Unravelling SO2 stress response in Brettanomyces/Dekkera bruxellensis using RNA-seq

Wine spoilage by Brettanomyces/Dekkera bruxellensis has increased in frequency because of the use of less-severe processing conditions, the great variety of diverse vinification tecniques and the tendency to reduce the use of preservatives, such as sulphur dioxide. Due to its antioxidant and antimicrobial effect, SO2 is a very common additive in many foods; however, it is also known for increased allergic reactions in humans and therefore consumer pressure to reduce the levels. The capability of B./D. bruxellensis to survive and to grow in wine can be partially ascribed to its high resistance to SO2, but no data are available on mechanisms involved in this metabolic trait.Triplicate batch fermentations were conducted in a wine-like medium using D. bruxellensis AWRI1499. Cells in exponential phase of growth were exposed to a molecular SO2 concentration of 0.35 mg/L and samples were collected for RNA-Seq analysis: i) before the SO2 pulse; ii) 5h thereafter; and iii) at cell growth recovery. Of the 4861 genes submitted to the gene set enrichment analysis (GSEA), 720 gene sets resulted upregulated. In particular, two distinct groups of gene sets resulted significantly enriched (p<0.01) in conditions ii) and iii), respectively. This result indicates that an early- and late response can be actuated by the yeast as a sequential strategy occurring under SO2 stress. Interestingly, SSU1, the main mechanism of sulphite detoxification in Saccharomyces cerevisiae, increased its expression during the late response up to 50 times. On the other hand, the data show that a long-term response, involving the carbohydrate biosynthesis and the sulphur compound metabolism, is activated and maintained by the yeast to counteract SO2 exposure.