The effect of Levucell SC, a strain of Saccharomyces cerevisiae marked as a feed additive for ruminants, was investigated in vitro on lactate metabolism by the ruminal bacteria Streptococcus bovis and Megasphaera elsdenii. The coculture between 10(7) live cells x mL(-1) of SC and a Streptococcus bovis strain in the presence of glucose reduced lactate production by the bacterial strain. Live yeast cells were able to compete with Streptococcus bovis for glucose utilization in strictly anaerobic conditions, so less glucose was available for the bacterium. SC also stimulated L-lactate utilization by a strain of M. elsdenii. The effect depended on the concentration of yeast cells added. Bacterial growth and fermentation end-product concentrations were also increased in the presence of SC. Some amino acids and vitamins, but not dicarboxylic acids, stimulated the bacterial specific activity of L-lactate uptake. SC was able to provide amino acids to M. elsdenii. In a coculture of Streptococcus bovis and M. elsdenii on glucose, the reduction of lactate concentration was improved by SC, the same trend being observed when maltose or soluble starch were used as carbon and energy source. These results indicate that SC can be a very useful tool to reduce lactate accumulation in vitro during fermentation of soluble sugars.

The effects of a live strain of Saccharomyces cerevisiae on hydrogen utilization and acetate and methane production by two hydrogenotrophic ruminal microorganisms, an acetogenic bacterial strain and an archaea methanogen, were investigated. The addition of yeast cells enhanced by more than fivefold the hydrogenotrophic metabolism of the acetogenic strain and its acetate production. In the absence of yeasts, and in a coculture of the acetogen and the methanogen, hydrogen was principally used for methane synthesis, but the presence of live yeast cells stimulated the utilization of hydrogen by the acetogenic strain and enhanced acetogenesis.

The effects of a live yeast strain of Saccharomyces cerevisiae have been investigated on zoospore germination, metabolism, and cellulolytic activity of the anaerobic rumen fungus Neocallimastix frontalis MCH3. The addition of yeast cells to a vitamin-deficient medium stimulated the germination of fungal zoospores, increased cellulose degradation and hydrogen, formate, lactate, and acetate production. Responses depended on the concentration of yeast cells added and on their viability. Yeast supplementation provided vitamins such as thiamine, which is essential for fungal growth and activity. These results demonstrate that yeasts could enhance plant cell wall colonization by N. frontalis. With certain diets, yeasts could therefore be a good tool to optimize the microbial degradation of lignocellulosic materials, but more research is needed to understand their mechanisms of action, so that they can be used with maximum efficiency as feed supplements.

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Previous results have shown that some rumen entodiniomorphid ciliates present cellulolytic and hemicellulolytic activities (Bonhomme, 1988). A comparative study of xylanolytic activity, one of the main enzymatic activities, was performed in Epidinium caudatum and Eudiplodinium maggii.The cytosolic soluble proteins were precipitated by ammonium sulfate from sonicated cell preparations of these 2 ciliates obtained free of cellulolytic and hemicellulolytic bacteria according to Bonhomme et al (1982). With non-denaturing polyacrylamide gel electrophoresis both extracts showed multiple active proteic bands against xylan and Remazol brilliant blue xylan. Preparative isoelectrofocalization showed that the active bands had acidic isoelectric points in both cases. The quantitation of reducing sugars was used to determine the enzymatic activity. The temperature for optimal activity was 45°C for both ciliates, and the pH optimal activity were 5 and 5.5 respectively for E caudatum and E maggii. Protein denaturation was observed > 50°C.The xylanolytic fractions obtained after gel filtration (Ultrogel ACA 34) and anion exchange chromatography (Biogel A) were pooled and used to determine the biochemical properties V max and K m of xylanases in both ciliates by using the Lineweaver and Burke plots (table I).In conclusion, the physicochemical and kinetic properties of these enzymes are clearly similar but we still require information on molecular weight and potential immunological cross-reactions to complete this study.