Microbiology of Dental Caries
Microbiology of Dental Caries
Dental caries can be defined as a localised and chemical loss of the tooth structure caused by the metabolic activity of dental biofilm that covers the tooth surface. This loss is reversible, especially in its early stages. The final outcome of dental caries is determined by multiple factors such as saliva, exposure to fluoride, and sugar dietary consumptions, which influence the dynamic balance between the demineralisation and remineralisation processes. Dental caries can be developed in any part of the tooth surface: enamel, dentine or cementum. However, it is more common in the sites wherein the dental biofilm is more protected, which allows it to mature and grow. Examples of these sites include: grooves, fissures, and pits on the occlusal surface and along the gingival margin.
The mouth, as with other parts of the human body, is a habitat for resident bacteria, which are called microbiota. They are not living passively, but rather contribute to maintaining the body’s health by contributing to immune system development and excluding pathogenic, exogenic microorganisms. This is called colonisation resistance and it happens as the resident microbiota are more competitive regarding attachment to the oral receptors and nutrient acquisition. The application of the molecular approach to identifying the resident oral microbiota brought an opportunity to estimate the high number of microbiota in the oral cavity. It is estimated that around 700 different species live in the mouth.
The development of caries was believed to be caused by only a few gram-positive bacterial species, such as Streptococcus mutans, Streptococcus sobrinus and lactobacillus. This understanding was based on cultivation studies by isolating these bacteria and determining their cariogenic properties. This is called the specific plaque hypothesis. It became evident that a caries lesion could happen in the absence of these putative pathogens. Current evidence states that population groups and individuals are susceptible to dental caries with a low level of S. mutans, and vice versa.
Dental caries happens mainly when the production of organic acid results in dental hard tissue decalcification. Thus, dental caries bacteria should be acid-tolerant in carrying out the dental caries process in an acidic environment. Acidogenic bacteria, e.g. S. mutans, are able to function at pH 6 and can carry out the glycolysis process at pH 4 or below. Lactobacillus spp., considered a moderate acidophilic bacterium, can function at a pH level from 3-4. In general, the acid tolerance property allows cariogenic bacteria to displace other acid-sensitive bacterial species, which would lead to enriching the aciduric bacteria and continuing the acidification process of dental caries, which is favourable for caries formation.
Although there are studies showing an association between dental caries and S. mutuns, lactobacilli and Actinomyces, most of the study samples were collected from caries sites wherein these groups of bacteria cannot be isolated. This could suggest that other microorganisms can contribute to the dental caries process. Therefore, it is the change in dental plaque ecology that leads to caries lesion development. Whenever there are non-shedding surfaces, bacterial cells will attach to saliva film called the pellicle; the bacterial cells start to multiply to form a microcolony. If left undisturbed the growth continues, resulting in a mature type of biofilm within a week. Different types of acids are created as a result of biofilm metabolism, which causes a shift in oral pH under the critical level. This reduction in the pH level will influence the chemical composition of the tooth surface. The cumulative result of many pH fluctuations over a long period of time is the loss of calcium and phosphate, which makes the enamel surface seem porous (which can be detected clinically as a white spot lesion). Thus, the caries lesion is a result of an imbalance in the equilibrium between tooth mineral loss and biofilm fluid.
Microbiology: Current Research
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