The large intestine houses over 700 species of bacteria , the largest bacterial ecosystem in the human body that perform a wide variety of functions. The large intestine absorbs some of the products formed by the bacteria inhabiting this region. For example, undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and then absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.
Bacterial flora
Escherichia coli is one of the many species of bacteria present in the human gut.
These bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual that depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with antibiotics that inhibit other species of bacteria as well as the disease-causing bacteria.
Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of hydrogen, methane, and hydrogen sulphide. These are produced as a result of bacterial fermentation of undigested polysaccharides. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.
Bacterial flora is also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion. The most prevalent bacteria are the bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.
A mucus layer protects the large intestine from attacks from colonic commensal bacteria. Factors that disrupt the microorganism population of the large intestine include antibiotics, stress, and parasites.