Phenotypic investigation of biofilm formation and transcriptional analysis of invasive growth of commercial wine Saccharomyces cerevisiae

Abstract

This study investigated the morphological properties, environmental effects on and gene expression of biofilms, more specifically referred to as mats, formed by laboratory and commercial wine strains of Saccharomyces cerevisiae. Two morphological assays were conducted: mat formation and plastic adhesion. Mat features varied between strains and included various architectures, cellular morphologies, and incidence of invasive growth. One commercial strain, L2056, formed mats where a sector produced a distinctive mat morphology, which was retained when subcultured. In considering the role of biofilms in winery conditions, mat formation assays were also performed with grape pulp and adhesion to the soft plastic of common winery hoses. All strains grew invasively on all agar media and appeared to conduct fermentation on the grape-pulp mat assay. Some strains also had the ability to adhere to winery hose plastic. When only limited nitrogen was available, both laboratory and commercial wine strains formed mats with a subpopulation of cells that switched to filamentous and invasive growth. Such invasive growth was influenced by nitrogen concentration, the presence of a neighbouring mat, and by the addition of yeast metabolites. Ethanol and hydrogen sulfide were found to enhance invasive growth of cells within mats exposed to low levels of nitrogen whereas tryptophol and 2-phenylethanol suppressed this enhancement. Sulfite was found to delay overall mat growth. In an effort to understand the cellular decision to switch morphology, changes in the transcriptome of invasively growing cells were studied. In this analysis, 272 genes were identified to be upregulated and 84 genes were downregulated in invasively growing cells. Of the ten largest differentially expressed genes, four were genes encoding hexose transporters (HXT3, HXT4, HXT6 and HXT7) which had an increase in transcript abundance up to 13-fold. One hypothetical gene (AWRI796_5153) with a 6-fold increase in transcript abundance, has translation sequence homologous to an amidase domain. Following differential expression and Gene Ontology analysis, five GO categories represented the 37 significantly enriched GO terms in the upregulated gene set of invasively growing cells, these being glucose import, carbohydrate metabolic process, fungal-type cell wall organisation, medium-chain fatty acid biosynthetic process and cellular water homeostasis. Since cellular water homeostasis has not previously been associated with invasive growth, and four out of five genes in this group were found to be significantly upregulated in the invasively growing cells, further analysis of deletion mutants of each of these confirmed that FPS1, encoding the glycerol export protein, is required for invasive growth of yeast mats in low nitrogen conditions. In summary, this work reports the phenotypic properties of commercial wine yeast biofilms in environments of both rich nutrient and low nitrogen, either in typical laboratory type agar media or in conditions simulating that of a grape or wine hose. The ability of these yeasts to form complex morphologies, grow invasively into grape solids and attach to winery hose plastic may confer their residency and survival in the vineyard and winery. The influence of different yeast metabolites and transcriptional changes in invasively growing cells provide further understanding of this morphogenetic program.