(234a) Adaptive Mutation of Zymomonas Mobilis On High Concentrations of Xylose for Ethanol Production

Compared to other ethanol producing microbes, Zymomonas mobilis has superior tolerance for high concentrations of glucose and ethanol. While rational metabolic engineering has been successful in broadening the substrate range of Z mobilis to include xylose, xylose fermentation by recombinant Z mobilis is still not as efficient as glucose fermentation due to lower specific growth rate and sensitivity to higher xylose concentrations. Even though there are several published studies on inhibition by high glucose concentration on ethanol fermentation by Z mobilis and ways to combat the inhibition, there have been very few studies on the inhibitory effect of high xylose concentrations.

Through a carefully designed adaptive mutation process, we have enhanced tolerance to high concentrations of xylose in a Z mobilis ZM4 strain rationally engineered to use xylose. The adaptation process involves subjecting the cells to successively increased concentrations of xylose and serially culturing them until a stable strain is obtained. Over a period of several months, the tolerance to high xylose concentration has significantly improved. There has been a reduction in the duration of lag phase leading to a several-fold increase in the consumption rate of xylose and the volumetric ethanol productivity. As a result, a strain capable of consuming 5% (w/v) xylose within 30 hours, was obtained, significantly outperforming those previously reported by other researchers.

The presentation will highlight the successful use of adaptive mutation, in combination with rational engineering, for developing strains tolerant to high xylose concentration, and discuss the significance of the improvement in the context of bioethanol production from lignocellulosic biomass.