Intestinal bacteria
The GI tract is home to a vast population of microbes, particularly in the more distal parts of the intestine. Gastrointestinal microbes are an important source of some vitamins.
For example, it has been estimated that approximately 1/4 of the total body folate in humans is derived from the bacterial flora of the gut. Also, the presence of a normal GI microflora is necessary for the development of normal intestinal mucosal structure. Finally, many species of bacteria assist in the fermentation and digestion of dietary fiber. Lactobacillus and Bifidobacter spp., for instance, are able to ferment complex polysaccharides such as inulin and fructo-oligosaccharides, generating short-chain fatty acids that are used as an energy source by the enterocytes.The GI tract microflora is made up of a mixture of aerobes, microaerobes, as well as facultative and obligate anaerobic bacteria. Much of the anaerobic flora is found in close association with the GI mucosa, making reproducible sampling of this flora difficult.
The normal canine intestinal microflora is highly complex, varying markedly from individual to individual and at differing points within the GI tract.8 Each individual appears to have a unique bacterial flora present within its GI tract, and based on longitudinal studies in several species, this flora is apparently quite stable in healthy individuals.
The normal bacterial content in the feline small intestine has not been thoroughly explored, but it appears that, in comparison to dogs, the total number of bacteria in the small intestine and the proportion of obligate anaerobic bacteria are greater in the cat.9,10 While the actual organisms present were not defined, experimental work investigating the effects of diet and antibiotic therapy on serum markers of intestinal bacterial biomass in cats, such as serum cobalamin and folate concentrations, suggest that the normal flora in the cat is stable and resilient, with serum markers returning to pre-treatment concentrations rapidly after withdrawal of antibiotic ther- apy.11
While the GI microflora is an integral part of the normal intestinal physiology, it also contains many organisms that are potential pathogens. Bacterial translocation across the GI mucosa is a major cause of sepsis in critically ill patients.
The GI tract itself is susceptible to infection by specific, identified pathogens from contaminated food or the environment, such as Salmonella spp., which can lead to GI disease.The presence of an increased biomass of bacteria in the GI tract may also be associated with intestinal disease (see 5.3.8). This increase in biomass may occur due to the loss of normal regulatory mechanisms that control the size of the intestinal bacterial population. Loss of gastric acid (for instance, from the long-term use of antacid medications), loss of antibacterial pancreatic secretions in exocrine pancreatic insufficiency, secretory IgA deficiency, and suppression of mucosal cellular immunity may all be associated with an increase in the total bacterial biomass present within the small intestine. This increased bacterial biomass may then exert adverse effects on the host animal through increased competition for limiting nutrients, increased production of toxic metabolites from bile acids and nutrients, or direct invasion of the mucosa and production of bacterial exotoxins.12
O Key Facts
■ Physiological regulation of the intestine involves complex interactions between the endocrine, neural, and immune systems that are specific to the intestine.
■ The villus is the functional unit of the small intestine. Diseases affecting the villus are associated with diarrhea and malabsorption.
■ The gastrointestinal microflora plays an integral part in normal intestinal physiology.