Bacterial contamination in starch-to-ethanol fermentations

Abstract

Increasing interest in the bioeconomy has spurred the development of integrated methods to convert organic waste streams, particularly starch-rich substrates, into bioethanol. However, starch-based ethanol fermentations are vulnerable to bacterial contamination, particularly by lactic acid bacteria (LAB). Severe contamination can cause significant economic losses due to stuck fermentations and ethanol plant shutdowns. Although bacterial contamination can be managed with antibiotics, this approach is not cost-effective at an industrial scale and may increase the risk of selecting for antibiotic-resistant strains. Natural antimicrobial peptides (AMPs) can inhibit LAB contaminants in yeast fermentations, but commercial applications are limited by their low abundance and high production costs. Engineering Saccharomyces cerevisiae to produce recombinant AMPs might provide a cost-effective strategy to control LAB, thereby boosting ethanol yields during fermentation. Despite a comprehensive toolkit for gene expression in S. cerevisiae, only a few successful cases of bacteriocin expression have been reported. Since starch-to-ethanol fermentation is a key application for recombinant AMPs, this review explores strategies to optimize the expression of bacteriocin-encoding genes in S. cerevisiae. The ideal scenario would be a single yeast strain capable of producing amylases for starch hydrolysis, fermenting glucose to ethanol, and expressing bacteriocins to inhibit LAB contaminants. One-sentence summary: Yeast strains can produce heterologous antimicrobial peptides that help prevent contaminating bacteria from interfering with the starch-to-ethanol fermentation process.