Gum disease caused by plaque. Close up of the teeth of a patient with dental plaque and brown tartar formation that has caused gum disease (gingivitis). Plaque consists of a film of bacteria embedded in a glycoprotein matrix formed from bacterial secretions and saliva. Plaque can cause inflammation and infection of the gums, and it is also the main cause of tooth decay. Plaque hardens into yellow or brown tartar.