7654.