Engineering and adaptive evolution of Escherichia coli W for l-lactic acid fermentation from molasses and corn steep liquor without additional nutrients

The d-lactic acid producing strain, Escherichia coli HBUT-D, was reengineered for l(+)-lactic acid fermentation by replacing the d-lactate dehydrogenase gene (ldhA) with an l(+)-lactate dehydrogenase gene (ldhL) from Pedicoccus acidilactici, followed by adaptive evolution in sucrose. The resulting strain, WYZ-L, has enhanced expression of the sucrose operon (cscA and cscKB). In 100 g L−1 of sucrose fermentation using mineral salt medium, WYZ-L produced 97 g L−1 of l(+)-lactic acid, with a yield of 90%, a maximum productivity of 3.17 g L−1 h−1 and an optical purity of greater than 99%. In fermentations using sugarcane molasses and corn steep liquor without additional nutrients, WYZ-L produced 75 g L−1 of l(+)-lactic acid, with a yield of 85%, a maximum productivity of 1.18 g L−1 h−1, and greater than 99% optical purity. These results demonstrated that WYZ-L has the potential to use waste molasses and corn steep liquor as a resource for l(+)-lactic acid fermentation.