The role of Flo proteins in yeast cell adhesion on glass.

Microorganisms might have the ability to adhere to surfaces, often leading to the formation of biofilms. Depending on the microorganism and the circumstances, biofilm formation may be harmful or advantageous. The yeast Saccharomyces cerevisiae may also show the ability to adhere to surfaces and initiate biofilm formation. As the mannoprotein layer of the cell wall is the primary interface with the environment, cell surface glycoproteins will mediate adherence. In this study, we focused on the role of adhesins or flocculins, which are cell surface proteins encoded by the FLO (flocculation) genes, in yeast cell adhesion on glass. An advantageous aspect of yeast cell adhesion on glass can be found in the formation of a stable yeast deposit during bottle conditioning or refermentation of specialty beers. Bottle refermentation of beer is a process in which mature beer is inoculated with yeast and fermentable extract, and subsequently bottled and fermented. They are often favoured because of an enriched flavour perception and a prolonged flavour stability. The formation of a stable yeast deposit can be difficult to achieve and may lead to less-attractive, hazy beers. In contrast, bottle adhesion in the champagne wine process is not desired.
To examine the different glass adhesion characteristics of yeast cells, a strong and constitutive TEF1 promoter was used to drive the expression of each of the five FLO genes in the laboratory strain S288C. Glass recipients containing synthetic medium or an adhesion buffer solution and glass bottles containing beer, were used to evaluate yeast adhesion and yeast deposit stability. After decantation, the cell layer was deflocculated and cells were counted. The physicochemical characteristics of the yeast cells were determined by the Microbial Adhesion to Solvents test (MATS).
The presence of each adhesin on the yeast cell wall resulted in altered physicochemical surface properties of the cells, such as hydrophobicity, electron-donor and -acceptor properties. These alterations affected the interaction forces between yeast cells and the glass surface. The expression of the different adhesins all increase the cell wall hydrophobicity and may favour a direct binding between their lectin-like terminal domains and specific targets. These findings demonstrate the role of Flo proteins in yeast cell adhesion on glass and may contribute to a better control of the yeast deposit in beer or champagne wine refermentations.