Epsilon Open Archive

Abstract

Dekkera bruxellensis has been shown to outcompete an initial inoculum of Saccharomyces cerevisiae in several ethanol production plants, which nevertheless had a high efficiency in one of the monitored processes. Co-occurrence of D. bruxellensis with lactic acid bacteria (LAB) Lactobacillus vini has been observed. The aim of this thesis was to broaden the knowledge on D. bruxellensis physiology in respect to its high competitiveness.

Global gene expression analysis of D. bruxellensis under conditions similar to those in which it outcompeted S. cerevisiae was performed by whole transcriptome sequencing. Low expression of genes involved in glycerol biosynthesis, and expression of NADH-ubiquinone reductase (complex I) are probably the basis for an efficient energy metabolism. Genes of putative high affinity glucose transporters might be involved in the efficient glucose transport of D. bruxellensis.

D. bruxellensis also has a good potential to ferment lignocellulose hydrolysate to ethanol. Adaptation to lignocellulose hydrolysate inhibitors by pre-cultivation was demonstrated. Adapted cells had a shorter lag phase and produced higher amounts of ethanol compared to non-adapted cells.

The role of L. vini during co-cultivation with D. bruxellensis or S. cerevisiae was also investigated. Formation of LAB–yeast cell aggregates consisting of a bacterial core with an outer layer of yeast cells was identified. It was noted that addition of mannose to the aggregates dissolved them, but higher mannose amounts were required to inhibit co-flocculation between L. vini and S. cerevisiae compared to L. vini and D. bruxellensis.

Growth and metabolite profiles of D. bruxellensis during cultivation on different combinations of carbon and nitrogen sources were studied. Repression of genes involved in nitrate assimilation in D. bruxellensis under oxygen-limited conditions in presence of ammonium was shown.

In conclusion, D. bruxellensis has a great potential for industrial ethanol production due to a highly efficient energy metabolism, adaptability to lignocellulose hydrolysate, utilisation of an alternative nitrogen source and robustness against bacterial contaminants.