A co-fermentation strategy to consume sugar mixtures effectively

Abstract

We report a new approach for the simultaneous conversion of xylose and glucose sugar mixtures into products by fermentation. The process simultaneously uses two substrate-selective strains of Escherichia coli, one which is unable to consume glucose and one which is unable to consume xylose. The xylose-selective (glucose deficient) strain E. coli ZSC113 has mutations in the glk, ptsG and manZ genes while the glucose-selective (xylose deficient) strain E. coli ALS1008 has a mutation in the xylA gene. By combining these two strains in a single process, xylose and glucose are consumed more quickly than by a single-organism approach. Moreover, we demonstrate that the process is able to adapt to changing concentrations of these two sugars, and therefore holds promise for the conversion of variable sugar feed streams, such as lignocellulosic hydrolysates.

Figure 1

Batch aerobic culture of E. coli MG1655. Glucose (hollow square), xylose (hollow triangle), and OD (solid circle) were measured over the course of fermentations.

Figure 2

Batch aerobic culture of individual substrate-selective E. coli strains. Glucose (hollow square), xylose (hollow triangle), and OD (solid circle) were measured over the course of fermentations inoculated with A) ZSC113 only or B) ALS1008 only.

Figure 3

Batch aerobic co-culture of two E. coli strains. ZSC113 and ALS1008 were grown simultaneously on a mixture of glucose (hollow square) and xylose (hollow triangle). The OD (solid circle) was measured over the course of the fermentation.

Figure 4

Fed-batch aerobic co-culture of two E. coli strains. ZSC113 and ALS1008 were grown simultaneously using a feed containing a varying mixture of glucose (dotted lines) and xylose (dashed lines). The OD (solid circle), the concentrations of glucose (hollow square) and xylose (hollow triangle), and the fraction of the total cell population which is ZSC113 (solid triangle pointing down) were measured over the course of the fermentation.

Figure 5

Batch anaerobic fermentation of E. coli MG1655. After aerobic growth, additional xylose (hollow triangle) and glucose (hollow square) were added and anaerobic conditions commenced (t = 0). The concentrations of formate (solid triangle), lactate (solid square), succinate (hollow circle), acetate (solid diamond) and ethanol (hollow diamond) were measured over the course of the anaerobic phase.

Figure 6

Batch anaerobic fermentation of individual substrate-selective E. coli strains. After aerobic growth, xylose (for ZSC113) or glucose (for ALS1008) was added and anaerobic conditions commenced (t = 0). The concentrations of glucose (hollow square), xylose (hollow triangle), formate (solid triangle), lactate (solid square) succinate (hollow circle), acetate (solid diamond) and ethanol (hollow diamond) were measured over the course of the anaerobic phase previously inoculated with A) ZSC113 only and B) ALS1008 only.

Figure 7

Batch anaerobic co-fermentation of E. coli strains. ZSC113 and ALS1008 were grown simultaneously on a mixture of glucose (□) and xylose (△). After aerobic growth, xylose and glucose were added and anaerobic conditions commenced (t = 0). The concentrations of formate (solid triangle), lactate (solid square), succinate (hollow circle), acetate (solid diamond) and ethanol (hollow diamond) were measured over the course of the anaerobic phase.