INTESTINAL FLORA
The large intestine houses over 700 species of bacteria that perform a variety of functions.
The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. The bicarbonate the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.
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 generally provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who 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 the gases hydrogen, methane, and hydrogen sulphide. Bacterial fermentation of undigested polysaccharides produces these. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.
Colon bacteria are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, which are also effective against related pathogens, thereby preventing infection or invasion. The most prevalent bacteria in the colon 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. This mucus layer is called the mucosal barrier.
