Tongue bacteria. Coloured scanning electron micrograph (SEM) of bacteria on the surface of a human tongue. Large numbers of bacteria can form a visible layer on the surface of the tongue. The mouth contains a large number of bacteria, most of which are harmless or even beneficial. However, some bacteria can cause throat infections or cause the formation of plaque deposits on the teeth, which may lead to decay. Magnification: ×10,000 at 10cm wide/STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY.