- The oral cavity is one of a complex, heterogeneous microbial habitat.
- Saliva contains microbial nutrients, but it is not an especially good growth medium because the nutrients are present in low concentration.
- Also the saliva contains antimicrobial substances.
- Saliva contains an enzyme lysozyme that cleaves glycosidic linkages in peptidoglycan present in the bacterial cell wall, thus, weakening the wall and causing cell lysis.
- Lactoperoxidase in both milk and saliva kills bacteria by a reaction in which singlet oxygen is generated.
- Despite the activities of these antibacterial substances, food particles and cell debris provide high concentrations of nutrients.
- These are present near surfaces such as teeth and gums, creating favorable conditions for the extensive local microbial growth, tissue damage and diseases.
The teeth and the dental plaque
- The tooth consists of mineral matrix of calcium phosphate crystals (enamel) surrounding living tooth tissue (dentin and pulp).
- Bacteria found in the mouth during the initial year of life (when teeth are absent) are predominantly aero-tolerant anaerobes such as streptococci and lactobacilli.
- However, some other and aerobic bacteria occur in small numbers.
- When the teeth appear, the balance of the micro-flora shift toward anaerobes that are specifically adapted to growth on surfaces of teeth and in the gingival crevices.
- Bacterial colonization of tooth surfaces begins with the attachment of single bacteria cells.
- Even on freshly cleaned tooth surface, acidic glycoproteins from the saliva form a thin organic film several micrometers thick.
- This film acts as attachment site for bacterial micro-colonies.
- Streptococci (primarily Streptococcus sanguis, S.sobrinus, S. mutans, S. mitis) can then colonize the glycoprotein film.
- Extensive growth of these organism results in a thick bacterial layer called dental plaque.
- Filamentous anaerobes such as Fusobacterium species begin to grow if plaque formation continues.
- The filamentous bacteria embed in the matrix formed by the streptococci and extend perpendicular to the tooth surface, making an ever thicker bacterial layer.
- The filamentous bacteria are spirochetes such as Borellia species, gram positive rods and gram negative cocci.
- In heavy plaque, filamentous obligately anaerobic organisms such as Actinomyces may predominate.
- Thus, dental plaque can be considered a mixed-culture biofilm that consists of a relatively thick layer of bacteria from several different genera as well as accumulated bacterial products.
- The anaerobic nature of the oral flora may seem surprising considering the intake of oxygen through the mouth.
- However, anoxia develops as per the metabolic activities of facultative bacteria growing on organic materials at the tooth surface.
- A dense matrix is produced by the plaque buildup that decreases oxygen diffusion to the tooth surface, forming an anoxic micro-environment.
- The microbial population with-in dental plaque exists in a micro-environment of their own making themselves in the face of wide variations in the conditions in the macro-environment of the oral cavity.
- The resident micro-flora produces locally the high concentrations of organic acids as dental plaque accumulates.
- This is responsible for causing decalcification of tooth enamel resulting in dental caries or tooth decay.
- Thus, dental caries is considered as an infectious disease
- The smooth surfaces of the teeth are very easy to clean and resist decay.
- The tooth surfaces in and near gingival crevice can retain food particles so they are the sites where dental caries typically begins.
- High sugar containing diets promote dental caries.
- Lactic acid is formed by lactic acid bacteria by fermenting sugars which dissolves some of the calcium phosphate in localized areas.
- The proteolysis of the supporting matrix occurs through the action of bacterial proteolytic enzymes.
- Slowly the bacterial cells penetrate further the decomposing matrix.
- The structure of the calcified tooth tissue has also a great role in the extent of dental caries.
- For example, fluoride incorporated into the calcium phosphate crystal tooth matrix increases the resistance to acid decalcification.
- Consequently, fluorides in drinking water and denitrifrices inhibit tooth decay.
- Two bacteria involved in dental cares i.e., S. mutans and S. sobrinus are lactic acid bacteria.
- S. sobrinus has specific affinity for salivary glycoproteins found on the smooth tooth surfaces due to which it is probably the primary organism causing decay of smooth surfaces.
- S. mutans produces dextran which is a strong adhesive polysachharides that is used to attach to tooth surfaces and these organisms are more commonly found in crevices and small fissures.
- Dextran is produced only in the presence of sucrose by the activity of enzyme dextransucrase.
- Susceptibility to tooth decay varies and is affected by genetic traits in an individual along with the diet content and other extraneous factors.
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- For example: sucrose is a substrate for dextransucrase which is highly cariogenic and is a part of the diet of most individuals in developed countries.
- In the United States and Western Europe, 80-90% of all individuals are infected by S. mutans and dental caries is nearly universal.
- By contrast, S. mutans is absent from the plaque of Tanzanian children, and dental caries does not occur.
- This is due to the presumption of absence of sucrose from their diets.
- Microorganisms in mouth are also responsible for other infections.
- The areas along the periodontal membrane at or below the gingival crevice can be infected with microorganisms.
- This causes inflammation of the gum tissues (gingivitis) leading to tissue and bone-destroying periodontal disease.
- Some of the genera involved include fusiform bacteria (long, thin, gram negative rods with tapering ends) such as facultative aerobe Capnocytophaga.
- The aerobe Rothia, and even strictly anaerobic methanogens such as Methanobrevibacter (Archae) may also be present.
Normal Microbial Flora of the Oral Cavity