Abstract: The study aimed to increase the spore production by Bacillus velezensis strain YC11 which showed good control effects on black shank of tobacco. Three media components, including corn flour, yeast powder and magnesium sulphate, which significantly influenced the spore production by YC11, were screened out through Plackett-Burman design. A nonlinear equation was fit by the interaction between the most significant factors and the spore production through the steepest ascend experiment and Box-Behnken design. An optimized medium was obtained for the spore production, of which the composition was as the followings:corn flour 13.74 g/L, fish meal 15.00 g/L, yeast powder 15.73 g/L, corn steep liquor 5.00 g/L, magnesium sulphate 0.59 g/L, and potassium dihydrophosphate 0.30g/L. The theoretical spore production of this nonlinear model was 1.144×10¹⁰ spores/mL, which was very close to the practical spore production with the optimized medium in shake-flask fermentation. After optimization, the spore production was increased by 64.7%. The spore production of strain YC11 with the optimized medium in 500L fermenter reached 1.50×10¹⁰ spores/mL.The results of indoor pot experiment showed that the average control effect of fermentation broth of strain YC11 on tobacco black shank could reach 100%.

Key words: Bacillus velezensis, fermentation medium, optimization, response surface methodology, spore production