Optimization of fermentation conditions for surfactin production by B. subtilis YPS-32

Surfactin produced by microbial fermentation has attracted increasing attention because of its low toxicity and excellent antibacterial activity. However, its application is greatly limited by high production costs and low yield. Therefore, it is important to produce surfactin efficiently while reducing the cost. In this study, B. subtilis strain YPS-32 was used as a fermentative strain for the production of surfactin, and the medium and culture conditions for the fermentation of B. subtilis YPS-32 for surfactin production were optimized.

First, Landy 1 medium was screened as the basal medium for surfactin production by B. subtilis strain YPS-32. Then, using single-factor optimization, the optimal carbon source for surfactin production by B. subtilis YPS-32 strain was determined to be molasses, nitrogen sources were glutamic acid and soybean meal, and inorganic salts were KCl, K₂HPO₄, MgSO₄, and Fe₂(SO₄)₃. Subsequently, using Plackett-Burman design, MgSO₄, time (h) and temperature (°C) were identified as the main effect factors. Finally, Box-Behnken design were performed on the main effect factors to obtain optimal fermentation conditions: temperature of 42.9 °C, time of 42.8 h, MgSO₄ = 0.4 g·L^- 1. This modified Landy medium was predicted to be an optimal fermentation medium: molasses 20 g·L^- 1, glutamic acid 15 g·L^- 1, soybean meal 4.5 g·L^- 1, KCl 0.375 g·L^- 1, K₂HPO₄ 0.5 g·L^- 1, Fe₂(SO₄)₃ 1.725 mg·L^- 1, MgSO₄ 0.4 g·L^- 1. Using the modified Landy medium, the yield of surfactin reached 1.82 g·L^- 1 at pH 5.0, 42.9 ℃, and 2% inoculum for 42.8 h, which was 2.27-fold higher than that of the Landy 1 medium in shake flask fermentation. Additionally, under these optimal process conditions, further fermentation was carried out at the 5 L fermenter level by foam reflux method, and at 42.8 h of fermentation, surfactin reached a maximum yield of 2.39 g·L^- 1, which was 2.96-fold higher than that of the Landy 1 medium in 5 L fermenter.

In this study, the fermentation process of surfactin production by B. subtilis YPS-32 was improved by using a combination of single-factor tests and response surface methodology for test optimization, which laid the foundation for its industrial development and application.