Fecha de publicación: -- Fuente: WIPO "tomato" Synthetic biology, metabolic engineering and cell cultivation. This disclosure provides a cell for production of a negatively charged, preferably sialylated, oligosaccharide wherein the cell is genetically engineered to possess or express, preferably to over-express a hydrolyzing UDP-N-acetyl-D-glucosamine-2-epimerase. This disclosure further provides use of the cell in a cultivation or incubation. This disclosure also describes methods for the production of a negatively charged, preferably sialylated, oligosaccharide using the cell as well as the purification of the negatively charged, preferably sialylated, oligosaccharide.