Fermentative production of oligosaccharides by total fermentation utilizing a mixed feedstock

A method for production of lactose or an oligosaccharide of interest which comprises a galactose-β1,4-glucose moiety at its reducing end, the method comprising:

• a) providing a genetically engineered microbial cell, wherein said microbial cell comprises:
• at least one glucose transporter for translocating glucose from the culture medium into the microbial cell's cytoplasm;
• an UDP-galactose biosynthesis pathway;
• at least one β-1,4-galactosyltransferase being able to galactosylate free glucose to intracellularly produce lactose;
• and
• one or more of:
• i) a fructose transporter, fructokinase-6 activity, and 6-phosphofructokinase-1 activity;
• and
• ii) a fructose specific phosphotransferase system and 1-phosphofructokinase;
• b) cultivating the microbial cell in a culture medium and under conditions that are permissive for production of said lactose or oligosaccharide of interest, wherein the culture medium comprises a mixture of glucose and fructose as main carbon source;
• and
• c) recovering the lactose or oligosaccharide of interest from the culture medium and/or the microbial cell.