Tannins as antimicrobial agents: Understanding toxic effects on pathogens

"Tannins" are compounds that belong to a group of secondary metabolites found in plants. They have a polyphenolic nature and exhibit active actions as first line defenses against invading pathogens. Several studies have demonstrated the multiple activities of tannins, highlighting their effectiveness as broad-spectrum antimicrobial agents. Tannins have reported as antibacterial, antifungal, and antiviral compounds by preventing enzymatic activities and inhibiting the synthesis of nucleic acids. Additionally, tannins primarily strengthen the plant cell wall, making it almost impenetrable to harmful pathogens. Most tannins are synthesized via the phenylpropanoid pathway to become secondary metabolites. Increased uptake of tannins has the potential to provide permanent immunity to subsequent infections by strengthening cell walls and producing antimicrobial compounds. Tannins also demonstrate a synergistic response with other defense-related molecules, such as phytoalexins and pathogenesis-related proteins, including antimicrobial peptides. Studying the mechanisms mediated by tannins on pathogen behaviors would be beneficial in stimulating plant defense against pathogens. This understanding could help explain the occurrence of diseases and outbreaks and enable potential mitigation in both natural and agricultural ecosystems.