at birth there are no bacteria in the gastrointestinal tract. during birth however bacteria from the mother's colon and vagina are swallowed by the infant and within a few weeks or months they populate the infant's gastrointestinal tract. the relationship between normal intestinal bacteria and their human host is complex. the relationship is symbiotic which means that each benefits from the other. the bacteria benefit from the warm moist environment of the small intestine that is ideal for growing as well as the constant flow of food passing down the gastrointestinal tract. the human host benefits in several ways. for example the normal bacteria stimulate the growth of the intestinal lining and the immune system of the intestine. they prevent the growth of disease-causing bacteria within the intestine. they produce vitamin k which is absorbed and used by the host. in fact the bacteria are important even for the muscular activity of the small intestine. without bacteria there is reduced muscular activity. there is a delicate balance between the bacteria of the gastrointestinal tract and the human host. the gastrointestinal tract particularly the small intestine contains an extensive immune system. the immune system protects the intestine from disease-causing viruses bacteria and parasites. (the effects of the immune response have been experienced by anyone who has experienced gastroenteritis. ) the interesting fact is that the intestine does not attack the normal bacteria within it only disease-causing bacteria. somehow the intestine becomes tolerant of the normal bacteria and does not mount an attack against them. the intestine has other traits that may be important in protecting it from bacteria both normal and disease-causing. muscular activity keeps the numbers of bacteria within the intestine at a low level. mucus that is secreted into the intestine coats the intestinal lining and prevents the bacteria from touching the lining. the intestine secretes antibodies that can block and sometimes kill bacteria as well as substances that prevent the growth of bacteria. finally the lining of the intestine can produce receptors for toxic substances produced by bacteria and can prevent the substances from having their toxic effects.
what is the normal relationship between bacteria and the small intestine?