Whitening Efficacy of Bird's Nest Filtrate Fermented With <i>Bacillus subtilis</i> FZU‐S10: In Vitro and In Vivo Studies

Edible bird's nest (EBN) is a valued nutraceutical, yet its bioactivity is limited by macromolecular complexity. This study examined the effects of Bacillus subtilis FZU-S10 fermentation on EBN composition, bioactivity, and anti-melanogenic mechanisms. Fermentation increased peptide concentration (5-11.5 mg·mL^-1) and low-molecular-weight peptides (500-1000 Da, from 14.0% to 75.1%), altered N-terminal amino acids, and elevated sialic acid and polysaccharides. These changes enhanced tyrosinase inhibition (40%-70%) and antioxidant activity. In cells, fermented EBN filtrate (F-EBNF) dose-dependently reduced melanin content and tyrosinase activity, and suppressed LPS-induced IL-6 more effectively than unfermented EBN (U-EBNF). In zebrafish, F-EBNF significantly inhibited melanin synthesis and tyrosinase activity. Transcriptomic analysis revealed that F-EBNF modulated the expression of a greater number of genes than U-EBNF, with KEGG analysis highlighting "melanogenesis" as the most affected pathway. The results suggested that fermentation enhances the whitening effect of EBN by downregulating key genes in the Wnt/β-catenin and Endothelin-1/PLC pathways, including AC, Frizzled, Gαi/q, PLC, and CAM, which led to suppression of MITFand tyrosinase. Overall, fermentation transforms the macromolecular composition of EBN and enhances its bioactivity by modulating multiple melanogenesis-related signaling pathways. PRACTICAL APPLICATIONS: The content of peptides in fermented bird's nest filtrate (F-EBNF) was increased. F-EBNF enhanced tyrosinase inhibition and antioxidant activity. F-EBNF suppressed melanin synthesis in cells and zebrafish models. Transcriptomics showed F-EBNF downregulated key melanogenesis pathways.