Normal Microbial Flora of the Oral Cavity

November 27, 2019

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.

Dental caries

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.

Image source: semantic

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.