Optimization of the Fermentation Medium of Streptomyces ahygroscopicus ΔnysB, a Tetramycin Producing Strain, by the Response Surface Method

Streptomyces ahygroscopicus ΔnysB is a genetically engineered strain that produces tetramycin by blocking the competition nystatin pathway. Response Surface Methodology (RSM) was used to optimize its fermentation medium composition. Taking the yield of tetramycin as the response value, the effects of corn starch, glucose, fish peptone, (NH₄)₂SO₄, KH₂PO_4, and CH₃COONa on the yield of tetramycin were investigated by the Plackett-Burman method and three main factors of corn starch, glucose and (NH₄)₂SO₄, were determined. The path of the steepest ascent test was used to approach the maximum response region and the center point was determined. Based on this, the three-factor three-level test was designed by the Box-Behnken center combination. Quadratic regression analysis was performed to calculate the medium with the highest yield of tetramycin and the optimal fermentation medium for tetramycin was formulated as corn starch 25.08 g/L, soybean cake meal 30 g/L, glucose 37.70 g/L, NaCl 0.2 g/L, MgSO4 0.2 g/L, K₂HPO₄ 0.1 g/L, FeSO₄ 0.2 g/L, CaCO₃ 5 g/L, (NH₄)₂SO₄ 0.24 g/L. The yield of tetramycin in the optimized medium reached 167.19 mg/L, which was close to the predicted value of the model. The yield of tetramycin was significantly increased, which was 5.5 times of that before optimization. The results provide a basis for improving the production level of tetramycin.