Les microorganismes sont essentiels au maintien et au fonctionnement des écosystèmes. Dans certains produits alimentaires, la communauté microbienne, composée principalement de levures et de bactéries lactiques, fermente les sucres, affecte positivement les qualités organoleptiques du produit et augmente sa durée de conservation. C’est le cas du levain de panification. Cette thèse visait principalement à décrire les patterns de diversité des levures de levains français produits à partir de farine issue de l’agriculture biologique et à conserver une partie de cette diversité. Grâce à des méthodes d’analyses culturales et non culturales de la diversité des levures, nous avons mis en évidence une diversité spécifique caractéristique de chaque levain étudié. Nous avons aussi montré la dominance du genre Kazachstania et la convergence des souches isolées de levain pour la capacité à consommer du raffinose et du saccharose. Bien que la densité et la composition en espèces varient entre deux levains, aucune structuration spatiale de la diversité n’a été mise en évidence. Les pratiques de panification et leurs effets sur la diversité des communautés microbiennes ont également été analysés. Deux typologies de pratiques de panification (plutôt « intensives » et plutôt « extensives »), affectant l’espèce dominante des populations de levures, ont été révélées. / Microorganisms are essential for the maintenance and functioning of the ecosystems. In some food products, microbial community, mainly composed by yeasts and lacid actic bacteria which ferment sugars, positively affects organoleptic qualities and shelf life of the product. It is the case of bread sourdough. This PhD aimed at describing yeast diversity patterns of organic French bread sourdoughs and conserving a part of this diversity. Using cultural-Based and non-Cultural based analyses of yeast diversity, we highlighted a unique specific diversity of each sourdough. We also shown the predominance of Kazachstania genus and a convergence of sourdoughs’ yeasts isolates for the ability to consume raffinose and sucrose. Although density and species composition varied between sourdoughs, no spatial pattern was highlighted. Bread-Making practices and their effects on microbial communities’ diversity were also analyzed. Two bread-Making practice typologies (quite « intensive » and quite « extensive »), affecting dominant yeast species were also revealed.

Long-time research of chronologically aging yeast populations of Saccharomyces cerevisiae laboratory strains revealed that yeasts are able to differentiate into specialized cell types. Differentiation of liquid cultures growing in glucose rich medium and differentiation of colonies growing on solid glycerol medium has been previously studied. These populations create two fractions of cells with diverse morphology which adapt their metabolism and physiology to enable a long-term survival of the yeast population in environment with limited nutritional potential. In this study, yeast subpopulations isolated from colonies growing on solid glucose medium and liquid cultures cultivated in glycerol medium were characterized. Newly isolated cell types were compared with already known cell types isolated from colonies and liquid cultures. Selected metabolic processes and stress resistance were analysed in studied populations. Based on previous studies of yeast differentiation, a spectrum of GFP-labelled marker proteins was choosen and production and localization of these marker proteins was monitored within yeast populations. Results of the analyses showed that type of medium and cultivation influence development and metabolism of each yeast cell type. Key words: Saccharomyces cerevisiae, BY4742, cell...