| Summary: | Many contaminant yeast strains that survive inside fuel ethanol industrial vats show detrimental cell surface phenotypes. These harmful effects may include filamentation, invasive growth, flocculation, biofilm formation, and excessive foam production. Previous studies have linked some of these phenotypes to the expression of <i>FLO</i> genes, and the presence of gene length polymorphisms causing the expansion of <i>FLO</i> gene size appears to result in stronger flocculation and biofilm formation phenotypes. We performed here a molecular analysis of <i>FLO1</i> and <i>FLO11</i> gene polymorphisms present in contaminant strains of <i>Saccharomyces</i> <i>cerevisiae</i> from Brazilian fuel ethanol distilleries showing vigorous foaming phenotypes during fermentation. The size variability of these genes was correlated with cellular hydrophobicity, flocculation, and highly foaming phenotypes in these yeast strains. Our results also showed that deleting the primary activator of <i>FLO</i> genes (the <i>FLO8</i> gene) from the genome of a contaminant and highly foaming industrial strain avoids complex foam formation, flocculation, invasive growth, and biofilm production by the engineered (<i>flo</i>8∆::<i>Ble</i><sup>R</sup>/<i>flo8</i>Δ::<i>kanMX</i>) yeast strain. Thus, the characterization of highly foaming yeasts and the influence of <i>FLO8</i> in this phenotype open new perspectives for yeast strain engineering and optimization in the sugarcane fuel-ethanol industry.
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