Efficient synthesis of optically pure D-lactate from mixed sugars by engineered E. coli consortium

In this study, we constructed two engineered E. coli strains that utilized glucose (E. coli GL10) and xylose (E. coli XL12) simultaneously via division of labor. Both two engineered strains blocked the synthetic pathway of several byproducts, including succinate, formate, ethanol, and acetate. XL12 demonstrated the alleviation of carbon catabolite repression effect and the improvement of xylose utilization in mixed sugar. GL10 and XL12 produced D-lactate from glucose and xylose with an optical purity of 100%, respectively. Microbial consortium using GL10 and XL12 with the inoculation ratio of 1:1 achieved complete glucose depletion and 53.21% xylose utilization simultaneously. Notably, when the fermentation was proceeded using mixed sugar for 36 h, the D-lactate titer of the consortium achieved 3.76 g/L, which is 5.96-fold higher than that of the wild-type (WT) fermentation. The effect of shaking speed and inoculum ratio on the fermentation performance were investigated. Specifically, the xylose utilization in the mixed sugars was improved with an increase of shaking speeds from 50 to 250 rpm, while the yield of D-lactate did not further increase at 250 rpm. When the inoculation ratio of GL10 and XL12 was 1:5, the consortium showed better performance in D-lactate production, which was 6.55-fold higher than that of the WT. The consortium was also utilized mixed sugars effectively in corn straw hydrolysate and produced D-lactate of 5.15 g/L, which was 41.87% higher than that of WT. Our study offers a feasible approach via metabolic engineering and artificial consortium construction for bio-based D-lactate production.