Enhanced anti-influenza activity of fermented yellow soybean extract and daidzein co-treatment on MDCK cells

The study investigated the effectiveness of pre- and co-treatment with fermented yellow soybean extract (FYSE) against anti-influenza A virus (IAV) on MDCK cells. FYSE, fermented with Bacillus subtilis, was evaluated for its anti-IAV activity by inhibiting the IAV PA gene expression. Daidzein was identified as a significant contributor to FYSE's antiviral effects. Co-treatment with FYSE and daidzein during IAV infection demonstrated superior anti-IAV activity compared to their respective pre-treatment (IC₅₀: FYSE; 8.65 vs 3.77 µg/mL, and daidzein; 6.01 vs 5.20 µg/mL). Both pre- and co-treatment with FYSE demonstrated higher therapeutic potential than daidzein (Selective index: pre-treatment; > 115.58 vs. 72.32 and co-treatment; > 265.04 vs. 83.56). Despite daidzein showing lower anti-IAV activity in both treatment methods compared to oseltamivir phosphate, it exhibited lower cytotoxicity (CC₅₀: 434.50 vs. 395.20 µg/mL). In conclusion, co-treatment with FYSE and daidzein presents a promising anti-IAV strategy with minimal cytotoxicity in vitro, potentially offering a safer alternative for IAV treatment.