http://www.oramd.com/recedinggums/2-gumsreceding.htm Our mouths are full of bacteria that, along with mucus and other particles, constantly form a sticky, colorless "plaque" on teeth, the cause of what is periodontal disease. Plaque that is not removed can harden and form bacteria-harboring "tartar" that brushing can not clean. Plaque is the major cause of what is periodontal disease.