Engineering Bacillus subtilis for highly efficient production of functional disaccharide lactulose from lactose.

Bioconversion of lactose to functional lactose derivatives attracts increasing attention. Lactulose is an important high-value lactose derivative, which has been widely used in pharmaceutical, nutraceutical, and food industries. Lactulose can be enzymatically produced from lactose by cellobiose 2-epimerase (CEase). Several studies have already focused on the food-grade expression of CEase, but they are all aimed at the biosynthesis of epilactose. Herein, we reported for the first time the biosynthesis of lactulose using the recombinant food-grade Bacillus subtilis. Lactulose biosynthesis was optimized by varying lactulose-producing CEases and expression vectors. Caldicellulosiruptor saccharolyticus CEase and pP43NMK were determined to be the optimal CEase and expression vector. Fine-tuning of CEase expression was investigated by screening a beneficial N-terminal coding sequence. After fed-batch cultivation, the highest fermentation isomerization activity reached 11.6 U/mL. Lactulose was successfully produced by the broth of the engineered B. subtilis with a yield of 52.1 %.