Relative rates of sugar utilization by an ethanologenic recombinant Escherichia coli using mixtures of glucose, mannose, and xylose.

The volumetric rates of glucose (G), mannose (M), and xylose (X) utilization by recombinant Escherichia coli B (pLO1297) were compared in pH-stat batch fermentations with Luria broth containing various combinations of two of these sugars at differing mass ratios. Using single substrate media, the rates of glucose, mannose, and xylose utilization were 3.0, 0.8, and 1.5 g/L/h, respectively. With all two substrate media, hexose and pentose sugars were consumed simultaneously. At a mass ratio of 2:1 (M or X:G), the rate of glucose utilization was reduced to 1.7 and 1.2 g/L/h by mannose and xylose, respectively. In media containing glucose and xylose, the rate of xylose utilization was inhibited when the glucose component exceeded about 40% of the total sugar mass in the medium. At a mass ratio of 2M:1X, mannose did not inhibit the rate of xylose utilization. At a mass ratio of 1:2 (G or X:M), the rate of mannose utilization was unaffected by either glucose or xylose. Synthetic media containing a mixture of hexose and pentose sugars were formulated to mimic different biomass hemicellulose hydrolysates. Relative to the rate in a single substrate medium, the respective rates of glucose and xylose utilization were 70% (2.1 g/L/h) and 40% (0.6 g/L/h) in a synthetic softwood prehydrolysate (SW) medium with a total reducing sugar (TRS) content of 45.7 g/L (20 wt% glucose, 30% xylose, and 50% mannose). However, the rate of mannose utilization in the SW medium was not inhibited. The respective rates of glucose and xylose utilization were 30% (0.9 g/L/h) and > 90% (1.4 g/L/h) in a synthetic crop residue prehydrolysate (CR) medium with a TRS content of 46.9 g/L (10 wt% glucose, 73% xylose, and 17% arabinose). Based on the results of this study, we suggest that the apparent "preference" for fermentation of hexose sugars by recombinant E. coli may be owing to the decreased rate of xylose transport caused by hexose sugars. Glucose is a more potent modulator of xylose utilization than mannose, but since xylose affects the rate of glucose utilization, this study also points to the importance of the concentration of the different sugars in terms of the relative rates of utilization by recombinant E. coli.