Microbial biotransformation of plant-derived bioactive compounds: structural modifications, enhanced bioavailability, and functional food applications

Plant-derived bioactive compounds, including polyphenols, polysaccharides, alkaloids, terpenoids/saponins, lignans and glucosinolates, are widely valued for their health-promoting potential, yet poor bioavailability and, in several cases, biological inactivity in native form limit their translation into functional food ingredients. Microbial biotransformation, carried out by naturally occurring gut microbiota, food-fermentation microorganisms and plant-associated endophytic fungi, offers a means of overcoming these limitations through mild, highly selective enzymatic modification. This review synthesizes current evidence on microbial biotransformation across five major classes of plant bioactive compounds, examining the natural microbial sources involved, the core enzymatic mechanisms, including glycosidases, esterases, oxidoreductases, decarboxylases and demethylases, responsible for these conversions, and the resulting changes in bioavailability and bioactivity for each compound class. Beyond the compound-specific discussion, we identify a smaller set of structural modifications, namely deglycosylation, molecular weight reduction, hydroxylation, demethylation, ring cleavage and decarboxylation, that recur across otherwise unrelated compound classes and account for much of the functional benefit associated with microbial processing. We discuss current applications in fermented functional foods and postbiotics, together with the principal obstacles, including strain stability, substrate specificity in complex matrices, and regulatory characterization, that constrain industrial-scale deployment, and outline multi-omics-guided and precision-fermentation strategies likely to shape future progress in this field.