Fig. 5

Fermentation optimization and optimization of the expression level of the manC gene. A and B Synthesis efficiency of 2′-FL was verified when the inducer xylose or carbon source (sucrose or lactose) was supplemented during 24-h fermentation using the BS12 strain. Symbol “o” indicates initial fermentation conditions (xylose 20 g/L, lactose 10 g/L, and sucrose 20 g/L). Symbol “ + ” indicates that 20 g/L xylose, lactose 10 g/L, and sucrose 20 g/L were supplemented when fermentation reached 24-h, and “–” means none of the three sugars were added. C Seven different constitutive promoters (P1–P7) were used to express the manC gene. D Optimization of fermentation conditions for strain BS20. All carbon sources were added at the beginning of fermentation. Symbol “ − ” means no added xylose. All data were the average of three independent studies with standard deviations