Flocculation of Saccharomyces cerevisiae: inhibition by sugars

Flocculation is governed by the competition between electrostatic repulsion (nonspecific interactions) and polysaccharide–protein bonds (specific interactions). In our study, the inhibition of flocculation by sugars for 12 strains of Saccharomyces cerevisiae leads us to extend the classification described in the literature and to define three groups of yeasts: flocculation mannose sensitive (MS), flocculation glucose–mannose sensitive (GMS), and flocculation mannose insensitive (MI). Only the first two groups showed specific interactions between proteins and mannans. In the MI group, the sugars tested did not inhibit flocculation. To characterize the particularities of the stereochemistry of the cell-wall proteic receptors of strains belonging to the MS and GMS groups, 31 sugars were used as inhibitor probes on two representative strains. The results show that the lectin specificity of strains belonging to the GMS group is less restricted regarding C-1 and C-2 hydroxyl groups than the lectin from strains belonging to the MS group, which interacts with all of the hydroxyl groups of mannopyranose. The two groups also differ with respect to inhibition by sugars: strains belonging to the MS group are partially inhibited whereas strains of the GMS group are completely inhibited. We observed that the presence of ethanol increases sugar fixation by strains from the MS group, but not from the GMS group. Moreover, both receptors interact with disaccharides, provided the two monomers are linked by an α(1 → 4), α(1 → 3), or α(1 → 2) bond. Key words: yeast flocculation, proteic receptors, sugars, lectins.