Mackie, R.I.: Mutualistic Fermentative Digestion in the Gastrointestinal Tract:Diversity and Evolution

Animals of a wide range of orders or classes have a portion of their digestive system adapted to accommodate a fermentation, which assists in digestion as well as providing a varietyof other benefits. Because of the refractory nature of the plant cell wall and the difficulty in digesting it, herbivores have anatomical and physiological adaptations of the digestive tract to allow assimilation of this material. Herbivorous reptiles, birds, and mammals usually have enlarged or elongated digestive tracts, often including fermentation chambers or sacs in the foregut and hindgut. Cecum-colon (hindgut) fermentors represent an older differentiation than foregut fermentors which, in turn, are older than ruminants. Advances in our understanding of fermentative digestion have tended to obscure the vital role that the gastrointestinal microbiota plays in the normal physiological, immunological and protective functions of the host animal. The association of microbes with tissues of the gastrointestinal tract of animals during evolution has resulted in a balanced relationship between resident microbes and the host. In fact, all animals including humans are adapted to life in a microbial world. Numerous biochemical, physiological and immunological features that are considered intrinsic characteristics of animal species are actually responses by the animal to the physical presence and metabolic activities of the normal indigenous microbiota. This microbial challenge has modified the course of evolution in animals resulting in the selection of animal-microbe relationships which are complex and which vary tremendously ranging from competition to cooperation. Its high population density, wide diversity and complexity of interactions characterize the gastrointestinal microbial community.