Mechanisms of tannin resistance and detoxification in the rumen

Tannins are naturally occurring protein-binding polyphenols that occur in many forage plants and which reduce nutritive value through inhibitory effects on ruminal and intestinal functions. Some animals have adapted to tannins through the synthesis of tannin-binding salivary proteins, the presence of tannin-resistant or tannin-degrading ruminal/intestinal microorganisms, or other potential adaptations in the lower intestinal tract. Streptococcus caprinus/gallolyticus is found ubiquitously in the rumen of many animals browsing tanninrich forage legumes. Biochemical studies have shown that this bacterium metabolises gallic acid to pyrogallol, although it does not metabolise pyrogallol, and produces extracellular polysaccharide (EPS) in response to tannins in the growth medium. Induction of EPS appears to be a bacterial defence mechanism that permits the bacterium to maintain its population when related species are dying. Selenomonas ruminantium K2 grows in the presence of hydrolysable or condensed tannins as a sole carbon source and secretes a tannin-inducible tannin acylhydrolase (TAH). This enzyme has been isolated and characterised, and the putative gene has been cloned and sequenced. Current studies focus on identifying the tannin-responsive element that control the gene. A number of other tannin-resistant bacterial species have been isolated and identified. In addition to effects of tannins on microbial function, intestinal function studies have revealed that tannins inhibit nutrient metabolism and uptake in the abomasum and small intestine of ruminants. Alkaline phosphatase and aminopeptidase-N activities are inhibited, intestinal microvilli structure is disturbed and signs of tissue fragility are evident. These studies indicate that the proteincomplexing action of tannins may have a more profound effect on intestinal rather than rumen function and that microbial interactions in the rumen may reduce but not eliminate tannin toxicity.