Plaque fluid and diffusion: study of the cariogenic challenge by computer modeling

Salivary elemental signature of dental caries: a systematic review and meta-analysis of ionomics studies

Trace- and macro-chemical elements are crucial for cellular physiological functioning, and their alterations in biological fluids might be associated with an underlying pathological state. Hence, this study aimed to examine and summarize the published literature concerning the application of salivary ionomics for caries diagnosis. An extensive search of studies was conducted using PubMed, EMBASE, Web of Science, and Scopus, without any language and year restriction for answering the following PECO question: "In subjects (i.e., children, adolescents, or adults) with good systematic health, are there any variations in the salivary concentrations of trace- or macro-elements between caries-free (CF) individuals and caries-active (CA) subjects?" A modified version of the QUADOMICS tool was used to assess the quality of the included studies. The Review Manager Version 5.4.1. was used for data analyses. The analysis of salivary chemical elements that significantly differed between CF and CA subjects was also performed. Thirty-four studies were included, involving 2299 CA and 1669 CF subjects, having an age range from 3 to 64 years in over 16 countries. The meta-analysis revealed a statistically significant difference (p < 0.05) in the salivary levels of calcium, phosphorus, chloride, magnesium, potassium, sodium, and zinc between CA and CF subjects, suggesting higher levels of calcium, phosphorus, potassium, and sodium in CF subjects while higher levels of chloride, magnesium, and zinc in CA patients. Half of the included studies (17/34) were considered high quality, while the remaining half were considered medium quality. Only zinc and chloride ions were found to be higher significantly and consistent in CF and CA subjects, respectively. Conflicting outcomes were observed for all other salivary chemical elements including aluminum, bromine, calcium, copper, fluoride, iron, potassium, magnesium, manganese, sodium, ammonia, nitrite, nitrate, phosphorus, lead, selenium, and sulfate ions.

In silico modelling to differentiate the contribution of sugar frequency versus total amount in driving biofilm dysbiosis in dental caries

Dental caries is the most prevalent infection globally and a substantial economic burden in developed countries. Dietary sugars are the main risk factor, and drive increased proportions of acid-producing and acid-tolerating (aciduric) bacterial species within dental biofilms. Recent longitudinal studies have suggested that caries is most strongly correlated with total sugar intake, contrasting with the prevailing view that intake frequency is the primary determinant. To explore this possibility, we employed a computational model for supragingival plaque to systematically sample combinations of sugar frequency and total amount, allowing their independent contributions on the ratio of aciduric (i.e. cariogenic) to non-aciduric bacteria to be unambiguously determined. Sugar frequency was found to be irrelevant for either very high or very low daily total amounts as the simulated biofilm was predicted to be always or never cariogenic, respectively. Frequency was a determining factor for intermediate total amounts of sugar, including the estimated average human consumption. An increased risk of caries (i.e. high prevalence of aciduric/non-aciduric species) was predicted for high intake frequencies. Thus, both total amount and frequency of sugar intake may combine to influence plaque cariogenicity. These findings could be employed to support public guidance for dietary change, leading to improved oral healthcare.

Reaction-diffusion theory explains hypoxia and heterogeneous growth within microbial biofilms associated with chronic infections

Reaction–diffusion models were applied to gain insight into the aspects of biofilm infection and persistence by comparing mathematical simulations with the experimental data from varied bacterial biofilms. These comparisons, including three in vitro systems and two clinical investigations of specimens examined ex vivo, underscored the central importance of concentration gradients of metabolic substrates and the resulting physiological heterogeneity of the microorganisms. Relatively simple one-dimensional and two-dimensional (2D) models captured the: (1) experimentally determined distribution of specific growth rates measured in Pseudomonas aeruginosa cells within sputum from cystic fibrosis patients; (2) pattern of relative growth rate within aggregates of streptococcal biofilm harboured in an endocarditis vegetation; (3) incomplete penetration of oxygen into a Pseudomonas aeruginosa biofilm under conditions of exposure to ambient air and also pure oxygen; (4) localisation of anabolic activity around the periphery of P. aeruginosa cell clusters formed in a flow cell and attribution of this pattern to iron limitation; (5) very low specific growth rates, as small as 0.025 h⁻¹, in the interior of cell clusters within a Klebsiella pneumoniae biofilm in a complex 2D domain of variable cell density.

Non-lethal control of the cariogenic potential of an agent-based model for dental plaque

Dental caries or tooth decay is a prevalent global disease whose causative agent is the oral biofilm known as plaque. According to the ecological plaque hypothesis, this biofilm becomes pathogenic when external challenges drive it towards a state with a high proportion of acid-producing bacteria. Determining which factors control biofilm composition is therefore desirable when developing novel clinical treatments to combat caries, but is also challenging due to the system complexity and the existence of multiple bacterial species performing similar functions. Here we employ agent-based mathematical modelling to simulate a biofilm consisting of two competing, distinct types of bacterial populations, each parameterised by their nutrient uptake and aciduricity, periodically subjected to an acid challenge resulting from the metabolism of dietary carbohydrates. It was found that one population was progressively eliminated from the system to give either a benign or a pathogenic biofilm, with a tipping point between these two fates depending on a multiplicity of factors relating to microbial physiology and biofilm geometry. Parameter sensitivity was quantified by individually varying the model parameters against putative experimental measures, suggesting non-lethal interventions that can favourably modulate biofilm composition. We discuss how the same parameter sensitivity data can be used to guide the design of validation experiments, and argue for the benefits of in silico modelling in providing an additional predictive capability upstream from in vitro experiments.

investigating acid production by Streptococcus mutans with a surface-displayed pH-sensitive green fluorescent protein

Acidogenicity and aciduricity are the main virulence factors of the cavity-causing bacterium Streptococcus mutans. Monitoring at the individual cell level the temporal and spatial distribution of acid produced by this important oral pathogen is central for our understanding of these key virulence factors especially when S. mutans resides in multi-species microbial communities. In this study, we explored the application of pH-sensitive green fluorescent proteins (pHluorins) to investigate these important features. Ecliptic pHluorin was functionally displayed on the cell surface of S. mutans as a fusion protein with SpaP. The resulting strain (O87) was used to monitor temporal and spatial pH changes in the microenvironment of S. mutans cells under both planktonic and biofilm conditions. Using strain O87, we revealed a rapid pH drop in the microenviroment of S. mutans microcolonies prior to the decrease in the macro-environment pH following sucrose fermentation. Meanwhile, a non-uniform pH distribution was observed within S. mutans biofilms, reflecting differences in microbial metabolic activity. Furthermore, strain O87 was successfully used to monitor the S. mutans acid production profiles within dual- and multispecies oral biofilms. Based on these findings, the ecliptic pHluorin allows us to investigate in vivo and in situ acid production and distribution by the cariogenic species S. mutans.

Heterogeneity in biofilms

Biofilms, accumulations of microorganisms at interfaces, have been described for every aqueous system supporting life. The structure of these microbial communities ranges from monolayers of scattered single cells to thick, mucous structures of macroscopic dimensions (microbial mats; algal-microbial associations; trickling filter biofilms). During recent years the structure of biofilms from many different environments has been documented and evaluated by use of a broad variety of microscopic, physico-chemical and molecular biological techniques, revealing a generally complex 3D structure. Parallel to these investigations more and more complex mathematical models and simulations were developed to explain the development, structures, and interactions of biofilms. The forces determining the spatial structure of biofilms, including microcolonies, extracellular polymeric substances (EPS), and channels, are still the subject of controversy. To achieve conclusive explanations for the structures observed in biofilms the cooperation of both fields of investigation, modelling and experimental research, is necessary. The expanding field of molecular techniques not only allows more and more detailed documentation of the spatial distribution of species, but also of functional activities of single cells in their biofilm environment. These new methods will certainly reveal new insights in the mechanisms involved in the developmental processes involved in the formation and behavior of biofilms.

Factors affecting the resting pH of in vitro human microcosm dental plaque and Streptococcus mutans biofilms

The aim was to examine factors that potentially control the resting pH, defined as the pH unaffected by meals, of microcosm dental plaques and Streptococcus mutans biofilms under standard conditions, and to examine the effect of supplying urea at concentrations found intraorally. Microcosm plaques were cultured from plaque bacteria-enriched saliva in an 'artificial mouth' with a continuous supply of a medium including 0.25% mucin [Basal Medium Mucin, (BMM), 3.6 ml/hr per plaque] and a periodic supply of sucrose. The steady-state resting pH was 6.4 (range +/- 0.1) in BMM containing no urea and supplied at the standard flowrate. This is a robust property of the ecosystem. In one experiment with a replicated (n = 9) set of measurements, the resting pH was approx. pH 6.3, 6.4, 6.7 and 7.3 with 0, 1, 5 and 20 mmol/l urea in the BMM. The magnitude of sucrose- and urea-induced pH responses was unaffected by elevating the resting pH to produce parallel pH curves. The sucrose-induced pH curves were analogous to those classically reported by Stephan that showed an association between caries activity and increasingly acidic plaque pH responses to glucose. Stopping the BMM flow caused a pH rise, indicating continuing net alkali generation from BMM components in the absence of a fluid flow. Step. mutans monoculture biofilms had an acidic resting pH of 5.0 to 5.3, which increased to 6.8 following an adventitious superinfection by Bacillus cereus. It was concluded that the resting pH in plaque results from a delicate balance between alkali and acid generation, which is in turn dependent both on the bacterial composition of the plaque and on the supply of substrates and buffers from, and metabolite clearance into, flowing oral fluid. In vivo the resting pH will vary with site-specific changing saliva flows. Urea continuously supplied at concentrations normal for saliva and gingival crevicular fluid can raise the resting pH of microcosm plaque by an amount tat in vivo would probably be significant in reducing dental caries.

The validity of models

The ubiquity of biofilm and its classification as a microbial aggregate is discussed. Investigations into any microbial ecological problem operate at four levels: (i) in situ investigations, (ii) the use of microcosms, (iii) experimental model systems, and (iv) mathematical models. Each of these is defined and their use in biofilm research illustrated. It is concluded that all these approaches are valid and that scientific research in general and biofilm research in particular must profit by the use widely different methods if a complete understanding of a system is to be achieved.

Mathematical modeling of biofilms

A set of mathematical equations constitutes a mathematical model if it aims to represent a real system and is based on some theory of that system's operation. On this definition, mathematical models, some very simple, are everywhere in science. A complex system like a biofilm requires modeling by numerical methods and, because of inevitable uncertainties in its theoretical basis, may not be able to make precise predictions. Nevertheless, such models almost always give new insight into the mechanisms involved, and stimulate further investigation. The way in which diffusion coefficients are measured for use in a model, particularly whether they include effects of reversible reaction, is a key element in the modeling. Reasons are given for separating diffusion from reversible reaction effects and dealing with them in a separate subroutine of the model.

Methods for microscopic characterization of oral biofilms: analysis of colonization, microstructure, and molecular transport phenomena

Assessment of the role of biofilm microstructure in biofilm-specific activities requires non-destructive measurement techniques for parameterization of structural characteristics in parallel with relevant biochemical and physiological data. This paper briefly reviews some current methods for biofilm structural analysis, with emphasis on new developments in optical imaging and mathematical modeling methods. Fluorescence imaging studies of bacterial colonization events occurring on exposed model tooth surfaces indicated that bacterial adhesion to sessile organisms was of central importance to the early colonization process and that this occurred in a non-random manner. Structural studies of mature biofilms by confocal microscopy demonstrated the spatial distribution of individual species using fluorescent antibodies. Biofilms grown under different physiological conditions exhibited differences in structure, and methods were developed for parameterizing the spatial orientations of the bacteria. Diffusive processes within biofilm microstructures were studied using a random walk model in both 2-D and 3-D. Modeling of convective flow within biofilm microstructures was achieved by application of lattice Boltzmann methodology.

Artificial dental plaque biofilm model systems

Difficulties with in vivo studies of natural plaque and its complex, heterogeneous structure have led to development of laboratory biofilm plaque model systems. Technologies for their culture are outlined, and the rationale, strengths, and relative uses of two complementary approaches to microbial models with a focus on plaque biodiversity are analyzed. Construction of synthetic consortia biofilms of major plaque species has established a variety of bacterial interactions important in plaque development. In particular, the 'Marsh' nine-species biofilm consortia systems are powerful quasi steady-state models which can be closely specified, modified, and analyzed. In the second approach, microcosm plaque biofilms are evolved in vitro from the natural oral microflora to the laboratory model most closely related to plaque in vivo. Functionally reproducible microcosm plaques are attainable with a biodiverse microbiota, heterogeneous structure, and pH behavior consistent with those of natural plaque. The resting pH can be controlled by urea supply. Their growth patterns, pH gradient formation, control of urease levels by environmental effectors, and plaque mineralization have been investigated. Microcosm biofilms may be the only useful in vitro systems where the identity of the microbes and processes involved is uncertain. Together, these two approaches begin to capture the complexity of plaque biofilm development, ecology, behavior, and pathology. They facilitate hypothesis testing across almost the whole range of plaque biology and the investigation of antiplaque procedures yielding accurate predictions of plaque behavior in vivo.

A mathematical model of potassium ion diffusion in dentinal tubules

Desensitizing agents containing potassium ions (K+) are believed to inactivate intradental nerves by raising extracellular [K+]. A mathematical model was used to investigate factors affecting [K+] in dentinal tubules. The most important factors affecting the steady-state tubular [K+] were the tubular fluid-flow velocity, salivary [K+] and the permeability to potassium (k) of the barrier between the tubule and the pulp. Tubular [K+] decreased with increasing outward flow velocity and increasing k. whereas the dimensions of the tubule and odontoblast process had little effect. Following a 1 min simulated application of 500 mmol/1 K+ to the dentine surface, [K+] at the inner end of the tubule increased above steady-state levels for 20-30 min. The maximum [K+] attained at the inner end of the tubule was around 30 mmol/l for an impermeable barrier (k = 0) and flow velocity of 1.4 microns/s, but lower maximum tubular [K+] were achieved when either the outward flow velocity or k was increased. The model suggests that applying potassium-containing preparations to dentine may increase [K+] at the inner ends of dentinal tubules to levels sufficient to inactivate intradental nerves. However, the localized increase in [K+] is transient, and the concentration change will be lessened by conditions that increase the tubular fluid-flow velocity or the permeability of the barrier between the tubule and pulp.

Inhibition by ethanol of the growth of biofilm and dispersed microcosm dental plaques

Inhibition of microcosm plaque biofilm growth by periodic application of ethanol was compared with the minimum inhibitory concentration (MIC) and bactericidal effects of ethanol on liquid cultures of dispersed plaque bacteria. Microcosm plaques were cultured from saliva in a multiplaque 'artificial mouth' and their growth in wet weight measured daily. Nutrient conditions included: a continuous supply of a medium containing 0.25 percent mucin, and 8-hourly 5 percent (w/v) sucrose (1.5 ml over 6 min). Plaque biofilm growth was strongly inhibited by exposure to 40 percent (v/v) ethanol applied in volumes of 3.75 ml over 15 min, six times daily. Application of 1.5 ml over 6 min inhibited much less or not at all. Ethanol concentrations lower than 40 percent caused less inhibition, with 10 percent having almost no effect. The pH response to sucrose was unchanged by prior application of 40 percent ethanol for 30 min. Some evidence was obtained for either bacterial adaptation to ethanol or selection of ethanol-resistant bacteria. The MIC and bactericidal effects of ethanol were assessed by growth of dispersed plaque in liquid culture; the bactericidal effect was measured as the induced delay in growth. The aerobic and anaerobic MIC of ethanol for growth was 10 percent and 8 percent; 50 percent inhibition of growth rate occurred at 3.7 percent and 2.8 percent. Ethanol (40 percent) was bactericidal within 1-2 min, but 10 percent had almost no effect. It was concluded that, despite the well-known high ethanol sensitivity of dispersed plaque bacteria, prolonged application of ethanol concentrations in the order of 40 percent are necessary to inhibit growth of plaque biofilms.

Computer modeling of the effects of chewing sugar-free and sucrose-containing gums on the pH changes in dental plaque associated with a cariogenic challenge at different intra-oral sites

Variation in salivary access to different intra-oral sites is an important factor in the site-dependence of dental caries. This study explored, theoretically, how access is modified by chewing sugar-free and sugar-containing gums. A finite difference computer model, described elsewhere, was used. This allowed for diffusion and/or reaction of substrate, acid product, salivary buffers, and fixed-acid groups. Site-dependent saliva/plaque exchange was modeled in terms of a 100-microns-thick salivary film covering the plaque (a) flowing directly from the salivary ducts, (b) flowing from the intra-oral salivary pool, or (c) exchanging with the pool. Computed flow-velocities or rates of exchange were based on previous intra-oral measurements. The model was also tested against an in vitro study conducted by two of the authors. In addition, the three proposed models of saliva/plaque interaction were compared, and the effect of salivary film thickness investigate. Results suggested that: (1) although sugar-free gum chewed during a cariogenic challenge causes a rapid rise in plaque pH, sucrose-containing gums cause the pH, after a temporary rise resulting from increased salivary flow, to stay low for an extended period; (2) the computer model reproduced in vitro tests reasonably well; (3) although the three models of the plaque/saliva interaction start from different assumptions, two lead to closely related predictions; and (4) increasing the assumed salivary film thickness by a large amount (e.g., from 50 to 200 microns) caused no change in modeled Stephan curves, as long as these changes were accompanied by appropriate reductions in film velocity, in accord, theoretically, with the practical clearance data.

The diffusion and enzymic hydrolysis of monofluorophosphate in dental plaque

Although the ability of dental plaque to hydrolyze sodium monofluorophosphate (MFP) has been known for some time, its effect on the F- concentration at the plaque-enamel interface is undefined. We have determined enzyme kinetic values for MFP hydrolysis and diffusion coefficients so that the penetration and degradation of MFP in plaque can be modeled by computer. The KM and Vmax values for natural human plaque were 1.77 mmol/L and 41.4 nmol/min/mg protein, respectively, at pH 8.0. At pH 6.0, the Vmax was lower, 15.6 nmol/min/mg, but KM was not significantly different. Competitive inhibition by orthophosphate gave a Ki of 4.55 mmol/L. The diffusion coefficient for MFP in artificial plaque was 1.91 x 10(-6) cm2/sec. When these data were used for mathematical modeling of the effects of rinsing with MFP and F- solutions, compared with an equivalent NaF application, the concentration of F- from MFP was lower at the inner surface of plaque, and the peak occurred later. Both pH and plaque thickness had a marked effect on the amount of MFP that could penetrate: At pH 8.0, almost none reached the inner surface of a 1-mm-thick plaque intact. At pH 6.0, however, more MFP was able to penetrate, due to lower MFPase activity. While MFP diffusion is inherently slower than that of F-, enzymic degradation increases the gradient for inward diffusion. If the conventional view that MFP in toothpaste acts as a source of F- is true, then MFP toothpaste should be formulated to optimize MFPase activity in dental plaque.

The pH response to urea and the effect of liquid flow in 'artificial mouth' microcosm plaques

This study examined in detailed the pH response of microcosm plaque biofilms to the application of 500 mmol/l urea, and the effect of modifying the flow rate of BMM (a basal medium containing 0.25% mucin). Microcosm plaques were cultured from the mixed salivary bacteria in a multi-plaque 'artificial mouth' supplied continuously with BMM at 3.6 ml/h per plaque, and periodically with sucrose (5 or 10%). Urea (500 mmol/l) induced a pH response that was the inverse of the Stephan pH curve induced by sucrose. In thicker plaques the ureolytic pH response was delayed and slower. With no BMM flow, the urea-induced pH curve reached a maximum and then slowly decreased indicating loss of ammonia. A flow of BMM reduced the magnitude of the pH response. Urea dilution explained (r2 = 0.97) the reduction in the maximum rate of pH rise caused by an increasing BMM flow. There were, however, additional flow-rate effects on the magnitude of the pH rise, the curve areas and the maximum rate of pH decrease back to the resting pH. These effects were greatest at low BMM flow rates, indicating that ammonia clearance may be limited at higher flow rates by the rate of intraplaque diffusion and metabolism. Application of 50 instead of 500 mmol/l urea reduced the rate of pH rise about 10-fold, and the area of the curve about seven fold. Metabolism of arginine (50 mmol/l) generated only about half the pH response of the same amount of urea.(ABSTRACT TRUNCATED AT 250 WORDS)

pH gradients induced by urea metabolism in 'artificial mouth' microcosm plaques

Evidence was sought for urea-induced pH gradients in dental plaque microcosm biofilms cultured from the mixed salivary bacteria in a multi plaque 'artificial mouth'. Application of 500 mmol/l urea for short periods (6 min) to 5-8 mm maximum-thickness plaques induced intraplaque pH gradients of up to 0.7 pH units with the surface alkaline relative to the inner plaque. These pH gradients persisted for more than 5 h in the absence of a flow of fluid. With 30-min urea applications and a flow of a basal medium containing mucin (BMM, pH 7.0), the pH of the inner (deeper) plaque regions also increased. Although the pH gradient initially formed was alkaline at the plaque surface, the BMM flow lowered the surface pH to neutrality whilst the inner layers were still alkaline, thereby reversing the pH gradient. In thick microcosm dental plaques, urea-induced pH gradients can therefore form and last many hours. They probably result from the significant time taken for urea to penetrate to the inner layers of plaque, its rapid metabolism by the outer plaque layers, and a rate-limiting clearance of ammonia. Even a slow BMM flow over the plaque greatly increased the rate of return to the resting pH, causing the gradients to change polarity.

Effect of the bathing fluid on measurements of diffusion in dental plaque

The suggestion that loss of labile components from dental plaque into contacting aqueous buffer during some diffusion measurements might change the plaque's diffusion behaviour, and therefore invalidate the results, has been tested using tracer clearance. (a) Diffusion of [14C]-acetate and 3H2O was determined before and after a 3-4 h equilibration with neutral buffer. No differences or trends were found. (b) Mean normalized [14C]-acetate clearance curves from live plaque for measurements before and after such equilibration were identical in shape and showed no change in diffusion coefficient during clearance, as evidenced by their fits to the theoretical clearance curve. No significant difference was found between coefficients for the two periods (paired sample test). In addition, [14C]-lactate and 3H2O were found to permeate plaque in a miniature diaphragm diffusion cell at rates independent of whether the plaque was bathed either in saliva supernatant, plaque fluid or neutral buffer. This lent some further support to the main findings. It is concluded that diffusion through dental plaque is little affected by the bathing fluid.

A quantitative study of calcium binding and aggregation in selected oral bacteria

By means of micro-equilibrium dialysis, calcium binding capacities and affinities were measured in three different oral bacteria, and the effects of extracellular polysaccharide, pH, and aggregation were investigated. Binding capacities of 31.0 +/- 2.1 (C. matruchotii), 34.7 +/- 3.7 (S. sanguis), and 41.5 +/- 5.4 (S. downei) mumol calcium/g wet weight of cells were found at pH 7.0, falling to 21.4 +/- 0.8 mumol calcium/g wet wt. cells at pH 5.0 for S. downei. Dissociation constants were found to vary between 0.78 +/- 0.24 and 1.77 +/- 0.30 mmol/L (at pH 7.0, depending on species), and between 0.62 +/- 0.04 and 1.77 +/- 0.30 (in the pH range 5.0 to 7.0, for S. downei only). Examination suggested that at pH 7.0 calcium-facilitated bacterial association occurs in the streptococci with calcium uptake curves analogous with those of positively cooperative systems. Desorption of calcium from aggregated S. downei suggested that the mechanism of desorption differed from that of uptake. This may be an important factor in plaque formation and in the binding of cells to the surface of formed plaque. Plaque calcium forms a reservoir, readily released by a pH drop, which may increase plaque fluid saturation and reduce demineralization.

pH responses to sucrose and the formation of pH gradients in thick 'artificial mouth' microcosm plaques

Artificial microcosm plaques were grown in a five-plaque culture system for up to 6 weeks, reaching a maximum depth of several mm. Procedures for long-term pH measurement with glass electrodes were established; they showed that the application of 5 or 10% sucrose for 6 min with a slow continuous flow of a basal medium containing mucin (BMM) generated the pH changes characteristic of in vivo Stephan curves. These pH responses were reproducible between plaques. Plaque mass and thickness were critical variables. Successive, sucrose-induced pH curves in plaques up to 4 mm thickness showed minor reductions only in the amplitude and rates of pH change. In plaques over 4 mm thick there was a pronounced reduction in pH response to successive sucrose applications, indicating increased diffusion limitations--a result of plaque growth to seal in the freshly-inserted pH electrode. In plaques of 6 mm maximum thickness, 10% sucrose induced a decrease to below pH 5.5 lasting 24 h, compared to the pH response in 2 mm thick plaque, which returned to the resting pH in 2 h. Differences in pH of up to 0.9 units were identified in thick plaques between inner and outer layers. The BMM flow rate was a critical determinant of the amplitude of the pH response to sucrose and subsequent return to resting pH. These results confirm, for microcosm plaque, the importance of clearance dynamics and diffusion-limited gradients in regulating plaque pH.

Effects of salivary film velocity on pH changes in an artificial plaque containing Streptococcus oralis, after exposure to sucrose

Results from a computer model suggest that following exposure of dental plaque to sucrose, the rate of clearance of acids from plaque into the overlying salivary film will be greatly retarded at low film velocities. This was investigated with an in vitro technique in which artificial plaque containing S. oralis cells was exposed to 10% sucrose for one min. The pH at the proximal (P) and distal (D) undersurfaces of the plaque (0.5 or 1.5 mm thick) was then monitored during the passage of a 0.1-mm-thick film of a sucrose-free solution over the surface. Over the range of salivary film velocities that have been estimated to occur in vivo (0.8-8 mm/min), lower minimum pH values and increased times for the pH to recover toward neutrality occurred at the lower salivary film velocity. Lower pH values were also reached with the 0.5- than with the 1.5-mm-thick plaque. P/D pH gradients, with a lower pH distally, developed at film velocities of 0.8 and 8 mm/min, and the gradients were much more pronounced at the lower velocity. No P/D pH gradients developed when the film velocity was 86.2 mm/min. Incorporation of dead S. oralis cells into the plaque at percentages up to 57% reduced the extent of the pH fall and prolonged the recovery of the pH toward neutrality. The results support the prediction that, other factors being equal, plaque located in regions of the mouth with low salivary film velocity will achieve pH values lower than those of plaque of identical dimensions and microbial composition located in areas where salivary film velocity is high.

An assessment of recent advances in the study of the chemistry and biochemistry of dental plaque fluid

This paper discusses key points made during the symposium in the light of work carried out in other laboratories. It is emphasized that the unique importance of plaque fluid is that the net result of chemical changes induced by microbial activity is reflected in this medium, which is in intimate contact with the enamel surface, and that this medium is accessible to chemical and biochemical analyses. However, in order to assess the cariogenic potential of plaque, we must consider the properties of both whole plaque and plaque fluid together. Although it is apparent that results of plaque fluid composition are sensitive to both isolation and the storage procedures utilized, plaque fluid appears to be a distinct entity within the oral cavity. Technical advances have been made which allow for the determination of the activity of selected ions (hydrogen, calcium, phosphate, potassium, fluoride) in plaque fluid obtained from a single site within the mouth. It appears, however, that such data alone may be insufficient to define the cariogenic potential of plaque appropriately. Evidence is presented from which it can be concluded that, with use of pooled samples of plaque obtained from individuals with clear differences in caries experience, results on plaque and plaque fluid composition can be obtained which are consistent with noted differences in caries susceptibility. The importance of base production is also discussed, and it is noted that few studies have been carried out to elucidate the role of proteins found in plaque fluid. In conclusion, recent advances in the study of plaque fluid have provided new insights into the mechanism of caries formation which are also germane to the formation of dental calculi.