Caries Management - Science and Clinical Practice. Группа авторов

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СКАЧАТЬ a series of jumps results in net movement of the molecule (double arrow, d = net distance moved).

      b Since more molecules move from regions of high concentration into regions of low concentration than vice versa, there is net transfer of molecules down gradients of concentration. This is illustrated for molecules diffusing from saliva (left) into plaque, via the plaque fluid.

      Fig. 2.12 Diffusion-with-reaction in plaque. Molecules of a nutrient such as sugar or oxygen (small red circles) diffusing into the surface of the plaque (left) are utilized immediately by superficial bacteria and this leaves less to diffuse deeper into the plaque. This results in a much steeper fall in concentration than in the case of simple diffusion (cf. Fig. 2.11). If metabolism is rapid, little or no nutrient will reach the deepest parts of the plaque. In plaque containing little extracellular polysaccharide (EPS) (above), the bacteria are closely packed and quickly utilize sugar molecules diffusing in from the saliva, allowing little to reach the interior. If EPS is abundant (below) the bacteria are more widely spaced, more sugar can diffuse into the interior and bacteria near the tooth surface can produce acid, which causes a greater pH drop in the environment of the tooth mineral.

      The interstitial fluid bathing the matrix, referred to as plaque fluid, is the component of plaque in direct contact with the tooth surface and is therefore the medium for the flux of H⁺ ions and mineral ions during the caries process. Analysis of plaque fluid isolated by centrifugation shows that its electrolyte composition differs markedly from that of saliva³⁷ (Table 2.2), mainly because exchange of ions between the two fluids is diffusion-dependent and slow. Thus, the elevated potassium concentration is due to slow clearance of K⁺ ions released by bacterial lysis. In plaque that has not recently been exposed to nutrients, the predominant organic acid is acetic acid (Table 2.2), which is an end product of metabolic pathways that derive the maximal ATP from the low amounts of available carbohydrate³.

      A cariogenic challenge is initiated by exposure to fermentable carbohydrate, which provokes a characteristic pattern of change in plaque pH, known as the Stephan curve^3,4 (Fig. 2.13), which can be recorded by micro-electrodes inserted into the plaque. The duration of a Stephan curve varies, but is typically 30–60 minutes. The curve can be divided into two phases: an initial rapid pH fall from the resting value (approximately pH7), followed by a slower recovery of pH. These phases reflect the underlying pattern of bacterial metabolism. In the initial phase, the high intraoral sugar concentration typical of confectionery or sweetened drinks drives diffusion of sugar into the plaque, where it is rapidly metabolized to produce energy. The main metabolic pathway is conversion of sugar by glycolysis to pyruvic acid and then directly to lactic acid (see Chapter 11), which reduces the pH within the plaque fluid³ (Table 2.2). The rate of pH fall is slowed by combination of H⁺ ions with plaque buffers, made up mostly of macromolecules associated with the bacterial cell walls. Simple sugars are cleared from the mouth by saliva within 1–2 minutes but the pH fall in plaque lasts for somewhat longer because the bacteria continue to metabolize the sugar that diffused inward initially. Eventually the sugar is used up and acid production ceases, so the initial phase of the Stephan curve comes to an end. The minimum local pH during a Stephan curve is about 4.0, which represents the lowest value at which even the most aciduric bacteria can produce acid, but average values will be higher than this.

      Although plaque pH is lower than under “resting” conditions for the whole duration of a Stephan curve, only part of it represents the cariogenic challenge. At neutral pH, plaque fluid is actually supersaturated. This means that the pH must fall to a certain value before the plaque fluid becomes just saturated with respect to the dental mineral. This pH is called the critical pH.²⁴ For enamel mineral, this is calculated from analyses of plaque fluid to be 5.2–5.5. Dentin mineral is known to be more soluble than enamel mineral,²⁷ so the critical pH must be higher than for enamel, but as the solubility of dentin is not accurately known a good estimate has yet to be made. Once the pH of plaque fluid falls below the critical pH, demineralization can occur. Conversely, once the pH of plaque fluid rises above the critical pH, dissolution will cease. Therefore, only the fraction of the Stephan curve between the critical pH and the minimum pH represents a cariogenic challenge (see Figs. 2.2 and 2.13).

      During the second phase of the Stephan curve, pH returns to the resting value, but this typically takes longer than the pH fall in the first phase of the curve. During this phase, H⁺ ions are released from the immobile buffers associated with the bacterial cell walls and diffuse slowly outward into the saliva. H⁺ ions are also removed from plaque by mobile buffers—small, diffusible ions such as phosphate and bicarbonate—which act as H⁺ “carriers” between the plaque fluid and the saliva³⁸ (Fig. 2.14). During the second phase of the Stephan curve, the pH will rise above the critical pH and precipitation of mineral will become possible. This can occur either as crystal growth within the caries lesion—where it will help to replace lost mineral (remineralization)—or as precipitation within the plaque (calculus formation).

      Acids are not the only products of sugar utilization by plaque bacteria. In particular, many plaque bacteria synthesize polysaccharides from dietary sugar (see Chapter 11). These include extracellular polymers of glucose (glucans) or fructose (fructans) and intracellular glycogen-like storage polysaccharides. All are relevant to the caries process. Intracellular polysaccharides (IPS) can be utilized for energy between intakes of sugar, as can soluble extracellular fructans, and the resulting acid production may extend the recovery phase of the Stephan curve. IPS-producing bacteria are more abundant in the plaque of caries-active individuals, owing to the selection pressure of frequent sugar ingestion.¹⁹ Extracellular glucans tend to be insoluble and are not utilized for energy. The insoluble, sticky glucans synthesized by S. mutans are important in the adhesion of these bacteria to tooth surfaces and in their retention in plaque. They may also increase the cariogenicity of plaque in another way. In plaque with abundant extracellular glucans, the number of bacteria per unit volume is reduced. Consequently, sugar diffusing into plaque at the start of a cariogenic challenge is utilized less rapidly and so can diffuse further into the plaque. This brings acid production closer to the tooth surface, thereby creating a more cariogenic environment³⁹ (see Fig. 2.12). Sucrose might induce other alterations in plaque matrix composition which lead to depletion of mineral ions.³⁹

      Fig. 2.13 Stephan curve. The dashed horizontal lines indicate the range for the critical pH. The shading indicates the period of the cariogenic challenge, when the plaque fluid is undersaturated with respect to enamel mineral. The denser СКАЧАТЬ