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.

Large-scale microbial-biotechnology processes for production of chemicals almost exclusively rely on pure cultures of microbial strains. Especially for extensively engineered pure cultures, process performance can be negatively affected, which can be caused by issues such as pathway imbalance, deterioration of productivity caused by genetic instability and enzyme promiscuity. An increasing number of studies demonstrate that, under ‘academic’ laboratory conditions, the use of defined co-cultures (i.e. deliberate mixtures of known microbial strains) offers unique possibilities for mitigating such drawbacks. These advantages differ for dissimilatory products, whose synthesis from one or more carbon substrates provides cells with free energy, and assimilatory products, whose synthesis requires a net input of free energy. Based on advances in experimental and theoretical research, this paper highlights how defined co-cultures can address several limitations of mono-cultures for production of low-molecular-weight compounds. From this largely academic perspective, we outline the key challenges for scaling these systems to industry, which underscore the need for innovative solutions and continued research in this area.