Effect of overexpression of partial TDH1 and TDH2/3 gene sequences in a starter strain of industrial bioethanol fermentation on the Brettanomyces bruxellensis contaminant growth.

Selected Saccharomyces cerevisiae strains, such as the commercial Ethanol-Red (ER) strain, are used as starters in the bioethanol industry. Yet, bioethanol fermentations are prone to microbial contaminations, mainly by Brettanomyces bruxellensis and lactic acid bacteria. Chemicals, such as sulphuric acid and antibiotics, are commonly used to combat those contaminations, but they have negative environmental impacts. Recently, ER strain was found to secrete antimicrobial peptides (AMPs) active against B. bruxellensis. Therefore, the partial TDH1 and TDH2/3 genes sequences that codify those AMPs were inserted into the pSR41k plasmid and cloned in ER strains. The relative expression levels (plasmidic/genomic) of those sequences in the respective modified ER strains were quantified by RT-qPCR, confirming its overexpression. The effect of the modified strains on B. bruxellensis (Bb) growth was then evaluated during synthetic must- (SM) and carob syrup (CS) fermentations, co-inoculated with 105 cells mL-1 of ER and Bb in SM and with 106 of ER and 5 × 103 cells mL-1 of Bb in CS. Results showed that modified ER strains exerted a much higher inhibitory effect against B. bruxellensis (72-fold in SM and 10-fold in CS) than the non-modified ER strain. In those fermentations 90-100 g L-1 of ethanol were produced in 3-6 days.