Dental caries, commonly known as a cavity, is a destructive process that breaks down the tooth structure. This damage is caused by acid-producing bacteria, primarily Streptococcus mutans, which feed on sugars and fermentable carbohydrates in the mouth. The resulting acid strips away the minerals from the tooth’s hard outer layers, a process called demineralization. While a cavity is not an infection that jumps from person to person like a cold, the damage spreads in two ways: physical progression deeper into a single tooth, and elevated microbial risk leading to new lesions on other, previously healthy teeth.
The Progression of Tooth Decay
Tooth decay begins with the demineralization of the enamel, the hardest substance in the human body, which first appears as a small, chalky white spot on the surface. If this initial stage is not reversed, the lesion penetrates through the enamel, creating a small hole or cavity. At this point, the decay is still progressing relatively slowly due to the density of the enamel layer.
The speed of destruction accelerates dramatically once the acid-producing bacteria breach the enamel and reach the underlying layer, the dentin. Dentin is softer and contains microscopic tubules that lead directly toward the center of the tooth. This porous structure allows the decay to spread much faster and more widely within the tooth structure.
Continued progression through the dentin eventually leads the decay to the pulp chamber, the innermost layer containing the nerves and blood vessels. Once the bacterial infection reaches the pulp, it causes significant irritation, swelling, and often intense pain. If left untreated, this deep infection can lead to the formation of an abscess at the tooth’s root tip, which can spread into the surrounding jawbone and tissues.
Bacterial Transmission and New Lesions
The external spread of decay relates to the microbial environment in the mouth, where having one active cavity increases the risk of developing others. An existing cavity acts as a reservoir, concentrating these acid-tolerant bacteria within the biofilm, or plaque, that covers the teeth. This elevated bacterial load creates a high-risk factor for new lesions to form, particularly on adjacent teeth.
The bacteria themselves are transmissible and can be shared through saliva, such as from parent to child or by sharing utensils. While the physical hole of the cavity is not contagious, the transfer of the high concentration of decay-causing bacteria can initiate the disease process in a healthy mouth.
The bacteria attach to the tooth surface using special enzymes that convert dietary sugars into sticky, water-insoluble substances. These substances allow the bacteria to form a robust, difficult-to-remove biofilm, which shields them while they produce acid that demineralizes the enamel. This microbial shift transforms the oral environment into one that is more favorable for the development of multiple new carious lesions.
Halting Decay and Preventing Further Damage
For early-stage lesions confined to the enamel, professional interventions often include the use of high-concentration fluoride varnish or gels. Fluoride helps reverse the demineralization process by promoting remineralization, where the tooth structure rebuilds itself by drawing in minerals. Dentists may also apply dental sealants, which are plastic coatings painted onto the chewing surfaces of back teeth to create a physical barrier against plaque and food particles.
Once the decay has created a hole in the tooth, the infected material must be physically removed by a dentist before the damage can spread further. The resulting space is then sealed with a filling, which restores the tooth’s structure and prevents bacteria from re-entering. If the decay has progressed deep into the pulp, a root canal or, in severe cases, tooth extraction may be necessary to remove the infection and halt the spread.
Reducing the intake of sugary and starchy foods deprives Streptococcus mutans of the fuel it needs to produce damaging acid. Consistent brushing twice a day with fluoride toothpaste and daily flossing physically removes the bacterial biofilm from the tooth surfaces. This disrupts the acid production cycle and is fundamental to preventing the formation of new lesions.
Saliva also plays a significant protective role by naturally rinsing away food debris and acting as a buffer to neutralize acid in the mouth. A reduced flow of saliva, often caused by certain medications or dehydration, increases the risk of decay progression because the mouth loses its ability to naturally restore a neutral pH balance. Maintaining good hydration helps support the natural protective mechanisms of the mouth.