Caries Management - Science and Clinical Practice. Группа авторов

Чтение книги онлайн.

СКАЧАТЬ

      Fig. 1.15 Acidic metabolic end products and change of pH in dental plaque before and after intake of a lump of sugar [2]. L: lactic acid; A: acetic acid; P: propionic acid.

      The microorganisms need energy for their survival and replication. They can use many different methods for obtaining energy, which is influenced by the substrate available in the mouth that comes from saliva and the host's diet. The pioneers accumulated on the teeth after 3–6 hours are arranged in a monolayer. The pioneers, which primarily are aerobes or facultative anaerobes, will most likely use oxygen from the surrounding salivary film, which enters via the cell membrane, and the tricarboxylic acid cycle of Krebs (see Ref. [1] or other biochemical textbooks) to get intracellular energy. The end products leaving the cells are CO₂ and water, which are not harmful to the teeth.

      Through multiplication of the pioneers and arrival of newcomers, over the next few hours a rapid increase in the number of microorganisms accumulating on the teeth is seen (6–12h). Thus, the monolayer of microorganisms is replaced by multiple layers.⁴⁰ As the thickness of the layers increases (at a certain stage it becomes visible, and thus, as plaque), the oxygen tension in the inner layer (against the tooth surface) will drop, and the microorganisms in that layer will shift their metabolism and become more facultative anaerobic or strictly anaerobic.

      In the case of no access to dietary sources of nutrition, the microorganisms get energy primarily from the glycoproteins in the saliva. Under such conditions, the byproducts of metabolism by microorganisms on the teeth are evenly distributed among lactic acid, acetic acid, and propionic acid (Fig. 1.15). The concentration and strength of these acids do not harm the teeth, mainly owing to the action of the buffering systems. In the case of access to fermentable carbohydrates, the pH will drop in the liquid phase of the plaque within 3minutes, and it takes about 20–30 minutes for the pH to return to normal. The reason for the pH drop is that some microorganisms are able to convert the available sugar—which due to its very high concentration enters the cell membrane passively—via the glycolytic pathway and metabolize it to lactate¹ (Fig. 1.16). The fraction of lactate increases eightfold² during the first couple of minutes after starting to eat breakfast. This process requires that the microorganism has a system of constitutive enzymes, and in this case it is the lactate dehydrogenase that enables the microorganism to transform pyruvate to lactate, which then is released through the cell membrane (lactate gate) to the environment¹ (Fig. 1.16). During the metabolic process energy in the form of ATP is created. The microorganisms use the energy from ATP mainly for cell functions and replication. Microorganisms that do not possess lactate dehydrogenase may die, caused by substrate (sugar) killing. Some microorganisms can also synthesize intracellular polysaccharides to be used as “fuel” when there is no sugar in the surroundings to be metabolized.¹ Finally, some microorganisms also have constitutive enzymes, such as glucosyltransferases and fructosyltransferases, which can convert sucrose to glucans and fructans (extracellular polysaccharides), respectively. Glucans serve to glue the micro-organisms together, and fructans are easily metabolized and can act as a reserve source of nutrients.¹

       NOTE

      Microorganisms in cariogenic plaque have the following characteristics³⁵:

      • Anaerobe or facultative anaerobe

      • Acidogenic (produce acid, mainly lactic acid)

      • Aciduric (can survive under low pH conditions)

      • Produce intracellular polysaccharides

      • Produce extracellular polysaccharides

      Plaque Stagnation Areas

      Plaque development can only happen on areas of the teeth where there is no mechanical or chemical disturbance.^9,34,35,41 Examples of mechanical disturbance on the teeth are movements of the tongue and lips, and oral hygiene practices such as tooth brushing, flossing, etc. Due to mechanical disturbance from chewing, we don't see plaque accumulation on the incisal third of the incisors. Functional wear and tear is limited along the marginal gingiva, below or above the contact points between approximal surfaces and on the entire occlusal surface including the cusp areas during the eruption period of molar and premolar teeth, and after full occlusion is established the groove–fossa system of these teeth, while plaque will accumulate in these locations (Fig. 1.17) if it is not removed by oral hygiene methods. These locations are called plaque stagnation areas in dentistry. Interestingly, it is exactly in these areas that caries can develop.^9,41

      Fig. 1.16 Schematic illustration of how polysaccharides and disaccharides (carbohydrates) enter into the cells of the microorganism and are broken down into monosaccharides (glucose and fructose), which are used to create energy by glycolysis or to create intracellular polysaccharides (JPS) as an energy reservoir. The end products of fermentation are acids which the microorganism excretes. When carbohydrates are present in high concentration, lactic acid is created which can demineralize the dental hard tissues.

      Fig. 1.17a, b Illustration of plaque stagnation areas.

      a After staining in approximal and cervical areas.

      b On the occlusal surface of a molar without antagonist.

      Plaque Composition and Structure in Stagnation Areas

      Different methods, including light and electron microscopy (Fig. 1.18) as well as cultivation studies, have been used to examine the composition and structure of plaque in stagnation areas, from which it is clear that the composition and structure of plaque vary in different locations. In fissure parts of the groove–fossa system, on smooth surfaces, and on approximal surfaces the microorganisms are arranged in palisades perpendicular to the tooth surface and composed of many dividing cells^40,42 (Fig. 1.18 d). This indicates a vital flora.⁴² In contrast, the content in the bottom part of the fissures consists of an unorganized material of dead microorganisms, with inter- as well as intracellular evidence of minerals⁴⁰ (Fig. 1.18e). This arrangement of the microflora in the fissures suggests that the unfavorable conditions for bacterial growth deep in the fissures account for the findings that caries in fissures develops at the entrance to the fissures.^42–44

      Chapter 2 will СКАЧАТЬ