Diffusion in supported lipid bilayers: influence of substrate and preparation technique on the internal dynamics.

The diffusion law of DMPC and DPPC in Supported Lipid Bilayers (SLB), on different substrates, has been investigated in details by Fluorescence Recovery After Patterned Photobleaching (FRAPP). Over micrometer length scales, we demonstrate the validity of a purely Brownian diffusive law both in the gel and the fluid phases of the lipids. Measuring the diffusion coefficient as a function of temperature, we characterize the gel-to-liquid phase transition of DMPC and DPPC. It is shown that, depending on the type of substrate and the method used for bilayer preparation, completely different behaviours can be observed. On glass substrates, using the Langmuir-Blodgett deposition technique, both leaflets of the bilayer have the same dynamics. On mica, the dynamics of the proximal leaflet is slower than the dynamics of the distal leaflet, although the transition temperature is the same for both layers. Preparing bilayers from vesicle fusion in same conditions leads to more random behaviours and shifted transition temperatures.

Fluctuations and destabilization of single phospholipid bilayers.

Supported phospholipid bilayers are interesting model systems for biologists and present fascinating physical properties. The authors present an extensive experimental study of the dynamic properties of supported bilayers. The structure and the equilibrium properties of single and double supported bilayers were investigated with neutron reflectivity. The submicronic fluctuation spectrum of a nearly free "floating" bilayer was determined using off-specular x-ray scattering: the surface tension of the bilayer, its bending modulus, and the intermembrane potential could be determined. Using fluorescence microscopy, the authors showed that this well-controlled single bilayer can form vesicles. Destabilization occurred either at the main gel-fluid transition of the lipids and could be interpreted in terms of a decrease in the bending rigidity or under an ac low-frequency electric field applied in the fluid phase. In the latter case, the authors also studied the effect of the electric field at the molecular length scale by neutron reflectivity. In both cases, destabilization leads to the formation of relatively monodisperse vesicles. This could give further understanding on the vesicle formation mechanism and on the parameters that determine the vesicle size.

Effect of an electric field on a floating lipid bilayer: A neutron reflectivity study.

We present here a neutron reflectivity study of the influence of an alternative electric field on a supported phospholipid double bilayer. We report for the first time a reproducible increase of the fluctuation amplitude leading to the complete unbinding of the floating bilayer. Results are in good agreement with a semi-quantitative interpretation in terms of negative electrostatic surface tension.

[Emphysematous pyelonephritis]. / La pyélonéphrite emphysémateuse.

Pyelonephritis is a frequent infection of variable severity. Mortality is low since the era of antibiotics. Emphysematous pyelonephritis is however a rare but life threatening form of kidney infection, which affect more frequently diabetic subjects, characterized by gas producing bacteria. These gaseous lesions can be localized in the renal parenchyma or in the perirenal or retroperitoneal space. Diagnosis became easier with the advent of CT scan. Treatment usually requires surgery and antibiotics. We report a case of emphysematous pyelonephritis in a diabetic patient. The CT scan strongly suggested the diagnosis which was confirmed per-operatively. Proteus mirabilis is exceptionally described in emphysematous pyelonephritis.