Abstract: GameXPeptide-A (GXP-A), a cyclic peptide natural product, exhibits significant inhibitory activity against the innate immune systems of agricultural pests, making it a highly promising candidate for pest control. In this study, we systematically optimized the fermentation medium for Xenorhabdus budapestensis XBD8, an entomopathogenic nematode symbiotic bacterium isolated by our team, to boost GXP-A production and facilitated its large-scale application. Initially, single-factor and combinatorial experiments were used to identify the optimal carbon sources, nitrogen sources, inorganic salts, and their concentrations in the culture medium. The Plackett-Burman design was then employed to screen the key factors influencing bacterial growth, after which the steepest ascent experiment was used to approach the response surface center. Finally, a regression model was established based on the Box-Behnken design, which allowed us to determine the optimal fermentation medium composition: yeast extract 25.77 g/L, ferric chloride 0.30 g/L, sodium chloride 5.49 g/L, tryptone 10.0 g/L, magnesium sulfate 2.0 g/L, and beef extract 14.0 g/L. The optimized medium significantly enhanced GXP-A production. In 250 - mL shake flask experiments, GXP-A yields reached 8.92 mg/L, and in a 5-L bioreactor setup, yields further increased to 20.87 mg/L. Compared to pre-optimization yields, this represents remarkable increases of 869.57% and 2168.48%, respectively.

Key words: Xenorhabdus budapestensis XBD8, GameXPeptide-A, response surface analysis, fermentation optimization