Dental plaque

Dental plaque is a complex microbial biofilm that grows on surfaces within the mouth. It is a sticky, colorless deposit that can eventually harden into dental calculus, also known as tartar.

This biofilm is a highly organized community of microorganisms, including bacteria and fungi. It attaches to tooth surfaces, dental restorations, and prosthetic appliances like dentures or bridges. Because plaque is a major cause of dental decay and gum disease, understanding its biology is essential for maintaining oral health.

The formation of plaque occurs through a specific sequence of biological steps. First, an acquired pellicle forms on the tooth surface. This is a layer of saliva composed mainly of glycoproteins that appears shortly after cleaning. Within hours, bacteria undergo adhesion, loosely binding to this pellicle layer. Next, the bacteria begin proliferation, spreading and multiplying throughout the mouth. They then form microcolonies, where species like Streptococcus secrete a protective slime layer. Finally, these microcolonies develop into a mature biofilm that can even possess a primitive circulatory system.

Different types of bacteria dominate the plaque at various stages of its development. The early biofilm consists primarily of Gram-positive cocci, such as Streptococcus species, which make up 60% to 90% of the early colonizers. As the biofilm matures over 3 to 4 days, the number of filaments and fusiform bacteria increases. By 7 to 14 days of undisturbed growth, the community becomes more complex, including Vibrio species, spirochetes, and more Gram-negative organisms. Some bacteria, known as late colonizers, such as Porphyromonas gingivalis, join the community later. Fusobacterium nucleatum plays a vital role by linking the early and late colonizers together.

Plaque is categorized by its location in the mouth, which changes its composition. Supragingival biofilm forms above the gumline in areas like the pits, grooves, and spaces between teeth. This type of plaque is mostly made of aerobic bacteria, which require oxygen to survive. In contrast, subgingival biofilm forms below the gumline in the pockets of the gums. This environment lacks oxygen, allowing anaerobic bacteria to thrive.

This downward growth from the supragingival area to the subgingival area is a key part of how plaque progresses.

The chemical makeup of plaque is quite specific. Approximately 80% to 90% of the total weight of plaque is water. Of the remaining dry weight, 70% consists of bacteria, while 30% is made of polysaccharides and glycoproteins. A key player in this ecosystem is Streptococcus mutans. This bacterium uses an enzyme called glucansucrase to convert sucrose into a sticky, extracellular, dextran-based polysaccharide. This process allows the bacteria to cohere and stick firmly to the tooth.

If plaque is not removed through regular brushing and flossing, it leads to several health issues. One consequence is dental caries, or tooth decay. This is the localized destruction of tooth tissues caused by acid produced during the bacterial degradation of fermentable sugar. Another result is gingivitis, an inflammatory response where the gums appear red and puffy.

If left untreated, gingivitis can progress to periodontitis. This is a serious infection that leads to the destruction of the bone supporting the teeth.

In periodontitis, bacteria release enzymes that attack the bone, while cells called osteoclasts also break down bone to prevent the spread of infection.

The oral environment provides a unique setting for these microorganisms. Unlike other parts of the body, tooth surfaces are hard and do not shed. The mouth is warm and moist, which is ideal for bacterial growth. Saliva acts as a buffer to maintain a pH between 6 and 7, and it provides nutrients like amino acids and proteins. However, when bacteria ferment dietary sucrose, they produce acids that lower the pH. This acidity can cause demineralization of the enamel. Furthermore, the mouth's temperature, usually between 35°C and 36°C, can shift dominant species if it changes by even two degrees.