Walnut Pellicle Polyphenols Trigger <i>Lactiplantibacillus plantarum</i> DX7 to Produce Indole-3-Lactic Acid

Indole-3-lactic acid (ILA), a gut microbial metabolite, plays an important role in gut health. However, the number of bacterial strains that can produce ILA remains limited. This study investigated the synergistic relationship between <i>Lactiplantibacillus plantarum</i> DX7 and walnut pellicle polyphenols (WPP) for enhancing ILA production. Genomic analysis confirmed that the strain, though equipped with genes necessary for converting tryptophan (Try) to ILA, did not produce ILA under the standard conditions. However, cofermentation with WPP stimulated bacterial growth and ILA biosynthesis. <i>L. plantarum</i> DX7, in turn, modulated the WPP phenolic profile and enhanced its antioxidant activity. Specific phenolic acids, including ferulic, gallic, dihydrocaffeic, and cinnamic acids, were identified as key WPP components stimulating bacterial growth and activating the Try-to-ILA pathway. These results demonstrated a mutualistic interaction where WPP activated the metabolic potential of <i>L. plantarum</i> DX7 for ILA production, while the bacterium biotransformed WPP to improve its functionality, positioning their combination as a promising synbiotic strategy.