Applied Microbiology and Biotechnology, Vol.104, No.22, 9565-9579, 2020
Combining metabolic engineering and evolutionary adaptation inKlebsiella oxytocaKMS004 to significantly improve optically pure D-(-)-lactic acid yield and specific productivity in low nutrient medium
In this study,K. oxytocaKMS004 (Delta adhE Delta pta-ackA) was further reengineered by the deletion offrdABCDandpflBgenes to divert carbon flux through D-(-)-lactate production. During fermentation of high glucose concentration, the resulted strain namedK. oxytocaKIS004 showed poor in growth and glucose consumption due to its insufficient capacity to generate acetyl-CoA for biosynthesis. Evolutionary adaptation was thus employed with the strain to overcome impaired growth and acetate auxotroph. The evolvedK. oxytocaKIS004-91T strain exhibited significantly higher glucose-utilizing rate and D-(-)-lactate production as a primary route to regenerate NAD(+). D-(-)-lactate at concentration of 133 g/L (1.48 M), with yield and productivity of 0.98 g/g and 2.22 g/L/h, respectively, was obtained by the strain. To the best of our knowledge, this strain provided a relatively high specific productivity of 1.91 g/gCDW/h among those of other previous works. Cassava starch was also used to demonstrate a potential low-cost renewable substrate for D-(-)-lactate production. Production cost of D-(-)-lactate was estimated at $3.72/kg. Therefore, it is possible for the KIS004-91T strain to be an alternative biocatalyst offering a more economically competitive D-(-)-lactate production on an industrial scale.