Dental caries and saliva

Teeth are constantly exposed to saliva and bacteria in the oral cavity. Through a complex scries of events the teeth are coated with salivary proteins, resulting in an acquired pellicle, which both protects the surface of the tooth and facilitates the binding of specific oral bacteria to the tooth. Attachment of free bacteria to adherent bacteria and their proliferation result in the growth and maturation of deposits of bacterial plaques on the exposed surfaces of teeth. The development of bacterial plaque deposits on teeth is a dynamic event with net results being a continuous increase in size and complexity of the plaques. Unlike the epithelium, which sloughs off the outer cell layer or the mucus—bacterial aggregates, mechanical removal is required to clean teeth of plaque deposits and within hours after cleaning the colonization process repeats itself. Consequently, there is a chronic source of microbial challenges to the teeth and to the connective tissues abutting the teeth, resulting in oral diseases.

To protect teeth, the body uses a combination of immune mechanisms contained in saliva. Caries, a major oral infectious disease, is due to the accumulation of acid-producing bacteria, Streptococcus mutatis, in plaque and a high sucrose diet, which result in acid demineralization of localized areas of the enamel tooth surface (i.e. dental caries) under the plaque deposit. Saliva originates from the submandibular, sublingual, parotid and other minor salivary-glands. It contains components common to other mucosal secretions, which vary as to the specific functions of the epithelium to be serviced. Saliva in the oral cavity is referred to as whole saliva since it contains the glandular secreted materials plus serum proteins from the gingival crevicular fluid. In addition to the rinsing effect of saliva, salivary mucins (MG1 and MG2), parotid agglutinin, lyso-zyme, and 32-rn'croglobulin agglutinate bacteria into large aggregates which arc easily cleared. Secretory immunoglobulin A (slgA), the adaptive immune component, is generally derived from slgA-secreting plasma cells in the salivary glands which were stimulated through the common mucosal immune response system, although local stimulation and synthesis in the salivary gland can occur. slgA is antigen-specific and can prevent the initial binding of bacteria, virus or fungi to epithelial cells or the acquired pellicle on teeth. slgA can also specifically agglutinate bacteria and neutralize viruses. IgA consists of two isotypic forms, IgAl and IgA2. IgAl is predominant in serum as a monomer and IgA2 in saliva and other secretions in a dimeric form held together by a J chain and also having an attached secretory peptide. IgAl is susceptible to degradation to Fab and Fc fragments by an IgA protease produced by a number of microbial pathogens. The role of slgA specific for Strept. mutans in the control of dental caries remains controversial. Serum-derived IgG is also found in whole saliva, but its role in host defense in the oral environment is not defined. Other salivary proteins with antimicrobial activities are lysozymc, histatins, Iactoferrin and proline-rich proteins. Histatins and acidic proline-rich proteins are only found in saliva. Interestingly, histatins appear to be tailor-made for the protection of the oral cavity, as they are potent candidacidal agents and neutralize lipopolysac-charides, as well as having antimicrobial activities against strains of Strept. mutans and Porphyromonas gingivalis, two oral pathogens. Acidic proline-rich proteins are antimicrobial as well as being active in mineral homeostasis and toxin neutralization. Lacto-ferrin binds iron, which is required for microbial proliferation, and lysozyme either lyses the bacterial wall or induces autolysis by the bacterium. In parallel to the antigen-specific and antigen-nonspecific antimicrobial activities, salivary proteins also help maintain the integrity of the fragile oral epithelium, buffer pH fluctuations in plaque thereby slowing acid induced tooth demineralization, and help maintain a calcium balance to counteract demineralization of the teeth.

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