Response Surface Methodology (RSM) Mediated Optimization of Medium Components for Mycelial Growth and Metabolites Production of Streptomyces alfalfae XN-04.

Streptomyces alfalfae XN-04 has been reported for the production of antifungal metabolites effectively to control Fusarium wilt of cotton, caused by Fusarium oxysporum f. sp. vasinfectum ( Fov ). In this study, we used integrated statistical experimental design methods to investigate the optimized liquid fermentation medium components of XN-04, which can significantly increase the antifungal activity and biomass of XN-04. Seven variables, including soluble starch, KNO 3 , soybean cake powder, K 2 HPO 4 , MgSO 4 ·7H 2 O, CaCO 3 and FeSO 4 ·7H 2 O, were identified as the best ingredients based on one-factor-at-a-time (OFAT) method. The results of Plackett-Burman Design (PBD) showed that soluble starch, soybean cake powder and K 2 HPO 4 were the most significant variables among the seven variables. The steepest climbing experiment and response surface methodology (RSM) were performed to determine the interactions among these three variables and fine-tune the concentrations. The optimal compositions of medium were as follows: soluble starch (26.26 g/L), KNO 3 (1.00 g/L), soybean cake powder (23.54 g/L), K 2 HPO 4 (0.27 g/L), MgSO 4 ·7H 2 O (0.50 g/L), CaCO 3 (1.00 g/L) and FeSO 4 ·7H 2 O (0.10 g/L). A verification experiment was then carried out under the optimized conditions, and the results revealed the mycelial dry weight of S. alfalfae XN-04 reaching 6.61 g/L. Compared with the initial medium, a 7.47-fold increase in the biomass was achieved using the optimized medium. Moreover, the active ingredient was purified from the methanol extract of S. alfalfae XN-04 mycelium and then identified as roflamycoin (a polyene macrolide antibiotic). The results may provide new insights into the development of S. alfalfae XN-04 fermentation process and the control of the Fusarium wilt of cotton and other plant diseases.