Enhancement of Mycelial Growth and Antifungal Activity by Combining Fermentation Optimization and Genetic Engineering in Streptomyces pratensis S10

The biocontrol strain <i>Streptomyces pratensis</i> S10 was isolated from tomato leaf mold. The fermentation broth of strain S10 can effectively control Fusarium head blight (FHB), caused by <i>Fusarium graminearum</i>. Enhancing antifungal activity is essential in advancing its commercialization. In this study, we aimed to improve the antifungal activity of S10 by integrating fermentation optimization and genetic engineering. Single-factor experiments revealed that seven parameters, namely corn flour, yeast extract, NaNO₃, CaCO₃, K₂HPO₄, KCl, ZnSO₄·7H₂O, and MnCl₂·4H₂O, were identified as significant components. A Plackett-Burman design (PDB) indicated that corn flour, yeast extract, and ZnSO₄·7H₂O were the most critical variables affecting its inhibitory activity and mycelial biomass. The fermentation medium was further determined based on the steepest climbing experiment and a Box-Behnken design (BBD), and the mycelial dry weight of <i>S. pratensis</i> S10 was improved from 2.13 g/L in Gauze's synthetic No. 1 medium to 8.12 g/L in the optimized medium, closely aligning with the predicted value of 7.98 g/L. Under the optimized medium, the antifungal rate of <i>F. graminearum</i> increased from 67.36 to 82.2%. The spore suspension of strain S10 cultured in the optimized medium substantially improved its biocontrol efficacy against FHB. Moreover, disruption of the key gene <i>tetR</i> led to increased antifungal activity of strain S10 against <i>F. graminearum</i>. Importantly, the antifungal activity of Δ<i>tetR</i> was greatly increased under the optimized fermentation medium. This study suggests that the gene <i>tetR</i> negatively regulates bioactive compound biosynthesis, and the optimized medium provides favorable conditions for the growth of S10. These observations establish an extended basis for the large-scale bioactive metabolite secretion of <i>S. pratensis</i> S10, providing a strong foundation for sustainable FHB management in agriculture.