Biofilm
The normal healthy oral environment will contain teeth which consist primarily of calcium hydroxyapatite containing phosphate and other minerals. Tissue fluid moves slowly outward through tooth structure because pressure within the tooth is greater than the outside pressure. The teeth are bathed constantly in saliva and this is also supersaturated with a similar variety of minerals. There is a constant ion exchange between saliva and tooth structure depending upon the local concentration and ambient pH.
There is a coating of biofilm constantly attached to the surface of all teeth which can be removed by hygiene procedures but will immediately reform. The biofilm is the total complex formed when pellicle adheres to the tooth surface and becomes populated with bacteria and their extracellular products. Demineralisation can only occur at the interface between biofilm and the tooth surface. There will be natural metabolic fluctuations within and beneath it leading to regular cycles of demineralisation and remineralisation of the tooth surface and this is perfectly normal and natural. Most of the ions released will be retained under the biofilm and may return into the tooth structure. However, the fluctuations, particularly of pH values, can vary considerably and the ions can be lost when the pH is lowered thus interfering with the cycle. A healthy flow of saliva will play a significant role in remineralisation. Ion concentration can also be therapeutically increased thus leading also to remineralisation.