**2.1. Microorganism and inoculum preparation**

*B. subtilis* CICC 10025 purchased from the China Centre of Industrial Culture Collection was used in this study. It was maintained on agar slants with the following medium (g/L): glucose 10, beef extract 10, peptone 10, sodium chloride 5, and agar 16 at pH 7.0. The seed culture was prepared by growing the bacterium in 50 ml of the following medium in a 250-ml shake flask for 10 h with agitation of 150 rpm and temperature at 37°C: glucose 60 g/L, beef extract 10 g/L, peptone 10 g/L, yeast extract 10 g/L, and sodium chloride 5 g/L at pH 7.0 [16].
