Relative Contributions of Bacteria, Protozoa, and Fungi to In Vitro Degradation of Orchard Grass Cell Walls and Their Interactions

Abstract

To assess the relative contributions of microbial groups (bacteria, protozoa, and fungi) in rumen fluids to the overall process of plant cell wall digestion in the rumen, representatives of these groups were selected by physical and chemical treatments of whole rumen fluid and used to construct an artificial rumen ecosystem. Physical treatments involved homogenization, centrifugation, filtration, and heat sterilization. Chemical treatments involved the addition of antibiotics and various chemicals to rumen fluid. To evaluate the potential activity and relative contribution to degradation of cell walls by specific microbial groups, the following fractions were prepared: a positive system (whole ruminal fluid), a bacterial (B) system, a protozoal (P) system, a fungal (F) system, and a negative system (cell-free rumen fluid). To assess the interactions between specific microbial fractions, mixed cultures (B؉P, B؉F, and P؉F systems) were also assigned. Patterns of degradation due to the various treatments resulted in three distinct groups of data based on the degradation rate of cell wall material and on cell wall-degrading enzyme activities. The order of degradation was as follows: positive and F systems > B system > negative and P systems. Therefore, fungal activity was responsible for most of the cell wall degradation. Cell wall degradation by the anaerobic bacterial fraction was significantly less than by the fungal fraction, and the protozoal fraction failed to grow under the conditions used. In general, in the mixed culture systems the coculture systems demonstrated a decrease in cellulolysis compared with that of the monoculture systems. When one microbial fraction was associated with another microbial fraction, two types of results were obtained. The protozoal fraction inhibited cellulolysis of cell wall material by both the bacterial and the fungal fractions, while in the coculture between the bacterial fraction and the fungal fraction a synergistic interaction was detected.Bacteria, protozoa, and fungi have been shown to be the microorganisms involved in plant cell wall digestion in the rumen. However, due to the difficulty of separating each microbial group in the rumen, to difficulties in measuring fungal biomass, and to the complex nature of the rumen ecosystem, the precise role and overall contribution of each microbial group to the degradation and fermentation of plant cell wall material is not understood. In spite of complicated interrelationships among the microorganisms (e.g., bacteria, protozoa, and fungi) in the rumen ecosystem, bacteria are believed to play a major role because of their numerical predominance and metabolic diversity (7). However, protozoa have been shown to digest from 25 to 30% of total fiber. The extent of the involvement of fungi, however, has not yet been estimated. Interaction effects between microorganisms can range from synergism to antagonism and depend on the microbial groups and species involved and the type of substrate used. In vitro examinations to estimate the role...

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