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PubMed2002

In vitro study of transmission of bacteria from contaminated metal models to stone models via impressions.

The purpose of this study was to determine the risk of microbiological transmission into the dental laboratory via impressions and dental stone models. Metal master models were contaminated with Bacillus subtilis and Streptococcus sanguis. Impressions in alginate, polyvinylsiloxane, and polyether were used, and models were cast in two brands of dental stone. Samples were taken from the impression surfaces before and after casting, and sections from the stone models were taken by a technique developed for this study. After incubation, the numbers of colonies were counted and the numbers of colony-forming units per milliliter were calculated. The reductions were statistically significant but considered to be clinically insignificant. The conclusion is that even after severe contamination, the risks to dental laboratory personnel are minimal. "Normal" hygienic procedures are recommended instead of disinfection.

Alginates↗

PubMed2005

Modeling natural attenuation of benzene with analytical and numerical models.

Natural attenuation of benzene is a much-accepted technology for remediating low risk sites. To date, numerous protocols have been developed for assessing natural attenuation and measuring indicator parameters. Many models have additionally been developed to describe the advection, dispersion, sorption and biodegradation processes involved. It is evident that while there is extensive guidance in natural attenuation protocols for field sampling methodologies, less emphasis is placed on analyzing natural attenuation data for supporting appropriate model development. This paper presents methodologies for data analysis and interpretation that may be undertaken to achieve data reduction for modeling purposes. A case study is presented to illustrate the use of an analytical and a numerical natural attenuation model at the same site for predicting the time required to achieve the remedial goal at the site.

Benzene↗

PubMed1994

Re-examination of hexose exchanges using rat erythrocytes: evidence inconsistent with a one-site sequential exchange model, but consistent with a two-site simultaneous exchange model.

(1). The kinetic parameters of zero-trans net uptake and infinite-trans uptake of 3-O-methyl-D-glucoside, 2-deoxy-D-glucose and D-mannose into rat red cells at 24 degrees C were measured after taking account of the linear diffusion components of flux. (2). Zero-trans exists of 3-O-methyl-D-glucoside and D-mannose from rat cells were also measured. (3). After correction for linear flux via non-specific routes, the Vmax of zero-trans uptake of 3-O-methyl-D-glucoside was significantly higher, (1.25 +/- 0.06 mumol (10 min)-1 (ml cell water)-1) than the corresponding parameters of mannose or 2-deoxy-D-glucose, (0.33 +/- 0.01 and 0.39 +/- 0.01 mumol(10 min)-1 (ml cell water)-1, respectively; P < 0.001). (4). After correction for linear flux via non-specific uptake routes, the Vmax of zero-trans exit of 3-O-methyl-D-glucoside is significantly higher (1.70 +/- 0.1 mumol (10 min)-1 (ml cell water)-1) than the corresponding value for mannose exit flux, (1.10 +/- 0.1 mumol (10 min)-1 (ml cell water)-1; P < 0.001). (5). The acceleration ratio, i.e., the ratio of infinite-trans influx Vmax/zero-trans influx Vmax of mannose by mannose (9.12 +/- 0.03) is significantly higher than that of 3-O-methyl-D-glucose by 3-O-methyl-D-glucose (2.77 +/- 0.14)(P < 0.001). (6). The one-site simple carrier model of glucose transport in which sugar exchange is viewed as a sequential process, predicts that the acceleration ratio of the more rapidly moving sugar 3-O-methyl-D-glucose by 3-O-methyl-D-glucose should be greater than that of the slower sugar, mannose by mannose. Hence, the observed findings are inconsistent with the one-site model, but confirm the earlier disputed studies of Miller, D.M. (1968; Biophys. J. 8, 1329-1338). (7). A two-site model, in which sugar exchange is considered as a simultaneous process, predicts that the acceleration ratio of mannose influx by mannose should be higher than for 3-O-methyl-D-glucose by 3-O-methyl-D-glucose. The data are, therefore, consistent with a two-site model.

Animals↗

PubMed1993

Simulation of stochastic micropopulation models--II. VESPERS: epidemiological model implementations for spread of viral infections.

This second paper concerning stochastic micropopulation simulations describes VESPERS, which can serve as a framework for simulation models of the epidemic spread of infection. The versions described are implemented using the simulation shell, SUMMERS, which includes the generic commonalities of several micropopulation models. Population members in VESPERS move through states related to the individual's status relative to the infective agent. Features of the models include mixing groups, member demographics, susceptibility and infectiousness, and co-circulation of infectious agents. The sensitivity of the simulation outcomes to quantitative features of the model has been analyzed. The user can select reports of desired distributions and averages of simulation outcomes.

Demography↗

PubMed1996

A model of the flexion-extension motion in the elbow joint some problems concerning muscle forces modelling and computation.

This paper represents an application of a general approach for modelling and mathematical investigation of the human upper limb considered in a previous paper (Raikova, 1992, J. Biomechanics 25, 857-867). Six ways of modelling the muscle force in a plane are suggested and compared to each other. A model of the flexion-extension motion in the elbow joint is proposed, which includes two extensor muscles and three flexor muscles. An optimization task is formulated and solved analytically using the Lagrange multipliers method with an objective function dependent on the n-th power of the muscle forces moduli: sigma c(i)F(i)/(n) (n > 1). The influence of a set of parameters (the power n in the objective function, the weight coefficients c i, the lever arms of the modelled muscle forces, an external force applied to the forearm) on the predicted muscle forces and on the joint reaction is investigated. The results demonstrate that strictly positive and continuous solutions for all muscle forces (for synergistic muscles, also for the antagonistic ones) may be simultaneously predicted if the coefficients C(i) are properly chosen.

Algorithms↗

PubMedDate not supplied

The stochastic SI model with recruitment and deaths. I. Comparison with the closed SIS model.

We compare the stochastic and deterministic versions of an SI model with recruitment, background deaths, and deaths due to the disease. For the stochastic version, analysis of the mean number of susceptibles, mx, and infecteds, m(y), and of the means conditioned on nonextinction of the infection, m*x and m*y, shows that (1) if R0 < or = 1, the disease dies out monotonically for the deterministic and stochastic models, and (2) if R0 > 1, the disease dies out early with a probability close to (1/R0)a, where a is the number of infecteds introduced, or m(y) rises to a peak and then dies out slowly. For small populations, N, the peak is an obvious maximum. If N > or = 100, the peak in m(y) is hidden in a long, nearly stationary plateau and m*y is close to the deterministic endemic level for a large range of parameter values. The analytical results are illustrated with simulations. The results for the SI model are motivated by and compared with the corresponding results for the closed SIS model.

Communicable Diseases↗

PubMed1996

Structural identifiability of models characterizing saturable binding: comparison of pseudo-steady-state and non-pseudo-steady-state model formulations.

A two-state variable model in which saturable binding takes place is studied. Two mathematical representations of the same model are considered, one a standard chemical kinetics type of polynomial system, the other a reduced model formed from the original system via a pseudo-steady-state approximation. For a particular experiment the structural identifiability of the set of unknown parameters of each model is examined using the similarity transformation approach. The analysis shows that when the pseudo-steady-state approximation is made a certain degree of structural identifiability is lost in the sense that fewer individual parameters can be uniquely identified.

Kinetics↗

PubMed1985

Genetical ESS-models. II. Multi-strategy models and multiple alleles.

The problem of evolutionarily stable strategies (ESS) in sexual populations can be investigated by means of genetical ESS-models which link common sense, phenotypic ESS-models to an underlying genetical system. Thorough results are obtained for multi-strategy models in diploid, panmictic populations on the basis of multi-allelic, one-locus systems. A sexual population will be maintained at a phenotypic ESS if this can possibly be produced by the genotypes currently existing. If there is enough allelic variation, the corresponding gene pool may either be an ESS itself, or belong to an attracting, continuous set of states, which all determine the same evolutionarily stable population. The latter case allows new alleles to enter and spread in the gene pool without disturbing the phenotypic ESS. If a phenotypic ESS cannot be established, ESSs of the genetical model may be found which give rise to stable populations alternatively. Since these depend on the phenotypes determined by the currently existing genotypes, they may be destabilized by the occurrence of new mutations. In this sense, they are less durable than populations maintained at a phenotypic ESS and can be expected to evolve, in the long run, towards a phenotypic ESS.

Alleles↗

PubMed1993

Direct and indirect methods for the quantification of leg volume: comparison between water displacement volumetry, the disk model method and the frustum sign model method, using the correlation coefficient and the limits of agreement.

Volume changes can be measured either directly by water-displacement volumetry or by various indirect methods in which calculation of the volume is based on circumference measurements. The aim of the present study was to determine the most appropriate indirect method for lower leg volume calculation using water displacement volumetry as a 'golden standard'. For 20 male volunteers, age range: 20-35 years, the volume of both lower legs was determined directly by water-displacement volumetry, and indirectly by the frustum sign model method and the disc model method. Calculation of the correlation coefficient and the limits of agreement showed that water-displacement volumetry and the disc model method are interchangeable (r = +0.99, mean +/- 2s = -45 +/- 78 ml), whereas this does not hold for the frustum sign model (r = +0.93, mean +/- 2s = 521 +/- 238 ml). In the clinical situation volume measurement can be valuable for monitoring of the severity of oedema or haematoma occurrence after surgery or severe trauma. This non-invasive diagnostic aid may be a valuable adjuvant means of diagnosis for several volume dependent disorders of the extremities.

Adult↗

PubMed1984

The Environmental Model Segment Approach for estimating potential environmental concentrations. II. Application of the model to p-dichlorobenzene and trichloroethene.

The Environmental Model Segment Approach introduced in Part I (R. Frische, W. Klöpffer, G. Rippen, and K. O. Günther, 1984, Ecotoxicol. Environ. Safety 8, 352-362) is applied to estimate potential environmental concentrations of p-dichlorobenzene and trichloroethene. Environmental concentrations of these chemicals can be estimated only with difficulty by other models because of many diffuse emission sources emitting mainly into the air. By comparison of the estimated with the measured environmental concentrations it is found that the claim of the model to yield values which apply to reasonable unfavorable environmental boundary conditions is satisfied for the medium air and for the biota (fish). Soil concentrations calculated by means of the model, assuming equilibrium with high air concentrations, are higher than the measured ones, while the calculated water concentrations are lower than the values often measured for river waters. The latter fact may be attributed to direct input of the substances into water which is not being taken into account.

Air Pollutants↗

PubMedDate not supplied

Simulation of linear compartment models with application to nuclear medicine kinetic modeling.

Several techniques are evaluated for solving the linear ordinary differential equations arising from compartment models. The methods involve approximating the matrix exponential of the state matrix (i.e. the transition matrix). The computational efficiencies of these techniques, together with that of a general purpose differential equation solver, are compared for several models arising from radiopharmacokinetic studies. The matrix exponential calculations are performed using both Ward's Padé approximation method and an eigenvalue-eigenvector decomposition (QR factorization) of the matrix A. These two algorithms have been incorporated as simulation options into the programs of the ADAPT package. ADAPT consists of a set of high-level programs for simulation, parameter estimation and experiment design, developed primarily for basic and clinical research modeling and data analysis applications involving pharmacokinetic and pharmacodynamic processes. The advantages and disadvantages of these simulation strategies for solving linear kinetic models within a parameter estimation setting are illustrated and discussed.

Algorithms↗

PubMed1994

A new model for the agonistic binding site on the histamine H2-receptor: the catalytic triad in serine proteases as a model for the binding site of histamine H2-receptor agonists.

The historical model for the agonistic binding site on the histamine H2-receptor is based on a postulated activation mechanism: it has been suggested that the histamine monocation binds to the histamine H2-receptor via the formation of three hydrogen bonds. The cationic ammonium group in the side chain and the -NH- group in the tau-position of the imidazole act as proton donors, whereas the =N- atom in the pi-position of the imidazole acts as a proton acceptor. Participation of the ammonium group in H-bonding with a presumed negative charge on the receptor leads to a decrease in positive charge, which is thought to induce a tautomeric change in the imidazole ring system from N tau-H to N pi-H. A consequence of this tautomeric shift is the donation of a proton from the receptor to the agonist on one side, while on the other side a proton is donated from the agonist to the receptor. The propose tautomeric shift has been suggested to trigger the H2-stimulating effect. However, this model for the constitution of the agonistic binding site and the accessory activation mechanism cannot explain the weak histamine H2-activity of beta-histine and the activity of several other recently synthesized H2-agonists. Based on a thorough literature study and with the aid of molecular electrostatic potentials (MEPs) we demonstrate that the sulphur atom present in histamine H2-agonists as dimaprit and 2-amino-5-(2-aminoethyl)thiazole does not function as a proton acceptor, which implicitly means that a tautomeric shift is not a prerequisite for H2-stimulation. As a consequence, the model for the agonistic binding site is adjusted, resulting in a strong resemblance to the nature and orientation of the amino acids constituting the catalytic triad in serine proteases. Within this concept, the N pi-H tautomer of histamine is the biologically active form, in contrast with the existing model in which the N tau-H tautomer is the active form.

Animals↗

PubMed1990

Cochlear preprocessing in analog models, in digital models and in human inner ear.

Data of spontaneous, delayed, and simultaneously evoked otoacoustic emissions produced in human subjects are compared with data produced in an analog and in a digital wave-parameter realization of our model. The high probability of the frequency distance of the emission's extreme values at 0.4 Bark, corresponding to a local distance of 0.5 mm along the basilar membrane is found also in the models. The influence of the lateral feedback coupling in the models on the frequency selectivity and on the appearance of emissions show the high quality of simulating human peripheral signal processing by the models.

Cochlea↗

PubMed2005

Three-dimensional computer-generated head model reconstructed from cephalograms, facial photographs, and dental cast models.

PURPOSE: Three-dimensional (3D) computer models of the human craniofacial structure have been constructed with computed tomography (CT). However, the high cost of CT and the radiation exposure are drawbacks to this method. Attempts to create a 3D reconstruction from lateral and frontal cephalograms have failed because of problems with magnification, distortion, and limitations of landmark identification, among others. We introduce a new method that creates a standard head model for a patient from anatomic measurement points extracted from x-ray images, facial stereo photographs, and dental casts. MATERIAL: To obtain precise 3D coordinates from cephalograms, several equations were introduced to compensate for radiographic image magnification and distortion. RESULTS: By comparing the constructed model and 3D-CT images, this method proved to be accurate. CONCLUSIONS: It is possible to produce a 3D head model on a personal computer and to view it from any desired angle; this will provide easy-to-understand information for patients and establish a diagnostic or therapeutic method for communication with other health care providers.

Adult↗

PubMed2005

Pharmacokinetic-pharmacodynamic modelling of antibacterial activity of cefpodoxime and cefixime in in vitro kinetic models.

The bacterial time-kill curves of cefpodoxime and cefixime against four bacterial strains (Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae/penicillin sensitive and S. pneumoniae/penicillin intermediate) were compared in in vitro infection models in which various human pharmacokinetic profiles of unbound antibiotic were simulated. This approach offers more detailed information than the minimum inhibitory concentration (MIC) does about the time course of antibacterial efficacy of an antibiotic. A pharmacokinetic-pharmacodynamic (PK-PD) model based on unbound antibiotic concentrations at the site of infection, and a sigmoid Emax-relationship with EC50 as the antibiotic concentration necessary to produce 50% of the maximum effect, effectively described the antimicrobial efficacy of both cefpodoxime and cefixime. The EC50 values of cefpodoxime and cefixime were consistent with their respective MIC values. Both antibiotics had similar high potency against H. influenzae (EC50: 0.04 mg/L) and M. catarrhalis (EC50: 0.12 mg/L), while the potency of cefpodoxime against S. pneumoniae strains was about 10-fold higher than that of cefixime (EC50s/sensitive strain: 0.02 mg/L versus 0.27 mg/L; EC50s/intermediate strain: 0.09 mg/L versus 0.69 mg/L). Applications of this model and unbound tissue PK profiles obtained from a previous clinical study performed in our group, showed that cefpodoxime has higher bacteriological potency than cefixime against S. pneumoniae. Simulations based on this model allow the comparison of antibacterial efficacy of different antibiotics and dosing regimens.

Anti-Bacterial Agents↗

PubMed2004

Modelling effect of physical and chemical parameters on heat inactivation kinetics of hepatitis A virus in a fruit model system.

While thermal destruction of pathogenic bacteria has been thoroughly studied in food industry, heat inactivation of viruses in food has been poorly investigated. Experiments were carried out to characterize the effects of controlled physical and chemical characteristics of a food matrix upon heat resistance parameters (D and z values) of hepatitis A virus (HAV), taken as model because of its reported heat resistance. Sucrose content (28-52 degrees Brix), calcium concentration (90-1700 mg kg(-1)) and pH (3.3-4.3) were selected for possible influence on thermal inactivation of HAV in strawberry mashes and thus included in an experimental design according to a Doehlert matrix. Use of this design not only allowed to detect and quantify the direct influence of sucrose concentration upon the D85 degrees C value to be higher than the one of pH, but also to reveal a sucrose concentration/pH specific interaction, while no effect of calcium concentration was evidenced. Although the model cannot be directly used to predict heat resistance in real fruit systems, because of differences observed between predicted and measured D85 degrees C values, it is useful for predicting the trends and relative changes in D values due to sucrose concentration and pH variations. Results suggested possible effects of other constituents of strawberry products on heat resistance of HAV and confirmed the importance of experimental validation of any model-derived process. Nevertheless, such a modelling approach using response surface methodology provides a rapid answer to heat resistance evaluation of a food-borne virus as a function of specific physical and chemical parameters of specific food products.

Calcium↗

PubMed2005

Cell division theory and individual-based modeling of microbial lag: part II. Modeling lag phenomena induced by temperature shifts.

This paper is the second in a series of two, and studies microbial lag in cell number and/or biomass measurements caused by temperature changes with an individual-based modeling approach. For this purpose, the theory of cell division, as discussed in the first part of this series of research papers, was implemented in the individual-based modeling framework BacSim. Simulations of this model are compared with experimental data of Escherichia coli, growing in an aerated, glucose-rich medium and subjected to sudden temperature shifts. The premise of a constant cell volume under changing temperature conditions predicts no lag in cell numbers after the shift, in contrast to the experimental observations. Based on literature research, two biological mechanisms that could be responsible for the observed lag phenomena are proposed. The first assumes that the average cell volume depends on temperature while the second assumes that a lag in biomass growth occurs after the temperature shift. For a lag in cell number caused by an increased average cell volume, the cell biomass always increases at the maximal rate. Therefore, cells are evidently not stressed and do not have to adapt to the new conditions, as opposed to a lag in biomass growth. Implementation and simulation of both mechanisms are found to describe the experimental observations equally well. Therefore, further research is needed to distinguish between the two mechanisms. This can be done by observing, in addition to cell numbers, a measure for the average cell volumes. In conclusion, the individual-based modeling approach is a good methodology to investigate and test biological theories and assumptions. Also, based on the simulations, suggestions for further experimental observations can be made.

Algorithms↗

PubMed2006

Probabilistic biokinetic modelling of radiocaesium uptake in Arctic seal species: verification of modelled data with empirical observations.

The necessity to provide information about radionuclide concentrations in Arctic marine species has been heightened in recent years due to a number of accidents in Arctic regions involving nuclear vessels and the presence of a large number of potential radioactive contamination sources. The provision for such information is largely dependent on the use of radionuclide uptake and transfer models. The uptake of radionuclides in Arctic seal species in this study has been modelled using a probabilistic biokinetic approach. In this paper, model results are compared with empirical data from relevant samples taken within the Arctic region. Results indicate that the model performs well when estimating concentrations of (137)Cs in two seal species for both median values and reproduction of the distribution of data values, but not as well for a third seal species. Likely factors affecting the results are the probability density functions used for the input parameters.

Animals↗