Microbial Biotransformation of Plant Bioactive Compounds: Mechanisms, Enhanced Bioactivities, and Applications in Functional Food Development

Plant-derived bioactive compounds, including polyphenols, flavonoids, alkaloids, terpenoids, and saponins, possess diverse health-promoting properties but are often limited by poor bioaccessibility, structural complexity, and low stability. Microbial biotransformation provides a green and food-compatible strategy for modifying these compounds through enzymatic reactions and metabolic processes, generating derivatives with improved accessibility and potential functional properties. This review summarizes recent advances in microbial transformation mediated by lactic acid bacteria, Bacillus, yeasts, and filamentous fungi, focusing on key transformation pathways, including deglycosylation, hydrolysis, oxidation–reduction, methylation, and glycosylation, as well as their enzymatic mechanisms. The applications of transformed products in fermented foods, functional ingredients, and nutraceuticals are discussed, along with their potential antioxidant, anti-inflammatory, metabolic regulatory, and bioavailability-related effects. Current challenges, including incomplete mechanistic understanding, process standardization, safety evaluation, scale-up, and regulatory issues, are highlighted. Future integration of multi-omics, synthetic biology, enzyme engineering, and intelligent fermentation technologies will enhance the efficiency and industrial potential of microbial transformation systems for developing safe and evidence-based functional food ingredients.