The GI-tract and its microbiota represent a very complex ecosystem. The gastrointestinal microbiota of monogastrics is composed primarily of bacteria, and particularly anaerobic Gram-positive bacteria. The intestinal bacteria make important contributions to host metabolism and physiology. The relationship between host and microbiota in a healthy individual is a homeostatic symbiosis that primarily involves nutrient acquisition. But, beyond digestion, GIT microbes potentially contribute to the overall health status of the host. The commensal bacteria alert the immune system and allow the host to distinguish between commensal and pathogenic bacteria. The dynamic interactions and overall composition of the microbial community in the gastrointestinal tract of animals and humans ultimately reflect the co-evolution of microorganism with their host and the environments, and diets adopted by the host. Due to the impact on nutritional, physiological, and immunological processes, the microbial community of the gut markedly influences health and performance of the host. The intestinal epithelium has the capacity to ensure optimal absorption of nutrients, simultaneously it exclude and neutralize or detoxify pathogenic microbes. This epithelial function is influenced by direct host/microbiota interactions, and through microbial activities on the diet, accordingly, a microbiota ensuring the best possible operation of the epithelium may be considered advantageous.
The dietary components affect intestinal microbiota of the host. Both domestic cats and dogs are members of Carnivora, but their intestinal microbial diversity varies to some extent. In felines, Firmicutes have been shown to be the most predominant bacterial group, followed by Proteobacteria, Bacteroidetes, Fusobacteria, and Actinobacteria respectively. The predominant phylum of dogs, humans and mice are Fusobacteria and that dogs have a relatively greater percentage of Firmicutes. Among the groups of archaea, the methanogenic archaea were also the most abundant group in dogs, human, mice and chicken.
Microbiota and nutrient metabolism
Microbiota and carbohydrate metabolism
The sophisticated relationship between the GI tract and gut microbes allows for efficient use of dietary carbohydrates. The monogastrics and humans can adsorb monosaccharides (eg. glucose and galactose) in the small intestine, and can also hydrolyze certain disaccharides such as sucrose and lactose to their constituent monosaccharides. However, they have limited enzymatic ability to degrade complex polysaccharides from the diet. Therefore, simple sugars are absorbed in the proximal small intestine by active transport, and undigested dietary polysaccharides enter into the distal small intestine and colon, where they are degraded by microbial action. The degradation of these undigested dietary polysaccharides is a fermentation process involving numerous gut microbes, including Bacteroides, Bifidobacterium, Ruminococcus, and Roseburia spp., as well as some microbes from Clostridium, Eubacterium, and Enterococcus genera. The major end products of polysaccharide fermentation are SCFA i.e. acetate, propionate, and butyrate, which provide energy for the host and are involved in a number of physiological functions. These SCFAs can be used as the energy source for epithelial cells or peripheral tissues. The SCFAs stimulate the colonic sodium and fluid absorption, modify the microbial composition, and regulate the glucose and energy homeostasis. Acetate and propionate can be taken up by the liver and be used as substrates for liver cholesterol and fatty acid biosynthesis; propionate can also act as a substrate for gluconeogenesis. Butyrate has been shown to improve insulin sensitivity, and modulate immune responses by macrophages.
Microbiota and protein metabolism
The gut microbes can affect nitrogen balance by de novo synthesis of amino acids and intestinal urea recycling. The gut microbes contribute to the circulating pool of essential amino acids. It has been stated that up to 20% of circulating lysine and threonine in nonruminant mammals, including adult humans, is synthesized by gut microbes. The intestinal microbiota also contributes to nitrogen balance by participating in urea nitrogen salvaging (UNS). Increased urease expression in gut microbes results in metabolism of urea in the GI tract into ammonia and carbon dioxide. Some of the ammonia can be used for microbial synthesis of amino acids. Perhaps more importantly, the nitrogen generated during this process (urea nitrogen) can re-enter the host circulation and serve as a substrate for synthetic processes. Reduced urea recycling has been reported in germ free animals and in humans receiving antibiotic therapy.
Microbiota and lipid metabolism
Triglycerides are a prominent source of energy during critical illness. Their body supply is tightly linked to the intestinal microbiota. The conventionalization of adult germ-free (GF) mice with a normal microbiota harvested from the distal intestine (caecum) of conventionally raised animals produces a 60% increase in body fat content and insulin resistance within 14 days despite reduced food intake. This has been attributed to microbiota that promotes absorption of monosaccharides from the gut lumen, with resulting induction of de novo hepatic lipogenesis. Colonization of GF mice with just a single gut microbe (B. thetaiotaocmicron) significantly increases total body fat content, although the increase in fat content was less than that seen with transfer of the complete mouse microbiota. In addition, microbiota stimulates increased hepatic triglyceride production and promotes storage of adipocyte triglycerides by suppressing the activity of a circulating inhibitor of lipoprotein lipase.
Microbiota and Vitamins
The intestinal gut microbiota also acts as an important supplier of various vitamins. Several common bacterial genera in the distal intestine (eg, Bacteroides, Bifidobacterium, and Enterococcus) are known to synthesize vitamins. Thiamine, folate, biotin, riboflavin, and panthothenic acid are water-soluble vitamins that are plentiful in the diet, but that are also synthesized by gut bacteria. Similarly, it has been stated that up to half of the daily vitamin K requirement is provided by gut bacteria. The molecular structure of bacterially synthesized vitamins is not always identical to the dietary forms of the vitamins. The Bifidobacterium and Lactobacillus spp. have been verified to have folate biosynthetic properties; Lactobacillus reuteri also has the ability to produce cobalamin (vitamin B12). Several specialized epithelial transporters have been recognized to participate specifically in the absorption of vitamins derived from gut bacteria.
Gut microbiota should be considered a key aspect in animal nutrition. It influences many areas of animal health from innate immunity to appetite and nutrient metabolism.