Biomedical subjects

R Labarbe

Publications and source records attributed to R Labarbe.

6 recordsLinked to original sources

PubMed2001

A continuous culture biofilm model of cariogenic responses.

AIMS: To validate an in vitro model for the analysis of physiological and ecological responses to sugar challenge in bacterial populations, and subsequent changes in enamel mineralization. METHODS AND RESULTS: A seven-organism bacterial consortium was grown in a biofilm mode on enamel and hydroxyapatite (HA) surfaces in a continuous culture system and exposed to repeated sucrose challenges. This produced 'pH-cycling' conditions within the system. Populations on HA surfaces were enumerated. Changes in relative proportions of the different populations, and in the total viable count, were observed, between different treatments. Microradiography of the enamel sections showed increasing demineralization with increasing sucrose concentration. The lesions formed were similar to 'white-spot' lesions found in vivo. Differences in the quality of biofilms formed were also observed using Confocal Laser Scanning Microscopy. CONCLUSION: An in vitro model has been validated for the analysis of both physiological and ecological responses to sucrose challenges in bacterial populations, and subsequent changes in enamel mineralization. SIGNIFICANCE AND IMPACT OF THE STUDY: This model should facilitate the study of changes in bacterial populations in response to application of putative anticaries agents and concomitant changes in enamel mineralization.

Bacteria↗

PubMed1998

Calorimetric investigation of ethidium and netropsin binding to chicken erythrocyte chromatin.

We have investigated the thermodynamic aspects of the ligand binding to chromatin, using isothermal titration calorimetry. Two classical DNA ligands were used: an intercalator, ethidium bromide, and a minor groove binder, netropsin. Stoichiometry, affinity constant, and thermodynamic parameters were determined at various salt concentrations and different temperatures. The effect of ionic strength was analyzed according to the Record theory applied to chromatin. We also compared the binding parameters on naked DNA, H1/H5-depleted chromatin, and chromatin. We demonstrated that the presence of histones on DNA still allows the ligand binding that takes place according to a simple one single-site model. For both ligand types, the thermodynamic driving force is enthalpic and the association is characterized by a somewhat weaker affinity and more scattered ligand distribution than on naked DNA. The ligand affinity is weakly altered by the salt-induced compaction of the chromatin and the binding is accompanied by a release of one counterion per ligand molecule. The temperature-dependent studies revealed the existence of a small heat capacity change associated with ligand binding to chromatin, together with an enthalpy-entropy compensation that maintains the free energy constant over the investigated temperature range.

Animals↗

PubMed1996

Polyelectrolyte counterion condensation theory explains differential scanning calorimetry studies of salt-induced condensation of chicken erythrocyte chromatin.

The salt-induced chromatin condensation in chicken erythrocyte nuclei is studied by differential scanning calorimetry (DSC). The degree of chromatin condensation is measured for condensation induced by monovalent, divalent, trivalent, or tetravalent cations and by a mixture of sodium and magnesium. These last two cations show an evident competition effect. Salt-induced chromatin condensation is shown to be an entropy-driven process. A simple model of chromatin based on the polyelectrolyte counterion condensation theory is used in order to compute the charge neutralized by the cations in each chromatin domain. The degree of chromatin condensation is shown to be related to the weighed sum of the square of the phosphate charge of each domain. The model predicts the salt and the chromatin concentration dependence of the condensation and the effect of H1 removal.

Animals↗

PubMed1996

23Na NMR study of the effect of organic osmolytes on DNA counterion atmosphere.

The effect of different organic osmolytes on the DNA counterion condensation layer has been investigated by 23Na NMR relaxation measurements. The zwitterionic compounds glycine, beta-alanine, 4-aminobutyric acid, and 6-aminocaproic acid have shown an increasing capacity to decrease the amount of sodium ions in the vicinity of the macromolecule. The experimental data have been correlated with the dielectric constant increase in their corresponding solutions and have been compared with the prediction of counterion condensation theory. Polyols (sorbitol and mannitol) did not display the same effect. These compounds largely increase the relaxation rate of sodium ions in the proximity of DNA, unlike the zwitterionic compounds. This probably results from a perturbation of the water dynamic around the macromolecule, of the primary or secondary hydration shell of the sodium nuclei involved, or both.

Alcohols↗

PubMed1996

Dielectric constant and ionic strength effects on DNA precipitation.

We have investigated the effect of different zwitterionic compounds on DNA precipitation induced by spermine4+. Glycine, beta-alanine, 4-aminobutyric acid, and 6-aminocaproic acid have shown an increasing capacity to attenuate DNA precipitation. This protection effect has been correlated with the dielectric constant increase of their corresponding solutions. Calculations based on these experimental data and counter-ion condensation theory have confirmed the importance of this parameter for DNA-ion interactions and precipitation mechanisms. We have also observed a resolubilization of DNA in the presence of 6-aminocaproic acid at high spermine4+ concentration and in the presence of glycine at high spermidine3+ concentration. This could be explained by an increase of screening effect with polyamine concentration.

Aminocaproic Acid↗

PubMed1995

Osmotic effectors and DNA structure: effect of glycine on precipitation of DNA by multivalent cations.

We have investigated the effect of glycine (an organic osmolyte) on DNA precipitation induced by spermine4+, spermidine3+ and Tb3+ addition, using circular dichroism (CD), UV spectroscopy (UV), and electric linear dichroism (ELD) techniques. DNA precipitation by the three compounds is perturbed by glycine: more spermine4+, spermidine3+ and Tb3+ must be added to obtain the same extent of precipitation as compared to the behaviour in absence of this organic osmolyte. It seems that glycine has a general effect on the DNA environment. Calculations based on experimental results and Manning's counterion condensation theory show that glycine could modify the electrostatic environment of DNA as a consequence of a change in dielectric constant.

Animals↗