Optimization of fermentation medium and preparation of a bacterial agent from Serratia marcescens G4

Sclerotinia sclerotiorum causes severe yield losses in soybean. Overuse of chemical fungicides leads to pollution, residues, and resistance. Biological control offers a sustainable alternative. This study optimized fermentation of Serratia marcescens G4, developed microcapsules (G4Ms), and evaluated their efficacy against soybean sclerotiniosis.

Under initial culture conditions, the OD₆₀₀ of strain G4 reached 2.241 after 12 h. Sucrose and soybean peptone were identified as the most effective carbon and nitrogen sources, respectively. Response surface optimization determined the optimal medium as sucrose 13.21 g L^-1, soybean peptone 10.33 g L^-1, yeast extract 10.95 g L^-1, and NaCl 10 g L^-1, under which the predicted OD₆₀₀ increased to 2.474 (10.4% improvement). The optimal culture conditions were 35 °C, pH 7.0, 3% (v/v) inoculum, 155 rpm, and 50 mL working volume. Microcapsules prepared with 80% maltodextrin (G4Ms) exhibited enhanced storage stability, stronger biofilm formation, and improved inhibitory activity compared to free-living G4 cells. In pot experiments, G4Ms achieved a control efficacy of 55.84% against soybean sclerotiniosis, and their application increased soil enzyme activities and microbial diversity.

G4Ms is an effective, stable, and soil-friendly biocontrol agent against S. sclerotiorum-induced soybean sclerotiniosis, highlighting its potential as a sustainable alternative to chemical fungicides for integrated disease management. © 2026 Society of Chemical Industry.