Effect of a Saccharomyces cerevisiae culture on lactate utilization by the ruminal bacterium Selenomonas ruminantium.

The objective of this study was to examine the effects of a Saccharomyces cerevisiae culture (YEA-SACC) on lactate utilization by the predominant ruminal bacterium Selenomonas ruminantium. Lactate uptake was stimulated by YEA-SACC concentrations between 2.5 and 10 g/liter, and the 5-g/liter level increased uptake 3.8-fold. When YEA-SACC concentrations were increased above the 5-g/liter level lactate uptake was decreased, but 10 g/liter still stimulated uptake more than threefold. A filter-sterilized YEA-SACC filtrate also increased lactate uptake more than fourfold at all concentrations tested (10 to 100 microliters/ml), and the 25-microliters/ml level increased uptake ninefold. Growth of S. ruminantium in medium that contained 2 g/liter of DL-lactate was stimulated more than twofold by either 2 or 5% (vol/vol) YEA-SACC filtrate after 24 h. The YEA-SACC filtrate also increased the production of acetate, propionate, total VFA, and YLACTATE (grams of cells/mole of lactate) from lactate-grown cells. Because the increase in propionate production was greater relative to acetate, a decrease in the acetate:propionate ratio was observed. Growth on lactate and uptake of radiolabeled lactate by S. ruminantium was stimulated by a filter-sterilized YEA-SACC filtrate. The concentration of L-malic acid in the YEA-SACC filtrate was 4.9 mM, and it seemed that L-malic acid played a role in the stimulation of growth on lactate as well as lactate uptake by S. ruminantium treated with YEA-SACC.