New concepts of dental plaque

Dental plaque forms continually, and we strive (with greater or lesser success) to keep plaque levels low. Based on the modern concepts of dental plaque as a biofilm, a number of important questions arise for clinical practice: Is plaque equally pathogenic in all our patients? If not, how can we assess this? Is it necessary to remove (to the microscopic level) all deposits of plaque? If not, how can we encourage "healthy" plaque?

Our mouths have a complex ecology, and the conditions for microbial growth vary from site to site within the mouth. Ecological differences between sites (in terms of factors such as pH, oxygen levels, and salivary host defense systems) explain why plaque differs from site to site (for example, between buccal and proximal sites on the same tooth) and why certain habitats (such as proximal sites) are more likely over time to develop "thick/old plaque" which has higher levels of the facultative anaerobic bacterial species that are associated with caries and periodontitis.

"Thin/young" plaque does not have high levels of these disease-related species and its microbial constituents can be regarded as "normal" in the sense that they are present continuously on many tooth surfaces, even though this thin biofilm may not be clinically visible without the use of disclosing dyes. The use of combination products with two dyes to show "thick/old" versus "thin/young" plaque offers a significant advantage when assessing risk for dental caries or periodontitis, and when providing oral hygiene instruction. Such products use a mixture of the food dyes erythrosin and fast green to stain "young" plaque red, and "old" plaque blue, respectively (Figures 1-3).

The adjunctive use of biocides such as Triclosan and essential oil (Listerine) preparations in toothpastes and mouthrinses can provide a worthwhile clinical benefit to patients in maintaining adequate control of dental plaque and gingivitis, particularly in terms of reducing interproximal plaque and gingivitis. This is important since for the interproximal region the lack of salivary defences and poor access for mechanical cleaning means that "thick/old" plaque is very likely to develop here. Triclosan and essential oils have been shown to reduce plaque levels in interproximal sites, however these agents do not interfere with the balance of organisms in supragingival dental plaque, and microbial resistance does not occur even with daily usage for personal oral hygiene.

Two important factors which disrupt the normal endogenous dental plaque flora and can increase the risk of disease development are reductions in oral pH (from regular consumption of acidic drinks) and reduced oxygen tension (from smoking), which provide a growth advantage to acid tolerant and facultative anaerobes respectively. This leads naturally to the question, How often to brush? Regular (twice daily) brushing reduces the thickness of plaque. Plaque that is kept thin (with the biofilm brushed away regularly) does not have low oxygen micro-environments within it. Conversely, the oxygen tension in plaque falls if there is no brushing. As plaque thickens, the deeper layers with lower oxygen tensions favour the growth of facultative anaerobes, including mutans streptococci (which initiate caries), and Gram negative rods (associated with periodontitis).

As the microflora of "thick/old" dental plaque becomes dominated by pathogens linked to dental caries and periodontal diseases, its reduction and removal becomes more of a challenge. Well established biofilms resist environmental, physical and chemical stresses, and because of diffusion barriers and metabolic changes (such as dormancy) the bacteria within them are less prone to the actions of biocides than planktonic (free floating) forms of bacteria. A range of approaches have been developed to overcome the greater defensive capabilities of biofilms, including disruption using sonic energy, and photo-sensitization (with red or blue light). Methods which alter the structure and composition of biofilms as they develop include repellants, probiotics, replacement therapy, and vaccines. Many of these technologies are attracting interest as they undergo the transition from the laboratory bench to clinical practice.

Recently developed tests for plaque acid production, such as the 3M-ESPE ClinPro Cario Diagnosis test for lactate, and the GC Plaque-Check+pH fermentation test have an important place in risk assessment and in patient education. The GC test is based on the causal association between caries activity and the production of strong acids from plaque in response to sucrose, which was first identified by the work of Stephan more than 65 years ago. But, which acid to measure? Many dental plaque bacteria can ferment carbohydrate substrates, and this leads to the production of numerous organic acids (of varying potency for demineralization). Thus, it is logical to look at the intact plaque biomass and the net result of fermentation, rather than to focus narrowly on just one species or just one organic acid (such as lactate). The GC kit assesses plaque pH, using a colourimetric test to show the pH-drop which occurs in intact plaque samples when exposed to an excess supply of sucrose. As fermentation occurs, the greatest reduction in pH occurs after 5 minutes, and this can be seen as a colour change in a pH indicator.

In relation to dental caries, "good plaque" (formed in a low cariogenic environment) has limited fermentation capabilities, and produces primarily acetate, (with lesser quantities of propionate and butyrate). These weaker acids can effectively buffer plaque pH changes. In contrast, "bad plaque" (formed in a highly cariogenic environment) produces large quantities of lactate, formate and pyruvate, stronger organic acids that can readily demineralize enamel.

The use of simple chairside tools such 2-tone plaque disclosing and plaque fermentation tests are a low cost method for (1) assessing caries risk; (2) engaging patients in a discussion of whether their plaque is "young/good" or "old/bad", and (3) assessing their compliance with oral hygiene and lifestyle changes designed to reduce the cariogenic potential of their plaque.

The decision of the clinician to recommend an antiseptic mouthrinse to patient as an adjunct to mechanical oral hygiene, should take into account their oral hygiene status and the level of risk for various oral diseases. A range of studies (Figure 4) has shown antiseptic mouthrinses can provide a worthwhile clinical benefit to those patients who cannot maintain adequate control of dental plaque and gingivitis through conventional mechanical methods alone.

References

1. Foster JS, Pan PC, Kolenbrander PE. Effects of antimicrobial agents on oral biofilms in a saliva-conditioned flowcell. Biofilms. 2004;1:5-12.
2. Data on file, Pfizer Consumer Healthcare.