Optimization of fermentation conditions and extracellular metabolic profiling of Streptomyces rochei strain W71

This study aimed to determine the optimal fermentation conditions and to characterize extracellular differential metabolites of Streptomyces rochei strain W71. The inhibition rate of its fermentation supernatant against Phytophthora nicotianae was used as the evaluation index to assess anti-oomycete activity. The fermentation conditions of strain W71 were optimized through single-factor experiments combined with response surface methodology (RSM: Plackett–Burman design, steepest ascent, and Box–Behnken design). Untargeted liquid chromatography–mass spectrometry (LC–MS) metabolomic analysis was then performed to identify extracellular metabolites and elucidate the key metabolic pathways involved. The optimal fermentation conditions for strain W71 were as follows: maltose: 20.0 g/L, yeast extract powder: 2.5 g/L, NaCl: 1.0 g/L, working volume: 100 mL/250 mL, inoculum size: 5.55%, agitation speed: 200 rpm, initial pH: 8.66, temperature: 29.8 ℃, and fermentation time: 8 days. Under these conditions, the inhibition rate of the fermentation broth reached 92.90%, representing a 6.78-fold increase compared with the pre-optimization level. LC–MS metabolomic analysis identified 1,771 differential metabolites in extracellular extracts after optimization, of which 1,479 were upregulated and 292 were downregulated. The major metabolite classes included organic halogen compounds, organic acids and derivatives, and aromatic metabolites. KEGG enrichment analysis revealed 20 significantly altered pathways, among which ABC transporters, amino acid biosynthesis, and the pentose phosphate pathway were identified as key metabolic routes. Optimization of fermentation conditions for strain W71 significantly enhanced the antifungal activity of its fermentation broth against P. nicotianae and altered extracellular metabolic pathways, leading to substantial regulation of extracellular metabolite profiles.