Microbial Synthesis of Lactose, Galactose, and Galactitol by Metabolically Engineered <i>Escherichia coli</i> Strains

Lactose production is generally dependent on extraction from whey in the industry. In this study, a de novo lactose pathway was constructed in Escherichia coli MG1655 by introducing Neisseria meningitidisβ-1,4-galactosyltransferase and disrupting the glucose and lactose catabolism pathway. Multiple copy integrations of key pathway genes and deletion of competitive genes were performed to enhance the microbial synthesis of lactose. Galactose and galactitol pathways were further constructed by retaining the β-galactosidase and introducing specific aldose reductase. Using glycerol as the carbon source and glucose as the acceptor substrate, the highest titers of lactose, galactose, and galactitol reached 9.45, 7.63, and 5.92 g/L, respectively. Furthermore, the fed-batch cultivations of the corresponding engineered strains produced 32.56 g/L lactose, 20.16 g/L galactose, and 17.52 g/L galactitol, respectively. This study can also provide ideas for the de novo biosynthesis of human milk oligosaccharides and other functional lactose derivatives, especially galactitol and tagatose.