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G H Dibdin

Publications and source records attributed to G H Dibdin.

14 recordsLinked to original sources
PubMed1992

A finite-difference computer model of solute diffusion in bacterial films with simultaneous metabolism and chemical reaction.

This finite-difference computer model is designed to simulate complex diffusion/reaction events in bacterial films. It is modular, each module mirroring closely a particular physical, chemical or biochemical factor. It is capable of handling > 20 diffusing/reacting species, but can be easily expanded or simplified to match particular systems. It was originally designed for modelling the events in dental plaque leading to tooth decay, but should find application in other fields. It allows for ion-exchange interactions with, for example, fixed charges on bacterial surfaces, which can act as pH and cation buffer sites. pH-dependent utilization of substrate is modelled implicitly, combining Michaelis-Menten kinetics with diffusion in a single iterative procedure. Advantages are given for computing diffusion of all other species explicitly using single-species diffusion coefficients, with charge-coupling by means of the algorithm Q-COUPLE. Activity corrections and enzyme pH-dependence are included. Chemical equilibria and mineral deposition/dissolution are computed iteratively node by node. The program is tested against some problems having analytical solutions, and an example is given of its application to demineralization of teeth as a result of bacterial action in dental plaque.

Algorithms↗
PubMed1991

Precise charge-coupling calculations for finite difference diffusion problems using a modification of the add-on algorithm Q-COUPLE.

This note describes a minor modification to the recently published algorithm 'Q-COUPLE' designed for adding charge-charge interactions between diffusing species to time-dependent one-dimensional finite difference diffusion calculations. The original proposal concerned a simple way of doing this for Crank-Nicolson central time-difference schemes, and gave useful, but only approximate, agreement with theory in tests where the charge-coupling could also be calculated analytically. The new, slightly modified algorithm, when used with explicit (forward time-difference) modelling, gave analytically exact results for the charge interaction part of a similar trial calculation.

Algorithms↗
PubMed1990

Effect on a cariogenic challenge of saliva/plaque exchange via a thin salivary film studied by mathematical modelling.

Computer models can be powerful tools for studying complex interacting processes. The computer model of events in dental plaque during a cariogenic challenge described here simulates diffusion and metabolism of substrate, plus coupled diffusion/reaction of fourteen other species, charged and uncharged, including acidic metabolic products and fixed buffers. Its extension to deal with the effects of poor contact with bulk saliva when the plaque is presumed covered by a thin salivary film is here considered. Site-specific mixing rates between film and salivary pool were modelled phenomenologically, using data from the literature. Fast mixing was assumed during an initial carbohydrate intake phase (2 min sugar rinse), followed by site-dependent mixing and logarithmic clearance. The analysis also suggested a possible way of estimating local salivary film thickness. Increasing the halving time for exchange between film and bulk saliva was shown to prolong the pH minimum greatly, and to increase mineral loss. The respective roles of fixed buffers as stores of protons and of mobile buffers (especially bicarbonate) as exporters of protons from the inner plaque were emphasised.

Buffers↗
PubMed1990

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

Every cariogenic challenge involves a mixture of convective transport, diffusive transport, and biochemical reactions, plus physico-chemical reactions (including charge-coupling of diffusion), all of which together require numerical methods for their analysis. This presentation describes a one-dimensional finite-difference computer model of the cariogenic process, and some conclusions obtained from it. Sugar clearance from the mouth, together with site-dependent exchange between the bulk saliva and plaque surface via a salivary film, is combined with a finite-difference model of events occurring within the dental plaque. The latter includes: sugar diffusion and pH-dependent acid production; diffusion and dissociation equilibria for two acid end-products of fermentation and their anions (acetate and lactate); diffusion and dissociation equilibria of phosphate buffer; diffusion of potassium and chloride; diffusion of protons and simultaneous equilibration with fixed and mobile buffers. So that proper concentration distributions consistent with local charge neutrality can be ensured, an algorithm called Q-couple is used to impose charge-coupling between the fluxes of different ions including fixed charges. Mineral dissolution and precipitation are modeled as part of the same equilibrium calculations. The predictions of the model are compared with those of an earlier, much simpler one, in which fixed buffers were not included. It is shown that the known concentration of fixed buffer greatly extends the low pH of a Stephan curve. The isoelectric point of the plaque bacteria also appears to be of importance. The effects of various concentrations of mobile buffers, including acetate, are investigated. It is also shown that varying plaque/saliva contact over the known range derived from published studies has a profound effect on the modeled cariogenic challenge.

Computer Simulation↗
PubMed1989

A simple add-on algorithm to extend one-dimensional finite difference diffusion calculations to include charge coupling.

The importance of interionic charge coupling in chemical and biological diffusion problems is discussed, and the Nernst-Planck (ionic) and Onsager-Fuoss (neutral component) methods are considered. A novel single pass charge-coupling algorithm 'Q-COUPLE' is proposed, which should be usable as a separate add-on subroutine with many one-dimensional finite difference diffusion calculations. Its mode of operation is explained with the help of elementary electrostatics and by reference to listings in BASIC. The algorithm is being applied in a finite difference model of diffusion-with-reaction in dental plaque, with 12 ions or ionizable molecules diffusing and interacting with fixed charges. It is shown to be invariant with respect to the direction of sweep, and in the simple case of coupled diffusion of a single polyvalent electrolyte is found to compare well with the analytical solution. Advantages and limitations of the proposal are discussed.

Algorithms↗
PubMed1989

The interpretation of CO2 equilibration data to obtain plaque fluid buffer capacities, and comparison with results obtained by titration.

Titration measurements of pooled plaque fluid buffering capacity (Shellis and Dibdin, 1988), which showed a broadly defined minimum at pH 7, were compared with recent curves published by Carey et al. (1988a), which they obtained by an ultra-micro CO2-equilibration technique and which suggested a quite different profile, peaking sharply at pH 7.1. When analyzed in a different, more conventional way, the raw measurements in the latter study become more consistent with our own results and with earlier findings of Tatevossian (1977). In particular, we conclude that the peak at pH 7.1 is an artifact, and that Carey et al. underestimated buffer capacities below pH 6.4 and above pH 7.4. Rationales for the two modes of analysis are compared, and possible reasons for the remaining differences between the re-analyzed CO2-equilibration results and the titration results are discussed. Suggestions for the improvement of the accuracy of the CO2-equilibration technique are put forward.

Bicarbonates↗
PubMed1989

Estimation of the velocity of the salivary film at some different locations in the mouth.

Previously, we studied the clearance rates of KCl from agarose gels positioned at different locations in the mouth, and showed that the rates were much slower than when clearance was into a well-stirred solution. We designed the present in vitro study to test the effect on KCl clearance of the velocity of a 0.1-mm-thick film of water flowing over an agarose gel of the same diameter and composition as those used in vivo. The thickness of the salivary film overlying dental plaque has been estimated to be about 0.1 mm, and we assumed that when clearance rates in vitro matched those found in vivo, velocities of the fluid film (in vitro) and the salivary film (in vivo) must be equal. On this basis, it was calculated in the present experiments that when salivary flow was unstimulated, the velocity of the salivary film at the level of the teeth varied between about 0.8 mm/min (upper-anterior buccal region) and 8.0 mm/min (lower-anterior lingual region). When salivary flow was stimulated, this was estimated to increase the velocity of the salivary film from 2 to 40 times, depending on the location in the mouth. It is postulated that the slow movement of the salivary film when flow is unstimulated allows for accumulation of diffusants from dental plaque, which reduces the concentration gradient for diffusion from plaque and prolongs the clearance time of such metabolic products as acid.

Chemical Phenomena↗
PubMed1988

Physical and biochemical studies of Streptococcus mutans sediments suggest new factors linking the cariogenicity of plaque with its extracellular polysaccharide content.

Cultures of Streptococcus mutans MFe28 (serotype h) were grown with differing extracellular polysaccharide (EPS) content. Biochemical and physicochemical characteristics considered relevant to caries were measured. Acid production parameters measured in a pH-stat were: Vm = 0.76 +/- 0.14 mumol/g/sec (wet weight); apparent Km (acid production) = 100 mumol/L; molar yield = 1.97 +/- 0.25 mol acid/mol glucose. Acid anion inhibition of acid production was also noted. Buffering by the pure washed bacterial residue required approx. 112 mumol of base/g (wet weight) of residue to change the pH from 4 to 6.5, and this dropped almost to zero as the EPS content increased to 100%. Diffusion coefficients (D) in the residues were independent of EPS content over a wide range. When the effusion method was used, De (glucose) and De (acetate) were (3.26 +/- 0.6) and (5.05 +/- 0.8) x 10(-6) cm2/sec, respectively. The extracellular fluid fraction, measured by inulin exclusion, increased from 0.33 for the pure bacteria to 0.78 for the pure EPS. It is shown how, by these factors alone, and without any need for diffusion restriction, plaque EPS may lead to a lower pH at the tooth surface, thus increasing the cariogenic challenge.

Chemical Phenomena↗
PubMed1986

A theoretical analysis of the effects of plaque thickness and initial salivary sucrose concentration on diffusion of sucrose into dental plaque and its conversion to acid during salivary clearance.

A mathematical model, written in FORTRAN, has been developed to simulate the interrelated processes of salivary sucrose clearance from the mouth, diffusion of sucrose into dental plaque, and conversion of sucrose to acid and glucan. Reaction of acid with enamel is not included in the model. A total of 28 parameters can be varied by the user, and the relative importance of the different factors affecting acid formation can be assessed. The output of the program gives sucrose and acid concentrations and pH at different depths within the plaque. The initial variables studied were plaque thickness, the salivary sucrose concentration, and the duration of exposure of the plaque to sucrose. Stephan curves typical of those recorded in vivo were generated by the model. With any particular salivary sucrose concentration, there was an optimum plaque thickness at which a minimum pH was achieved at the enamel surface, with very thin or thick plaque samples producing a smaller pH fall. With thick plaque, the minimum pH was often not achieved at the inner surface but at some intermediate depth, which may explain the location of early caries lesions in fissures. The extent of the pH fall at the inner surface and the duration of the pH-minimum region of the Stephan curve were directly related to the initial salivary sucrose concentration and to the duration of exposure to sucrose prior to normal salivary clearance. Simulation of a water rinse at as short a time as two min after the beginning of normal salivary sugar clearance showed that this procedure had only a very small effect on the shape of the Stephan curve.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetates↗
PubMed1986

A comparison of the potassium content and osmolality of plaque fluid and saliva, and the effects of plaque storage.

Previous determinations of osmolality and potassium concentrations in plaque fluid, much higher than those in saliva, suggest a restricted exchange between the two, which must be reconciled with recent findings of quite rapid diffusion in plaque. Possible reasons for the high values were considered, and of these the effect of solute leakage from bacteria to the plaque fluid during typical periods of storage was investigated. It was also shown that the osmotic pressure of plaque fluid could be measured quite accurately by vapor pressure osmometry on whole plaque samples without the need for centrifugation. Samples of plaque, or plaque fluid prepared by centrifugation at 12,000 g, were compared for osmolality or potassium content with matched samples prepared from plaque stored chilled or in liquid nitrogen. Saliva samples obtained just prior to plaque collection were also analyzed. Freshly collected plaque from overnight-fasted subjects had a plaque fluid osmolality of 156 +/- 35 as compared with 98 +/- 23 mOs/kg for saliva. Potassium in plaque fluid from freshly collected "mature" plaque was 40.6 +/- 5.1 as compared with 20.3 +/- 5.3 mmol/L for saliva, but for 1-2-day-old plaque from fasted subjects it was significantly lower (30.4 +/- 5.6 mmol/L). These values for plaque fluid are all much less than those previously found, and storage was found to cause a marked increase (range, 35-100%). Centrifugation at 12,000 g caused little change in plaque fluid osmolality but seemed to accelerate the rate of increase during subsequent storage.

Adult↗
PubMed1975

Gas chromatography of volatile fatty acids. Method involving separation from biological material by vacuum distillation.

A method is described for the quantitation of C2-C5 volatile fatty acids present in biological tissues. It involved recovery of the acids from their biological matrix by vacuum micro-distillation at room temperature, followed by gas phase separation of aqueous solutions on orthophosphoric acid-modified Phasepak Q columns. The subsequent gas chromatographic procedure resolved iso from normal isomers and showed a linear response for each volatile acid over the range 10-400 ng. There was no evidence of ghosting, isomer peak broadening, or peak tailing. Relative molar response values were shown to be linear with carbon number for all the volatile fatty acids studied.

Carbon↗
PubMed1975

An intra-oral telemetry system for the continuous recording of vertical jaw movement.

Continuous measurement of the separation between upper and lower dental arches, the so-called interocclusal distance (10D), is of interest in dentistry. Criteria for making such measurements by radio telemetry are discussed. It is concluded that a transmitter small enough to fit in a molar tooth gap is necessary in order to minimize interference with normal function; other design factors are related to obtaining adequate frequency stability, on which measurement accuracy depends. A transmitter fulfilling these requirements is described which sensed the instantaneous value of 10D by the frequency change produced when a metallic object (dental filling, gold crown, shorted turn of wire or piece of ferrite) in one dental arch moved relative to the transmitter in the other. Errors due to lateral and protrusive jaw movements were measured. Careful design and the use of high grade ceramic chip capacitors resulted in a transmitter of good frequency stability (+/- 0.03% in 10 h, +/- 0.02%/degrees c) and small size (10 X 7 X 5 mm). An example is given illustrating 10D movements due to swallowing, speech and respiration.

Humans↗