Antimicrobial properties of tannins

Tannin toxicity for fungi, bacteria and yeasts is reviewed and compared to toxicity of related lower molecular weight phenols. The dependence of toxicity on tannin structure is examined. The different mechanisms proposed so far to explain tannin antimicrobial activity include inhibition of extracellular microbial enzymes, deprivation of the substrates required for microbial growth or direct action on microbial metabolism through inhibition of oxidative phosphorylation. A further mechanism involving iron deprivation is proposed. Many microorganisms can overcome plant defences based on tannins. They may detoxify tannins through synthesis of tannin- complexing polymers, oxidation, tannin biodegradation or synthesis of siderophores.