Plaque formation on teeth, illustration. Plaque is a bacterial biofilm that can form on teeth and can cause tooth decay. It can develop into tartar, a mineralised and hardened form of dental plaque. If not treated, the condition can lead to gingivitis, peridontitis and tooth loss. Dental bacteria are shown in the inset at upper right. An example of a bacterium that metabolises sugar in the mouth in this way is Streptococcus mutans.