Cooperation and Interplay Among the Many Species of Microbes Give Rise to a Complex Ecosystem in the Forestomach and Hindgut
The digestive process in the rumen or colon involves the interplay among the many species of bacteria and other microbes. The ecosystem of fermentative digestion is extremely complex, with the waste products of one microbial species serving as substrate for another.
For example, Ruminococcus albus and Bacteroides rιιminieola appear to exist synergistically. R. albus digests cellulose (is cellulolytic) but cannot digest protein. B. rumiιιicolat on the other hand, can digest protein but cannot digest cellulose. When the microbes are grown together, cellulose digestion by R. albus provides hexoses for the energy needs of B. rιιminicolat and protein digestion by B. ruminicola provides ammonia and branch-chain fatty acids for the growth needs of R. albus.In addition to substrate needs, growth factor needs are also supplied synergistically within the rumen ecosystem. For example, B vitamins are necessary for the growth of several rumen microbes, but these nutrients are generally not necessary in ruminant diets. The synergistic effect of B vitamins results from cross-feeding between species of those microbes that produce various B vitamins and those microbes that require them.
Despite tremendous ecological complexity, however, the entire pattern of fermentation may be viewed as a holistic process, without consideration of the roles and interactions of individual microbial species. Fermentative digestion is examined here from this viewpoint, with the actions of the entire rumen biomass considered as an overall digestive process, irrespective of the specific needs and actions of individual microbial species.