Spherical bacteria and rod-shaped bacteria inside biofilm with quorum sensing molecules (small red circles), illustration. A biofilm is a colony of bacteria that forms a coating on a surface. Inside biofilm bacteria communicate through production of so called quorum sensing biochemical molecules. Common places for biofilms to develop are in the mouth, where they can cause tooth cavities and gum disease, on contact lenses, where they can cause eye infections, on rocks submerged in water, and on industrial equipment, where they can cause fouling.