Mechanisms of tannin resistance and detoxification in the rumen

Tannins reduce the nutritive value of forage 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 micro-organisms, or other potential adaptations in the lower intestinal tract. Streptococcus caprinus/gallolyticus is found ubiquitously in the rumen of many animals browsing tannin-rich 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. This enzyme has been isolated and characterised. A number of other tannin-resistant bacterial species, including Lactobacillus, Butyrivibrio and Enterobacteriacae have been isolated, although their mechanisms of tannin resistance are not known. 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 protein-complexing action of tannins may effect intestinal as well as rumen function and that microbial interactions in the rumen may reduce but not eliminate tannin toxicity.