Résumé
Microbial communities are ubiquitous across earth ecosystems. Thanks to metagenomic methods, new data is gathered every day about their species composition. However very little is known about the ecological processes governing their patterns of biodiversity and how the species forming these consortia interact together. Wheat sourdough's microbial communities are relatively simple communities which can be a useful model to address these questions. Sourdough communities are composed of an assemblage of lactic acid bacteria and yeasts of which species composition vary between sourdoughs. It has been hypothesized that yeasts and LAB interact together mainly in non-competitive ways through mutualistic supply of important metabolites, such as glucose release by LAB, or vitamins release by yeasts to the benefits of each other. However interspecific interactions in sourdough have not been extensively studied. Consequently, the validity and generality of these hypotheses, as well as the nature of interactions between yeasts or between LABs species remains to be tested.In this study, we have selected different communities sampled from farmer-bakers' organic sourdoughs. Then we used coculture experiments to characterized the nature of interactions (competition, facilitation, …) between pairs of species (LAB/Yeast) and to test for different mechanisms of interaction. We have shown that LAB presence tends to decrease yeast density whereas yeast presence tends to increase LAB density. However, results are contrasted depending on species and strains. To explore this pattern further, we built a mathematical model of yeast/LAB coculture adapted from a competition Lotka-Volterra model. In this model, we used the working hypothesis that species interactions are the net results of multiple mechanisms. To assess the minimal “mechanistic complexity” needed to explain the observed pattern, we are currently exploring the parameters space using stochastic simulations