Abstract: To optimize the fermentation medium of Bacillus velezensis CY30 for enhancing the spore counts in the fermentation broth, the key factors among the media components were investigated by using Plackett-Burman（PB） design. The central axises of these factors were determined by using steepest-ascent method. The optimized medium for Bacillus velezensis CY30 was determined by central combination design and response surface method. The results demonstrated that the key factors influcening the spore counts in fermentation broth were sweet potato powder and yeast extract. The optimized medium was obtained through response surface analysis was 37.52 g/L of sweet potato powder, 15.29 g/L of yeast extract. The estimated maximum spore counts was 9.12 billion CFU/mL, while the real spore counts was 9.75 billion CFU/mL in shake-flask fermentation. The spore counts of the fermentation broth with optimized medium was 53% higher than that before the optimization（6.38 billion CFU/mL）.

Key words: Bacillus vezelensis CY30, fermentation medium, optimization, spore count, response-surface method