Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/1997
Publication Date: N/A
Citation: Interpretive Summary: Cellulose is the most abundant component of forage and the major nutrient of forage-fed ruminants. The three most important species of bacteria that digest cellulose fiber in the rumen are known to make different fermentation products that are utilized differently by the dairy animal, but little information is available on what factors determine the outcome of competition among these species. We found that, in paired cultures fed cellulose at high concentrations, each species was able to grow to similar cell densities. However, when cellulose was steadily provided in limiting amounts, paired cultures usually contained one species that dominated the culture (80-95% of the population) and one species that was present in smaller numbers. The factors that were most important in determining the outcome of the competition were the rate and extent to which the cells attached to cellulose fibers, and the rate of growth on low concentrations of soluble product of cellulose digestion. The results also indicate that on the more complex substrate cellulose, the competition among the species is more complex than on the simpler sugar substrate, cellobiose. This research identifies factors that determine which species of fiber-digesting bacteria become predominant under defined growth conditions. This informa- tion provides new strategies for altering the proportions of digestion products, which in turn affects the composition of milk.
Technical Abstract: Three predominant ruminal cellulolytic bacteria (Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD-1, and Ruminococcus albus 7) were grown in different binary combinations in either cellulose-excess batch culture or in cellulose-limited continuous culture. Relative populations of each species were estimated using signature membrane-associated fatty acids and/or 16S rRNA-targeted oligonucleotide probes. Both S85 and FD-1 coexisted in cellulose-excess batch culture with similar population sizes. By contrast, under cellulose-limitation FD-1 predominated in coculture with S85, as expected from FD-1's more rapid adherence to cellulose and its higher affinity for cellodextrin products of cellulose hydrolysis. In batch cocultures of S85 and 7, the populations of the two species were similar. However, under cellulose limitation, S85 was the predominant strain. The results from batch cocultures of FD-1 and 7 were not consistent within or among trials: some experiments yielded monocultures of 7 (suggesting production of an inhibitory agent by 7), while others contained substantial populations of both species. Under cellulose limitation, FD-1 predominated over 7 (85% and 15%, respectively), as would be expected by the former's greater adherence to cellulose. The retention of 7 in the cellulose-limited coculture may result from a combination of its ability to utilize glucose; its demonstrated ability to adapt under selective pressure in the chemostat to utilization of lower concentrations of cellobiose, a major product of cellulose hydrolysis; and its possible production of an inhibitory agent.