Surfactin Production by Bacillus subtilis 55033 via Kitchen Waste Fermentation and Antagonistic Activity Against Aquatic Pathogens

Surfactin is a lipopeptide biosurfactant with significant industrial potential, but high production costs have hindered its commercialization. In this study, we developed a sustainable and cost-effective bioprocess for surfactin biosynthesis using Bacillus subtilis 55033, utilizing kitchen waste as the primary fermentation substrate without any previous pretreatment—an advantage that simplifies the process and reduces costs compared to previous reports. To maximize productivity, the fermentation parameters were systematically optimized through single-factor and orthogonal experimental designs. The optimal conditions were as follows: a fermentation temperature of 31 °C, a duration of 48 h, a 5% v/v inoculum, a 7% w/v solid-to-liquid ratio, an initial pH of 7.5, and an agitation speed of 200 rpm. Under these conditions, the surfactin yield reached 371.67 mg/L, an increase of 100.5% from 185.4 mg/L prior to optimization. We performed a comparative analysis of the surfactin biosynthetic gene clusters between B. subtilis 55033 and the model strain B. subtilis 168. The produced surfactin exhibited potent antagonistic activity against aquatic pathogens, significantly inhibiting the growth of several species of Vibrio and the division and hatching of Cryptocaryon irritans Tomonts. These findings demonstrate that our platform not only provides a high-value valorization route for organic waste but also enables preliminary exploration of surfactin applications in aquaculture, in line with the principles of a circular bioeconomy.