Medium optimization for production of antifungal active substances from antagonistic Brevibacillus laterosporus and its antifungal activity analysis

The antagonist bacterium Brevibacillus laterosporus ZQ2 with broad-spectrum antifungal activity was isolated from an apple rhizosphere at Mount Tai in China. The fermentation medium for the production of antifungal substances from ZQ2 was statistically optimized. In one-factor-at-a-time experiments, five components, including carbon sources, nitrogen sources, and inorganic salt, were selected. Two factors were determined as critical based on the Plackett-Burman design. Steepest ascent experiments were then used to determine the design space containing the optimum point, followed by application of the response surface methodology. A quadratic model fitting the actual production was established using a central composite design. The obtained optimum medium for the antifungal substances production consisted of the following (in g/L): glucose, 15.6; soybean meal, 8.2; starch, 20; ammonium sulfate, 2; and monopotassium phosphate, 1. Furthermore, the inhibition rate of the substances against Rhizoctonia solani increased by more than 132% compared with the control. B. laterosporus ZQ2 strongly inhibited the growth of many apple phytopathogens in vitro. The inhibition rates against different fungi ranged from 55.26 to 79.02%. As well, the strain significantly reduced the disease damage caused by R. solani in the in vivo tests. Maximum disease protection (46.89%) was recorded when the bacterium applied 3 days before pathogen inoculation.   Key words: Brevibacillus laterosporus, antifungal activity, fermentation optimization.