Fluorine is an element of the halogen family and the most reactive of the non-metals, with an atomic number of nine and an atomic weight of 19. Fluoride compounds are widely distributed throughout the soils of the earth, enter plants, are ingested by humans, and are absorbed from the gastrointestinal tract. Over the last half century, fluoride has become synonymous with preventive dentistry. There has been a significant decline in dental caries since the incorporation of fluoride in dentifrice (Burt, 1978). Preventing the initiation and progression of caries is also important in the success and longevity of dental restorations. Although its primary preventive role is due to its enhancement of mineral tooth structure, fluoride may also affect the cellular functions of cariogenic microorganisms and render them less cariogenic (Wilson and McLean, 1988; Komatsu and Shimokabe, 1993). Fluoride’s antibacterial effect should be common knowledge amongst dental practitioners. This paper is intended to describe the process by which fluoride exerts its inhibitory effect on the cariogenic bacteria. A brief review of the beneficiary effect of fluoride on the mineralized tooth structure will be presented first.
EFFECT OF FLUORIDE ON THE MINERALIZED TOOTH STRUCTURE
Fluoride may become incorporated into the tooth structure and make it more resistant to acidic dissolution. The ionic radius of fluoride ion (1.36 () is similar to that of the hydrogen ion (1.40 (). This has important consequences in that the fluoride ion replaces the hydroxyl ion in the hydroxyapatite lattice and forms the more acid-resistant fluorapatite (Fig. 1) (Smith and Peltoniemi, 1982; Wilson and McLean, 1988).
The ionic fluoride in saliva, in plaque, and within enamel and dentin shifts the equilibrium of demineralization-remineralization toward remineralization. Fluoride acts as a catalyst for uptake of calcium and phosphate ions and results in a greater efficiency in remineralizing the areas of enamel and dentin that have been affected by acidic attack (Ten Cate, 1990).
The porous nature of demineralized tooth structure also helps this remineralization process by allowing a greater penetration of minerals. It has been established that remineralized tooth structure is significantly more acid resistant than intact tooth structure (Brown et al., 1977). Aside from the formation of the more acid resistant fluorapatite crystals, many researchers have demonstrated the formation of other fluoride complexes, such as calcium fluoride (CaF2) and fluoridated carbonato-apatite (FCA) (Fig. 2) (Ten Cate et al., 1995; Rolla and Saxegaard, 1990; Saxegaard et al., 1987; Geiger and Weiner, 1993).
In vitro and in situ experiments have demonstrated that as a result of an acidic attack on the tooth structure calcium and phosphate ions are released (Ten Cate et al., 1995; Rolla and Saxegaard, 1990; Tveit et al., 1983). The availability of fluoride from some dental materials, plaque, or saliva during this demineralization stage results in the formation of calcium fluoride (CaF2) crystals that are deposited on the tooth surface or at the interface between the restorative material and tooth structure (Fig. 2) (Rolla and Saxegaard, 1990; Saxegaard et al., 1987; Ten Cate et al., 1995). When the pH of the environment rises, CaF2 crystals act as a reservoir for fluoride and calcium that can be re-deposited into the tooth structure in the form of fluorapatite during the remineralization phase (Fig. 3) (Saxegaard et al., 1987; Rolla and Saxegaard, 1990; Ten Cate et al., 1995).
FLUORIDE ACCUMULATION IN THE PLAQUE AND THE BACTERIAL CELL
Dental pellicle is formed as a result of the strong affinity of salivary proteins and glycoproteins for tooth surface. These proteins form a layer of pellicle that is poorly organized and is free of bacteria. Further deposition of salivary constituents, food debris, and inorganic compounds strengthen the structure of dental pellicle. This provides a matrix to which microorganisms become attached and release their products into the resultant meshwork. The resultant porous and non-calcified coating on the tooth surface is known as the dental plaque and it harbors oral microorganisms (Anusavice, 1996; Rose and Turner, 1998).
Fluoride application to the porous matrix of dental plaque results in its accumulation (Tatevossian, 1990; Iwami et al., 1995; Spets-Happonen et al., 1998). It has been demonstrated that after rinsing with a chlorohexidine gluconate-sodium fluoride-strontium (CXFSr) solution twice a day for two weeks, the fluoride and strontium content of the plaque remained high for at least three weeks after completion of rinsing (Spets-Happonen et al., 1998). Plaque fluoride accumulates in two pools. Most of it (95%) exists as bound fluoride either inside the bacterial cells or attached to the matrix of the plaque, whereas the remaining 5% is present in the plaque fluid as a free ion (Fig. 4) (Tatevossian, 1990).
Oral bacteria growing in the presence of fluoride accumulates fluoride (Jenkins and Edgar, 1977). The amount taken up by the cells is proportional to the fluoride level in the external fluid phase. Fluoride accumulation in Mutans streptococci occurs due to a concentration gradient of fluoride across the membrane and does not involve an active transport mechanism (Kashet and Rodriguez, 1976; Whitford et al., 1977).
A decrease in external pH, indicating a more acidic environment, also leads to an increase in fluoride accumulation (Whitford et al., 1977). This leads to the conclusion that fluoride was taken up into the cell as hydrogen fluoride (HF) (Fig. 4). A fall in the extra-cellular pH results in the accumulation of more fluoride by the bacterial cell in an attempt to neutralize the acidic environment. The important relationship between the change of pH and fluoride uptake, is known as “F/pH effect”, and has been confirmed by other workers (Eisenberg and Marquis, 1980; Vicaretti et al., 1984; Kashket and Preman, 1985).
Subsequent to the transfer of HF across the membrane into the bacterial cell, the more alkaline intracellular compartment results in the dissociation of HF to fluoride and hydrogen ions (Hamilton, 1990). As a result, the continued influx of fluoride and concomitant build up of intracellular protons (H+) acidifies the cytoplasm (Fig. 4) (Guha-Chowdhury et al., 1997).
EFFECT OF FLUORIDE ON THE HOMEOSTATIC PATHWAYS OF THE CARIOGENIC BACTERIA
Accumulation of intracellular protons reduces the intra-cellular pH below the pH threshold for both catabolic and biosynthetic enzymes (Hamilton, 1986). In this way, therefore, fluoride increases the acquisition of protons by cells and results in a reduction in the tolerance of oral bacteria to growth and metabolism in acidic environments (Bender et al., 1986; Bowden, 1990; Spets-Happonen et al., 1998).
Fluoride also has a direct inhibitory effect on the metabolic activity of cariogenic bacteria (Figs. 4 & 5). Glycolysis is the central metabolic pathway by means of which saccharolytic microorganisms thrive. Inhibition of glycolysis by fluoride is central to the concept that the anti-microbial effect of fluoride has a role in caries prevention. It has been demonstrated that fluoride exerts this inhibitory action through its interference with uptake and degradation of polysaccharides by the bacterial cell, and also by reducing the ability of the cell to maintain pH homeostasis (Hamilton, 1990).
Intracellular fluoride primarily acts on two enzyme systems that are essential to the metabolic activity of the saccharolytic microorganisms. These enzyme systems are the enolase and the active proton-transport ATP-ase systems (Fig. 6) (Hamilton, 1990; Belli et al., 1995; Iwami et al., 1995).
Fluoride interferes with the complete breakdown of glucose to pyruvic acid by inhibiting enolase, an intermediary enzyme in the cascade. This results in a reduction in the synthesis of pyruvate and ATP. A decrease in pyruvate synthesis not only results in a reduction in the synthesis of lactic acid, but also interferes with the sugar transport via the phosphoeno
lpyruvate phosphotransferase system. Build up of the intermediate compounds of the glycolysis pathway also interferes with further import of glucose. These processes result in a significant reduction in the metabolic activity of the saccharolytic microorganisms.
The interference of fluoride with the ATP-ase active proton transport system results in the accumulation of intracellular proton ions and a reduction in glucose import (Hamilton, 1990; Marquis, 1990). As a result of the accumulation of intracellular proton ions and reduced intracellular substrate, the metabolic activity of the cell is down regulated significantly (Marquis, 1990; Guha-Chowdhury et al., 1995; Lanender-Lumikai et al., 1997; Spets-Happonen et al., 1998).
BACTERIAL MUTATION
Very high levels of fluoride, approximating 0.16 to 0.3 mol/L (3040 to 5700 ppm) are needed to kill bacteria (Bowden, 1990). Mutans streptococcus is an unusually sensitive microorganism to fluoride because of the direct inhibitory effect of fluoride on its proton-transporting ATP-ase system (Marquis, 1990). At lower fluoride concentrations, however, this bacterium mutates to fluoride resistant strains (Bowden, 1990; March and Bradshaw, 1990). These fluoride resistant strains have decreased metabolic activity, and as a result, demonstrate a significant reduction in their cariogenic potential (Marquis, 1990). In vitro studies have also demonstrated that fluoride-resistant strains of Mutans streptococci are less cariogenic in rats (Van Loveren, 1990).
Other clinical investigations have demonstrated that even low fluoride concentrations in plaque (sub-MIC: Minimum Inhibitory Concentration) could decrease the cariogenic potential of saccharolytic microorganisms (March and Bradshaw, 1990). It has been shown that prophylactic concentrations of fluoride (19 ppm) in combination with a moderately low pH (~5) adversely affect the metabolic capacity of bacterial cells and result in a significant reduction in acid production by cariogenic bacteria (Arweiler et al. 2002; Arweiler et al. 2001; Bowden, 1990; March and Bradshaw, 1990).
In summary, the passive diffusion of fluoride across the cell membrane in the form of hydrogen fluoride, its interference with substrate uptake and breakdown in the cell, and its effect in hindering the cellular mechanisms involved in the expulsion of protons from the intracellular environment result in a significant reduction in the metabolic activity of cariogenic micro-organisms. Bacteria that lack the ability to resist such environmental disturbances either down-regulate their metabolic activity but survive, or become eliminated from the plaque. The antibacterial effect of Fluoride may be significant both in preventing initial caries lesions and in enhancing restoration longevity.
M-Reza Nouri is a specialist in pediatric dentistry, and an assistant clinical professor at the University of British Columbia. He practices with the Oakridge-Richmond-Delta Paediatric Dental Group in BC. Dr. Nouri is a contributing consultant to Oral Health.
Keith C. Titley is a specialist in pediatric dentistry, Professor, Department of Paediatric Dentistry at the University of Toronto. Dr. Titley is the paediatric board member of Oral Health.
Oral Health welcomes this original article. Complete references available upon request.
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