Microbial Flora of the Oral Cavity, Dental Caries (World of Microbiology and Immunology)
The microbial flora of the oral cavity are rich and extremely diverse. This reflects the abundant nutrients and moisture, and hospitable temperature, and the availability of surfaces on which bacterial populations can develop. The presence of a myriad of microorganisms is a natural part of proper oral health. However, an imbalance in the microbial flora can lead to the production of acidic compounds by some microorganisms that can damage the teeth and gums. Damage to the teeth is referred to a dental caries.
Microbes can adhere to surfaces throughout the oral cavity. These include the tongue, epithelial cells lining the roof of the mouth and the cheeks, and the hard enamel of the teeth. In particular, the microbial communities that exist on the surface of the teeth are known as dental plaque. The adherent communities also represent a biofilm. Oral biofilms develop over time into exceedingly complex communities. Hundreds of species of bacteria have been identified in such biofilms.
Development of the adherent populations of microorganisms in the oral cavity begins with the association and irreversible adhesion of certain bacteria to the tooth surface. Components of the host oral cavity, such as proteins and glycoproteins from the saliva, also adhere. This early coating is referred to as the conditioning film. The conditioning film alters the chemistry of the tooth surface, encouraging the adhesion of other microbial species. Over time, as the biofilm thickens, gradients develop within the biofilm. For example, oxygen may be relatively plentiful at the outer extremity of the biofilm, with the core of the biofilm being essentially oxygen-free. Such environmental alterations promote the development of different types of bacteria in different regions of the biofilm.
This changing pattern represents what is termed bacterial succession. Examples of some bacteria that are typically present as primary colonizers include Streptococcus, Actinomyces, Neisseria, and Veillonella. Examples of secondary colonizers include Fusobacterium nucleatum, Prevotella intermedia, and Capnocytophaga species. With further time, another group of bacteria can become associated with the adherent community. Examples of these bacteria include Campylobacter rectus, Eikenella corrodens, Actinobacillus actinomycetemcomitans, and the oral spirochetes of the genus Treponema.
Under normal circumstances, the microbial flora in the oral cavity reaches equilibrium, where the chemical by-products of growth of some microbes are utilized by other microbes for their growth. Furthermore, the metabolic activities of some bacteria can use up oxygen, creating conditions that are favorable for the growth of those bacteria that require oxygen-free conditions.
This equilibrium can break down. An example is when the diet is high in sugars that can be readily used by bacteria. The pH in the adherent community is lowered, which selects for the predominance of acid-loving bacteria, principally Streptococcus mutans and Lactobacillus species. These species can produce acidic products. The resulting condition is termed dental caries. Dental caries is the second most common of all maladies in humans, next only to the common cold. It is the most important cause of tooth loss in people under 10 years of age.
Dental caries typically proceeds in stages. Discoloration and loosening of the hard enamel covering of the tooth precedes the formation of a microscopic hole in the enamel. The hole subsequently widens and damage to the interior of the tooth usually results. If damage occurs to the core of the tooth, a region containing what is termed pulp, and the roots anchoring the tooth to the jaw, the tooth is usually beyond saving. Removal of the tooth is necessary to prevent accumulation of bacterial products that could pose further adverse health effects.
Dental caries can be lessened or even prevented by coating the surface of the tooth with a protective sealant. This is usually done as soon as a child acquires the second set of teeth. Another strategy to thwart the development of dental caries is the inclusion of a chemical called fluoride in drinking water. Evidence supports the use of fluoride to lessen the predominance of acid-producing bacteria in the oral cavity. Finally, good oral hygiene is of paramount importance in dental heath. Regular brushing of the teeth and the avoidance of excessive quantities of sugary foods are very prudent steps to maintaining the beneficial equilibrium microbial equilibrium in the oral cavity.
See also Bacteria and bacterial infection