DEKKERA BRUXELLENSIS: STUDIES ON CARBON AND NITROGEN SOURCES METABOLISM AND ITS RESPONSE TO ACETIC ACID STRESS.

The lineages of Dekkera bruxellensis and Saccharomyces cerevisiae separated approximately 200 million years ago, but they share several industrially relevant traits, such as the ability to produce ethanol under aerobic conditions (Crabtree effect), high tolerance towards ethanol and acid, and ability to grow without oxygen. Beside a huge adaptability, D. bruxellensis exhibits a broader spectrum of consumable carbon and nitrogen sources in comparison to S. cerevisiae. This yeast is famous as a spoilage yeast in food and beverage industries and contaminates ethanol production process. Despite its economic importance and physiological interest, D. bruxellensis has not been well studied yet in detail. To characterize its carbon metabolism and regulation, we investigated how galactose is used as carbon source by this yeast. Here we show that in D. bruxellensis under aerobic conditions and on ammonium-based media galactose is a not-fermentable carbon source, in contrast to S. cerevisiae which can ferment also this sugar. The expression of genes involved in different metabolic pathways was also analysed. We report that genes involved in galactose utilization, respiratory metabolism, TCA cycle, glyoxylate cycle and gluconeogenesis are repressed in glucose-based media. These results indicate that in D. bruxellensis glucose repression operates similarly to what occurs in S. cerevisiae. In contrast to Saccharomyces cerevisiae, D. bruxellensis can use nitrate as sole nitrogen source. Our experiments showed that in D. bruxellensis, utilization of nitrate determines a different pattern of fermentation products. Acetic acid, instead of ethanol, became in fact the main product of glucose metabolism under aerobic conditions. We have also demonstrated that under anaerobic conditions, nitrate assimilation abolishes the ‘‘Custers effect’’, in this way improving its fermentative metabolism. Acetic acid, due to its toxic effects, is used in food industry as a preservative against microbial spoilage. We investigated how this yeast responds when exposed to acetic acid. A detailed analysis of acetic acid metabolism was performed on three strains which exhibited a different resistance. Our studies show that D. bruxellensis behaves, from a metabolic point of view, more similarly to S. cerevisiae, being unable to metabolize acetic acid in presence of glucose. The presence of acetic acid affected the growth, causing a reduction of growth rate, glucose consumption rate, ethanol production rate as well as biomass and ethanol yield. Interestingly, the cells continued to produce acetic acid.