Cellular Coupling Fermentation Utilizing Engineered <i>Escherichia coli</i> and Yeast for Efficient Biosynthesis of Sialyllacto-N-tetraose a via Modular Strategy

for endogenous cytidine-5'-triphosphate (CTP) cofactor regeneration. LSTa was successfully synthesized via a modular biosynthesis strategy, utilizing N-acetylneuraminic acid (Neu5Ac) and lacto-N-tetraose (LNT) as substrates. Then through systematic optimization of induction parameters and catalytic reaction conditions, LSTa productivity and substrate conversion efficiency were substantially enhanced. Ultimately, a whole-cell catalytic process was implemented in a 5 L bioreactor, achieving a maximum titer of 38.03 g/L for LSTa, demonstrating the scalability and potential for the biosynthesis of complicated sialylated HMOs.