|Wells James E|
Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/1995
Publication Date: N/A
Citation: Interpretive Summary: Providing adequate energy to ruminant farm animals (cattle, sheep, goats) represents a major cost of milk and meat production. Ruminant animals digest cellulose, a common component of plant foods, by exploiting a symbiotic (cooperative) relationship with ruminal bacteria. Ruminal bacteria ferment cellulose at a rapid rate, but as much as 50% of the cellulose escapes ruminal digestion. The bacterium, Fibrobacter succinogenes is the most active digester of cellulose in the rumen. However, other ruminal bacteria that can not digest cellulose grow in combination with F. succinogenes when cellulose is the only substrate. This diversion of cellulose digestion products to other bacteria decreases the growth potential of F. succinogenes, thereby reducing the overall efficiency of cellulose digestion. The mechanism for this loss of efficiency was not known. In these studies, we found that F. succinogenes secreted cellodextrins (products of cellulose digestion) into the extracellular culture medium and that other species of bacteria used these intermediate products of cellulose digestion. Further work is needed to see if the secretion of cellodextrins can be prevented and if the rate of cellulose digestion in the rumen can be increased. An increased rate of cellulose digestion would improve the efficiency of milk and meat production by ruminant animals.
Technical Abstract: When glucose or cellobiose was provided as an energy source for Fibrobacter succinogenes, there was a transient accumulation (up to 0.4 mM hexose equivalent) of cellobiose or cellotriose in the growth medium. Non- growing cell suspensions converted cellobiose to cellotriose and longer chain cellodextrins with total cellodextrin concentration of as much as 20 mM (hexose equivalent). Cell free extracts of glucose- or cellobiose-grown cells cleaved cellobiose and cellotriose by phosphate-dependent reactions, glucose-1-phosphate was an end product and the ratio of cellodextrins to cellodextrins + one more hexose was approximately 4 to 1. Thus, it appears that cellodextrins were being produced by a reversible phosphorylase reaction. When F. succinogenes was grown in a cellobiose-limited chemostat, cellobiose & cellotriose could be detected in a ratio of approximately 4 to 1, respectively. Based on these results, cellodextrin production is an equilibrium function, not just an artifact of energy-rich cultural conditions. Cellodextrins could not be detected in slow dilution rate, cellulose-limited continuous cultures, but these cultures had a large number of non-adherent cells. Since non-adherent cells had a large reserve of polysaccharide and were observed at all stages of cell division, it appeared that they were utilizing cellodextrins as an energy source for growth. The non-cellulolytic bacterium, S. bovis, persisted in batch culture with F. succinogenes, with cellulose as the only energy source, at a ratio of about 1 to 4, respectively. Carbohydrate metabolism by F. succinogenes seems to be a compromise between the energetic advantage of a phosphorylase reaction and the potential loss of carbon and energy as extracellular cellodextrins.