Impact of Yeast Flocculation and Biofilm Formation on Yeast-Fungus Coadhesion in a Novel Immobilization System

A novel method of yeast immobilization, called “biocapsules”, has been developed in which cells of the yeast Saccharomyces cerevisiae become attached to the hyphae of the fungus Penicillium chrysogenum, remaining adhered following loss of viability of this fungus. Yeast immobilization facilitates higher cell densities than traditional fermentation methods, improves yield, and allows the reuse of the biocatalyst. Yeast cells may adhere to each other via specific cell-surface molecular interactions (flocculation) or may attach to surfaces (biofilm formation), and the roles of these two distinct attachment mechanisms in biocapsule formation is unknown. To elucidate the influence of biofilm formation versus flocculation on the yeast-fungus coimmobilization, a screening of selected strains from the Viticulture and Enology Department collection at the University of California, Davis, was carried out, and their ability to flocculate and form biofilm was quantified. Eighteen yeast strains capable of flocculation and biofilm formation were identified in this screening. Strains displaying differential capabilities in flocculation or biofilm formation and two control strains were further evaluated for their ability to specifically immobilize with P. chrysogenum. Seven strains exhibiting different patterns of flocculation and biofilm formation were identified. Biofilm-forming yeast strains displayed higher rates of immobilization with P. chrysogenum and formed more consistent biocapsules. In contrast, strains able to flocculate developed smaller, inconsistent biocapsules. Although the size and number of biocapsules formed varied by yeast strain, the total mass of biocapsules generated was similar for all strains. These results shed light on parameters that influence yeast-fungus coimmobilization, which may lead to an improvement of biocapsule consistency and advance the field of application for this new immobilization system.