Conserver un aliment vivant

Cet article s’intéresse aux pratiques et aux représentations des consommateurs en France d’une boisson fermentée, le kéfir. Cette boisson artisanale aux propriétés mal connues est produite à partir d’un ferment. Elle connaît un regain d’intérêt dans les médias qui la présente comme un aliment dont la consommation aurait des effets positifs sur la santé. Cette innovation alimentaire fondée sur sa qualité d’aliment-santé conduit à redécouvrir des pratiques et des savoir-faire qui étaient jusque-là cantonnés à la sphère domestique au sein de laquelle ils étaient transmis. En nous appuyant sur l’analyse des pratiques et des représentations des buveurs de kéfir et les annonces des personnes diffusant le ferment (Leboncoin), l’article décrit les logiques de consommation et dégage les modes de circulation de ce produit (les relations interpersonnelles et Internet). Il apparaît que la configuration actuelle (les arguments sur les bénéfices pour la santé, la diffusion sur internet des ferments et des informations) favorise une nouvelle dynamique de diffusion et de consommation tout en conservant, à l’échelle des buveurs, une très grande diversité de représentations et de pratiques.This article focuses on the practices and representations of consumers in France of a fermented beverage, the Kefir. This artisanal drink with little-known properties is produced from a ferment. There is a renewed interest in the media, which presents it as a food whose consumption would have positive effects on health. This food innovation, based on its qualification as a healthy-food, leads to the rediscovery of practices and know-how that were until recently confined to the domestic sphere in which they were transmitted. Based on the analysis of the practices and representations of the Kefir drinkers and the advertisements of the ferment within “leboncoin” website, the article describes the logics of consumption and describes the circulation patterns of this product (interpersonal relationships and Internet). It appears that the current configuration (the arguments on the health benefits, the diffusion on the Internet of ferments and informations) promotes a new dynamic of diffusion and consumption while maintaining, at the scale of the drinkers, a very great diversity of representations and practices

Relationship between damage and permeability in liner-less composite tanks : experimental study at the lamina level

The aim of this study is to provide a relevant description of damage growth and the resultant crack network to predict leaks in liner-less composite vessels. Tensile tests were carried out on three different laminates: [02/90n/02], [+45/-45]2s and [0/+67.5/-67.5]s. Number n varies from 1 to 3 in order to study the effect of ply thickness. Transverse crack and delamination at crack tips were identified with an optical microscope during tensile loading. A length of 100 mm was observed for several loading levels to evaluate statistical effects. Results highlight a preliminary step in the damage scenario with small crack densities before a second step where the crack growth speeds up. In bulk, cross-section examinations showed that no delamination occurred at crack tip in the material of the study (M21 T700). Cross-section examinations were also performed on [+45/-45]2s and [0/+67.5/-67.5]s layups in order to bypass the issue of free edge effects. Damage state in those layups was shown to be significantly different in the bulk than at the surface. Observations of the damage state in bulk for those layups demonstrated that there is no transverse crack in [+45/-45]2s specimens subjected to shear strains up to 4%, and that interactions between damage of consecutive plies strongly impact both the damage kinetics and the arrangement of cracks. These elements are fundamental for the assessment of permeability performance, and will be introduced in the predictive model.Co-financement CNES et Région Aquitain

A Physical Model for the Condensation and Decondensation of Eukaryotic Chromosomes

During the eukaryotic cell cycle, chromatin undergoes several conformational changes, which are believed to play key roles in gene expression regulation during interphase, and in genome replication and division during mitosis. In this paper, we propose a scenario for chromatin structural reorganization during mitosis, which bridges all the different scales involved in chromatin architecture, from nucleosomes to chromatin loops. We build a model for chromatin, based on available data, taking into account both physical and topological constraints DNA has to deal with. Our results suggest that the mitotic chromosome condensation/decondensation process is induced by a structural change at the level of the nucleosome itself

Chromatin Fiber Dynamics under Tension and Torsion

Genetic and epigenetic information in eukaryotic cells is carried on chromosomes, basically consisting of large compact supercoiled chromatin fibers. Micromanipulations have recently led to great advances in the knowledge of the complex mechanisms underlying the regulation of DNA transaction events by nucleosome and chromatin structural changes. Indeed, magnetic and optical tweezers have allowed opportunities to handle single nucleosomal particles or nucleosomal arrays and measure their response to forces and torques, mimicking the molecular constraints imposed in vivo by various molecular motors acting on the DNA. These challenging technical approaches provide us with deeper understanding of the way chromatin dynamically packages our genome and participates in the regulation of cellular metabolism

Rad51 Polymerization Reveals a New Chromatin Remodeling Mechanism

Rad51 protein is a well known protagonist of homologous recombination in eukaryotic cells. Rad51 polymerization on single-stranded DNA and its role in presynaptic filament formation have been extensively documented. Rad51 polymerizes also on double-stranded DNA but the significance of this filament formation remains unclear. We explored the behavior of Saccharomyces cerevisiae Rad51 on dsDNA and the influence of nucleosomes on Rad51 polymerization mechanism to investigate its putative role in chromatin accessibility to recombination machinery. We combined biochemical approaches, transmission electron microscopy (TEM) and atomic force microscopy (AFM) for analysis of the effects of the Rad51 filament on chromatinized templates. Quantitative analyses clearly demonstrated the occurrence of chromatin remodeling during nucleoprotein filament formation. During Rad51 polymerization, recombinase proteins moved all the nucleosomal arrays in front of the progressing filament. This polymerization process had a powerful remodeling effect, as Rad51 destabilized the nucleosomes along considerable stretches of DNA. Similar behavior was observed with RecA. Thus, recombinase polymerization is a powerful mechanism of chromatin remodeling. These remarkable features open up new possibilities for understanding DNA recombination and reveal new types of ATP-dependent chromatin dynamics

Intrinsic flexibility of B-DNA: the experimental TRX scale

B-DNA flexibility, crucial for DNA–protein recognition, is sequence dependent. Free DNA in solution would in principle be the best reference state to uncover the relation between base sequences and their intrinsic flexibility; however, this has long been hampered by a lack of suitable experimental data. We investigated this relationship by compiling and analyzing a large dataset of NMR 31P chemical shifts in solution. These measurements reflect the BI ↔ BII equilibrium in DNA, intimately correlated to helicoidal descriptors of the curvature, winding and groove dimensions. Comparing the ten complementary DNA dinucleotide steps indicates that some steps are much more flexible than others. This malleability is primarily controlled at the dinucleotide level, modulated by the tetranucleotide environment. Our analyses provide an experimental scale called TRX that quantifies the intrinsic flexibility of the ten dinucleotide steps in terms of Twist, Roll, and X-disp (base pair displacement). Applying the TRX scale to DNA sequences optimized for nucleosome formation reveals a 10 base-pair periodic alternation of stiff and flexible regions. Thus, DNA flexibility captured by the TRX scale is relevant to nucleosome formation, suggesting that this scale may be of general interest to better understand protein-DNA recognition

Nucleosome Chiral Transition under Positive Torsional Stress in Single Chromatin Fibers

Using magnetic tweezers to investigate the mechanical response of single chromatin fibers, we show that fibers submitted to large positive torsion transiently trap positive turns, at a rate of one turn per nucleosome. A comparison with the response of fibers of tetrasomes (the (H3-H4)2 tetramer bound with ~50 bp of DNA) obtained by depletion of H2A-H2B dimers, suggests that the trapping reflects a nucleosome chiral transition to a metastable form built on the previously documented righthanded tetrasome. In view of its low energy, <8 kT, we propose this transition is physiologically relevant and serves to break the docking of the dimers on the tetramer which in the absence of other factors exerts a strong block against elongation of transcription by the main RNA polymerase.Comment: 33 pages (double spacing), 7 figure

A Tale of Four Stories: Soil Ecology, Theory, Evolution and the Publication System

International audienceBACKGROUND: Soil ecology has produced a huge corpus of results on relations between soil organisms, ecosystem processes controlled by these organisms and links between belowground and aboveground processes. However, some soil scientists think that soil ecology is short of modelling and evolutionary approaches and has developed too independently from general ecology. We have tested quantitatively these hypotheses through a bibliographic study (about 23000 articles) comparing soil ecology journals, generalist ecology journals, evolutionary ecology journals and theoretical ecology journals. FINDINGS: We have shown that soil ecology is not well represented in generalist ecology journals and that soil ecologists poorly use modelling and evolutionary approaches. Moreover, the articles published by a typical soil ecology journal (Soil Biology and Biochemistry) are cited by and cite low percentages of articles published in generalist ecology journals, evolutionary ecology journals and theoretical ecology journals. CONCLUSION: This confirms our hypotheses and suggests that soil ecology would benefit from an effort towards modelling and evolutionary approaches. This effort should promote the building of a general conceptual framework for soil ecology and bridges between soil ecology and general ecology. We give some historical reasons for the parsimonious use of modelling and evolutionary approaches by soil ecologists. We finally suggest that a publication system that classifies journals according to their Impact Factors and their level of generality is probably inadequate to integrate "particularity" (empirical observations) and "generality" (general theories), which is the goal of all natural sciences. Such a system might also be particularly detrimental to the development of a science such as ecology that is intrinsically multidisciplinary

Structural plasticity of single chromatin fibers revealed by torsional manipulation

Magnetic tweezers are used to study the mechanical response under torsion of single nucleosome arrays reconstituted on tandem repeats of 5S positioning sequences. Regular arrays are extremely resilient and can reversibly accommodate a large amount of supercoiling without much change in length. This behavior is quantitatively described by a molecular model of the chromatin 3-D architecture. In this model, we assume the existence of a dynamic equilibrium between three conformations of the nucleosome, which are determined by the crossing status of the entry/exit DNAs (positive, null or negative). Torsional strain, in displacing that equilibrium, extensively reorganizes the fiber architecture. The model explains a number of long-standing topological questions regarding DNA in chromatin, and may provide the ground to better understand the dynamic binding of most chromatin-associated proteins.Comment: 18 pages, 7 figures, Supplementary information available at http://www.nature.com/nsmb/journal/v13/n5/suppinfo/nsmb1087_S1.htm