Mode of action of the yeastSaccharomyces cerevisiaeas a feed additive for ruminants

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Abstract
Two suggested modes of action of yeast in stimulating rumen fermentation were investigated. The first, that yeast respiratory activity protects anaerobic rumen bacteria from damage by O2, was tested using different strains of yeast that had previously been shown to have differing abilities to increase the viable count of rumen bacteria.Saccharomyces cerevisiaeNCYC 240, NCYC 1026, and the commercial product Yea-Sacc®, added to rumen fluidin vitroat 1·3 mg/ml, increased the rate of O2disappearance by between 46 and 89%. The same three preparations also stimulated bacterial numbers in anin vitrofermenter (Rusitec).S. cerevisiaeNCYC 694 and NCYC 1088, which had no influence on the viable count in Rusitec, also had no effect on O2uptake. Respiration-deficient (RD) mutants ofS. cerevisiaeNCYC 240 and NCYC 1026 were enriched by repeated culturing in the presence of ethidium bromide.S. cerevisiaeNCYC 240 and NCYC 1026 stimulated the total and cellulolytic bacterial populations in Rusitec, while the corresponding RD mutants did not. Rigorous precautions to exclude air from Rusitec resulted i“nS. cevevisiaeNCYC 240 no longer stimulating total bacterial numbers, although it still increased numbers of cellulolytic bacteria. The second hypothesis, that yeast provides malic and other dicarboxylic acids which stimulate the growth of some rumen bacteria, was examined by comparing the effects of yeast and malic acid on rumen fermentation in sheep. Three mature sheep were given 0·85 kg barley/d plus 0·55 kg chopped ryegrass hay/d either unsupplemented, or supplemented with 4 gS. cerevisiaeNCYC 240/d or 100 mgl-malic acid/d either mixed with the diet or in aqueous solution infused continuously into the rumen. Yeast increased the total viable count of bacteria (P< 0·05)whereas malic acid did not, and no other effect of the treatments reached statistical significance. It was concluded, therefore, that the stimulation of rumen bacteria byS.cerevisiaeis at least partly dependent on its respiratory activity, and is not mediated by malic acid.