17-26: Systems biology comparison of glucose/xylose fermentation by Saccharomyces cerevisiae 424A (LNH-ST) and 424A (LNH-ST)–AAR in the presence of acetic acid

Acetic acid is one of the major fermentation inhibitors in the cellulosic biomass. We have shown that acetic acid at levels found in cellulosic hydrolysates (up to15 g/L) greatly affect S. cerevisiae 424A(LNH-ST) xylose utilization, especially at a low pH (< 5.5), while minimally affecting glucose utilization. Through selective culturing, an acetic acid-resistant glucose/xylose co-fermenting strain was developed that is able to ferment 60% of initial xylose (60 g/L) compared to 25% consumption by the original strain in the presence of acetic acid (10 g/L).

Specific consumption rates and ethanol metabolic yields for both glucose and xylose were estimated for batch glucose/xylose fermentations.  The adapted strain did not show significant changes in both parameters compared to the original strain.  Differences between the strains were primarily that the xylose fermentation stalled in the original strain while the fermentation continued in the adapted strain.

Global gene expression and metabolomic analysis were used to determine the differences between the original and the adapted strain during glucose/xylose co-fermentation in the presence of acetic acid. We identified 653 genes that change at least five fold in gene expression between the two strains over the entire course of the fermentation. Of those, 182 genes had >5 fold changes in expression during the xylose-fermenting stage after glucose was fully consumed. Clustering analysis was used to find patterns in these differentially expressed genes over the entire fermentation.  Intracellular concentration or 6 metabolites (of 27 metabolites measured) from glycolysis and PPP pathways significantly vary between original and adapted strain.