Yeast biofilm-like behaviour in rich or poor media in the presence of polyphenols.

The yeast cell wall is a dynamic structure in which changes in protein composition occur depending on nutrient availability and environmental conditions. Cell wall mannoproteins are responsible both for yeast interaction with various environmental substances, such as wine phenolic compounds, and cell-cell recognition events which lead yeast to grow as a biofilm, mat colonies, or display invasive and pseudohyphal forms, here defined as biofilm-like behaviour. Significant differences in colour, polyphenolic index and anthocyanin content of wines fermented with different yeast strains have been reported. The use of the chromogenic medium Grape skin agar (Graski) to evaluate the aptitude of yeasts to adsorb coloured phenolic compounds has allowed the inheritability of the trait Wine Colour Adsorption to be demonstrated. Yeast biofilm-like phenotypes depend on the expression of the FLO gene family but also on carbon and nitrogen availability. The aim of this work was to study changes both in yeast biofilm-like behaviour and in Wine Colour Adsorption ability in order to understand the link between these phenotypes. The study was carried out in rich or poor media, concerning carbon and nitrogen concentration, supplemented with polyphenols. Eleven wine autochthonous Saccharomyces cerevisiae strains, belonging to the microbial collection of the Department of Scienze e Tecnologie Agro-Forestali e Ambientali, and two control strains S. cerevisiae Σ1278b, (MATa), which has adhesive and filamentous phenotypes and S. cerevisiae BY4742, (MATα, Δflo8), which has a non-adhesive phenotype, belonging to the Department of Agriculture and Food Systems, were used. The strains were tested for: a) biofilm formation in synthetic complete medium with 2% or 0.1% of dextrose, in Synthetic Low Ammonium Dextrose (SLAD) with 2% or 0.1% of dextrose, and in the same media supplemented with 100 mg/L of (+)-catechin; b) invasive growth in solid YPD with 2% or 0.1% of dextrose, in solid SLAD with 2% or 0.1% of dextrose, in the same media supplemented with 100 mg/L of (+)-catechin, and in solid Graski with 2% or 0.1% of dextrose, in solid SLAD with 2% or 0.1% of dextrose supplemented with skins from black grapes; c) growth as mat colonies in the same media used for invasive growth but using 0.3% of agar; d) polyphenolic compound adsorption in solid Graski with 2% or 0.1% of dextrose and in solid SLAD with 2% or 0.1% of dextrose supplemented with skins from black grapes. In order to check the experimental condition influences on cell morphology, the strains were microscopically observed. Results demonstrate that polyphenolic compounds modify in a different way the biofilm-like yeast phenotypes, and this behaviour varies amongst the yeast strains. The genetic basis of this difference is currently being explored and elucidated (Fig. 1). These phenotypes and the Wine Colour Adsorption phenotype could be considered not fully correlated strain-specific phenotypes, probably due to the simultaneous or not presence of the responsible cell wall proteins which determines a variety of phenotype combinations.