The human digestive tract begins in the mouth and extends to the anus. Gut flora refers to the microorganisms residing in the digestive tract. These microorganisms are generally harmless. Their distribution varies along the location of the gut and among individuals. They play an important role in the protective, structural, and metabolic functioning of the gut.
Bacteria are the dominant microorganisms in the gut. They dominate both the jejunum and ileum of the distal small intestine as well as the large intestine. Acidic, pancreatic, and bile secretions prevent their colonization of the stomach and the duodenum of the proximal small intestine. Aerobic bacteria, such as aerobic lactobacilli and enterococci, are the dominant gut flora in the jejunum, and the ileum and large intestine are dominated by anaerobic bacteria such as bifidobacteria, Escherichia coli, and anaerobic lactobacilli. In an adult gut there may be some alterations in bacterial population when there are changes in age, diet, lifestyle, and environment.
Under normal conditions, the human host and its gut flora have a symbiotic relationship wherein both are beneficial to each other. The human host provides food and a stable environment for the gut flora. Meanwhile, the gut flora provides benefits to its human host by preventing the colonization of harmful bacteria in the gut, preventing intestinal inflammation, contributing to the development of the immune system, enhancing food and water absorption, and synthesizing vitamins K and B12.
The growth of harmful bacteria is prevented through barrier effect, meaning the adherence of the gut flora to the intestinal lining competes with and inhibits the invasion of harmful bacteria. Its role in preventing intestinal inflammation and in contributing to the development of the immune system is attributed to the ability of the gut flora to stimulate the lymphoid tissues in the gut to produce antibodies to harmful bacteria. Growth of lymphoid tissues may also be caused by the action of short-chain fatty acids, which are products of carbohydrate fermentation by bacteria. In addition, short-chain fatty acids control the proliferation and differentiation of cells lining the intestine that help in preventing injury of the gut lining.
Aside from carbohydrate fermentation, enzymes produced by the gut flora also enhance the absorption of carbohydrates. These enzymes digest carbohydrates into a form of useful energy and nutrients for humans. In addition, the absorption of water and dietary minerals such as iron, calcium, and magnesium, which are also essential in the metabolic functions of the human body, is also enhanced.