Influence of cell-cell contact on yeast interactions and exo-metabolome

Abstract

Alcoholic fermentation is the main step for winemaking, mainly performed by the yeast Saccharomyces cerevisiae. But other wine yeasts called non-Saccharomyces may contribute to alcoholic fermentation and modulate the wine aroma complexity. The recurrent problem with the use of these non-Saccharomyces yeasts is their trend to die off prematurely during alcoholic fermentation, leading to a lack of their interesting aromatic properties searched in the desired wine. This phenomenon appears to be mainly due to interactions with S. cerevisiae. These interactions are often negatives but remain unclear because of the species and strain specific response. Among the non-Saccharomyces yeasts, Lachancea thermotolerans is a wine yeast naturally found in grape must and well known as a great L-lactic acid producer and an aromatic molecules enhancer, but its behavior during alcoholic fermentation can be completely different in co-fermentation with S. cerevisiae in function of strain used. Thus, S. cerevisiae/L. thermotolerans couple was used to unravel interactions between these two species during alcoholic fermentation. Thanks to a modified S. cerevisiae strain expressing a GFP allowing discrimination between yeast populations, both yeast viability was monitored by flow cytometry in pure and sequential fermentations of grape must with or without cell-cell contact. This reveal a decrease in cell viability for S. cerevisiae in both conditions with a greater decrease in case of cell-cell contact. Investigating the nature of the interactions, our results demonstrate a competition between species for nitrogen compounds, oxygen and for the first time a competition for must sterols. Volatile compounds analysis revealed changes in sequential fermentations compared to pure fermentations and showed also that cell-cell contact modify yeast metabolism since the volatile compound profile was significantly different from sequential fermentation without cell-cell contact. Yeast metabolism modifications associated with cell-cell contact were confirmed further by analyzing the exo-metabolome of all fermentations by FT-ICR-MS analysis. These analyses show for the first time a specific metabolite production and quantitative metabolite changes linked to each fermentation condition. This study shows that cell-cell contact not only impact cell viability as already reported but deeply changes the yeast metabolism.