A. NAYYAR (1), A. K. Adya (1), G. Walker (1), E. Canetta (2), F. Wardrop (3); (1) University of Abertay Dundee, Dundee, U.K.; (2) St. Mary’s University College, Twickenham, London, U.K.; (3) Lallemand Inc., Montreal, Canada

Poster

Adhesion properties are known to play important roles in governing many essential aspects of the life cycles of microorganisms, like sexual reproduction, cellular aggregation during processes such as flocculation and bio-film formation, invasion and/or pathogenic behavior, and many others. Adhesion properties, by far, are dependent on the characteristics of the cellular surface, usually the outer layer of the cell wall. Microorganisms can adjust their adhesion properties by changing the structure of their external cell surface. Flocculence is the ability of yeast cells to flocculate under optimal conditions, which is a cell wall property independent of its environment. Thus, when we study flocculation we need to consider the cell wall properties The flocculation behavior of four industrial Saccharomyces cerevisiae strains expressing either the Flo1 or NewFlo phenotype were examined. These are strains employed for brewing, champagne production, winemaking, and fuel alcohol production. The behavior of brewing and champagne strains differed in terms of their cell surface hydrophobicity, cell surface charge, and presence of adhesins and cell wall binding sites (mannose residues), which likely impinge on their flocculation behavior. The brewing yeast strain exhibited the highest degree of flocculation among all the strains, and it was accompanied with a concomitantly high hydrophobicity index of 66%. This supports our hypothesis that cell surface hydrophobicity plays a major role in controlling yeast flocculation behavior in the fermenter. Equally important is cell surface charge, which was shown in highly flocculent brewing strains to possess a very high negative charge. From the studies, it was observed that high cell surface hydrophobicity, bonds between the adhesins and mannose residues (stabilized by Ca²⁺ ions), and finally the surface topography of yeast strains are responsible for maintaining flocs during the fermentation process. We have additionally observed that in contrast to wine and fuel alcohol yeast strains, brewing and champagne strains exhibit increased cell wall mannose concentrations from the early stationary phase to the late stationary phase. This correlates with simultaneous increase in flocculation ability. Brewing yeasts, therefore, may be characterized by a high density of mannose residues on their outer cell walls. In addition, we found that the brewing yeast strain studied had a high lectin density (3.65 × 10⁶ lectins /cell) compared with the champagne strain (2.44 × 10⁶ lectins/cell). Yeast adhesion properties and cell wall physiology were further investigated at the nanoscale using atomic force microscopy (AFM). For example, surface roughness, Young’s modulus, and adhesion energy of industrial yeast strains determined by AFM provided new information regarding yeast cell walls and physiological behavior. The work will further aid in greater understanding of the onset of yeast flocculation, and the vital role that cell surface hydrophobicity, cell surface charge, and surface topography, together with the density of adhesins on the yeast cell surface, play in brewing processes during fermentation.

Ashima Nayyar received a B.S (honors) degree in microbiology from Delhi University in New Delhi, India. She received her master’s degree in microbial technology (gold medalist) from Amity University, Noida, Uttar Pradesh, India. She was employed as a product executive with a pharmaceutical company for six months. She is currently a research Ph.D. candidate at the University of Abertay Dundee, Scotland. She has attended and presented at international conferences on yeast biotechnology.

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