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One of the two dominant phylotypes of Crenarchaeota that colonizes the roots of tomato plants grown in soil from a Wisconsin field is selectively enriched in mixed cultures amended with root extract. Results indicate that the diversity of crenarchaeal soil assemblage is influenced by the rhizosphere and that mesophilic soil crenarchaeotes are associated with plant roots that show the growth of nonthermophilic crenarchaeotes in culture.

The authors review the publications on microbial cellulose utilization. The topics of interest include the structure and composition of cellulosic biomass, processing of cellulosic biomass and organism development for consolidated bioprocessing.

Competition of three predominant ruminal celluloytic bacteria is discussed as it occurs in the presence or absence of noncellulolytic bacteria. The three are Fibrobacter succinogens S85, Ruminococcus flavefaciens FD-1 and Ruminococcus albus 7. Data show that cellulolytic bacteria interactions, though complex, may be changed further by noncellulolytic species.

The essential role of ruminal micro flora in ruminant nutrition provides the potential for improvement in animal production via altering the numbers or activities of specific classes of microorganisms. Successful alterations will be facilitated by an understanding of the microbial ecology of the rumen based on its mechanistic underpinnings. Demonstrated improvements in ruminal fermentation can be traced to their consonance with well-established principles of microbial ecology (niche occupancy, selective pressure, adaptation, and interactions) and the thermodynamics and kinetics of substrate utilization. Application of these principles to several proposed alterations of the ruminal bacterial population allows a prediction of their relative feasibility. Improving fiber digestion, decreasing protein degradation, and detoxifying feed components that are present in low concentrations will be difficult to achieve in the rumen and are best approached by altering the feed, either genetically or with postharvest treatment. By contrast, the detoxification of feed components present in high concentration, and redirection of electron disposal away from methanogenesis, are more productive targets for microbiological research. Key Words: Acetate, Bacteria, Fiber, Methane, Rumen, Toxins

The ruminal cellulolytic bacteria Ruminococcus flavefaciens and Fibrobacter succinogens coexist in substrate-excess culture with almost equal population size. However, R. flavefaciens outcompetes the other strain for cellobiose in the substrate-limited condition, regardless of whether they are coinoculated or a steady-state culture of F. succinogens is challenged by R. flavefaciens. R. albus outcompetes F. succinogens and R. flavefaciens in substrate-excess and -limited conditions respectively. This is due to the selection in the chemostat of a population of R. albus with a higher affinity for cellobiose.

Fibrobacter succinogenes contains cellobiose and cellodextrin phosphorylases which can both breakdown and form cellodextrins. The cellodextrins moves out of the F. succinogenes cells and is used by the cells of F. succinogens which are not attached to the cellulose and by the non-cellulose using bacteria like Streptococcus bovis. If the concentration of cellulose is less, most of the F. succinogenes cells are not attached to the cellulose and thus cellulse digestion depends on the cellulose concentration and not on the cellodextrin efflux.

The relation of plant metabolites that inhibit the fermentation of plant structural polysaccharides to the variable digestibility of cicer milkvetch by ruminal microorganisms is discussed. Water extracted and unextracted cicer milkvetch neutral detergent fiber were fermented in vitro. Cellulose fermentation was inhibited by mixed ruminal microflora and ruminal cellulolyticbacteria. It was found that the presence of a water-soluble agent causes the inhibition of cicer milkvetch ruminal digestion.