Antimicrobial properties of tannins
Key Findings
- Comprehensive review of antimicrobial mechanisms of tannins
- Tannins inhibit microbial enzymes and deprive microorganisms of substrates
- Both condensed and hydrolyzable tannins show antibacterial and antifungal activity
A systematic review of tannin antimicrobial activity published in Phytochemistry (1991), establishing the mechanistic basis for how tannins inhibit microbial growth. Tannins are phenolic polymers — a significant component of sphagnum-derived peat alongside humic acids — and their antimicrobial contribution is distinct from but complementary to the humic acid antimicrobial activity documented in Ansorg 1978 and Wollina 2009.
Mechanisms Described
Scalbert identifies three ways tannins inhibit microbial growth:
- Enzyme inhibition — tannins bind to and inactivate microbial enzymes, particularly those involved in carbohydrate and protein metabolism. This disrupts the microorganism’s ability to process nutrients.
- Substrate deprivation — tannins complex with carbohydrates and proteins in the growth medium, making them unavailable to microorganisms.
- Membrane disruption — at higher concentrations, tannins interact with microbial cell walls and membranes, compromising structural integrity.
Both condensed tannins (proanthocyanidins) and hydrolyzable tannins (gallotannins, ellagitannins) show antibacterial and antifungal activity, though the potency and spectrum differ between types.
Relevance to Peat
Sphagnum peat contains condensed tannins as part of its complex phenolic matrix. The tannin contribution to peat’s antimicrobial activity is separate from the humic acid contribution and may act through different mechanisms on different organisms. Together, they help explain why peat bogs are highly antimicrobial environments — capable of preserving human bodies for millennia — and why therapeutic peat preparations have documented antimicrobial properties across multiple compound classes.