CELLULOSE DEGRADATION BY RUMINAL MICROORGANISMS

The rumen is a highly developed digestive organ in which feed material, particularly plant fiber, is efficiently digested by a complex microbial fermentation. Cellulose, the major component of plant fiber, is digested in the rumen by an assemblage of anaerobic bacteria, fungi, and protozoa. While pure cellulose itself is completely digestible under the proper environmental conditions, the matrix interactions among structural biopolymers in natural plant material prevent the complete digestion of the cellulose component. Ruminal bacterial are probably the most active cellulose degraders in the rumen, but the fungi also play an important role, particularly in the physical disruption of recalcitrant tissue to render the cellulose more accessible to degradative enzymes. Cellulose degradation in the rumen is enhanced by attachment of microbial cells to the substrate and by synergistic interactions with noncellulolytic microbes, but is inhibited by factors that produce a low ruminal pH. Recent advances in continuous culture techniques and in the molecular biology of cellulase enzymes have permitted a better understanding of the cellulolytic process. Enhancement of ruminal cellulose fermentation will be best achieved in the near term by exploitation of recent improvements in the technology of substrate pretreatment. However, advances in the development of improved rumen microbial strains and more digestible forage varieties may yield additional improvements in ruminant cellulose utilization.