Strain‐Specific Biotransformation of Grape Marc by Lactic Acid Bacteria: Genomic Validation and Gut Microbiota Modulation

ABSTRACT Grape marc (GM) is a polyphenol‐rich winery byproduct that remains underutilized due to its complex fiber matrix, which limits the bioaccessibility of bound phenolics. While lactic acid fermentation (LAF) can enhance the release of these compounds, the strain‐specific metabolic fate of GM phenolics during digestion and colonic fermentation, and the genomic basis underlying these differences, remain poorly understood. In this study, we established a multi‐tiered genomic validation framework to investigate the effects of four lactic acid bacterial strains, Lactiplantibacillus plantarum (LP), Lacticaseibacillus casei (CA), L. rhamnosus (RH), and Lactobacillus acidophilus (LA5), on the transformation of GM phenolics during in vitro digestion and colonic fermentation. LAF significantly enhanced the release of bound phenolics and antioxidant capacity. During colonic fermentation, fermented GM reshaped the gut microbiota by rapidly enriching Lactobacillaceae while suppressing the pro‐inflammatory Enterobacteriaceae. Strain‐specific metabolic signatures were evident: CA fermentation uniquely drove a > 3‐fold increase in propionic acid and early anthocyanin release, whereas LP favored butyrate production and sustained antioxidant activity. Genomic mining linked these phenotypes to specific functional determinants, including the phosphoketolase gene (xfp), associated with bifid shunt‐mediated acidogenesis, and S‐adenosylmethionine synthase (metK), associated with phenolic stabilization. Collectively, this study provides a predictive basis for selecting LAB starters to optimize the functional output of fermented GM and supports strain‐guided fermentation as a promising strategy to valorize winery waste into gut‐health‐promoting ingredients.