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Gut flora or, more appropriately, gut microbiota, consists of a complex community of microorganism species that live in the digestive tracts of animals and is the largest reservoir of microorganisms mutual to humans. In this context ''gut'' is synonymous with ''intestinal'', and ''flora'' with ''microbiota'' and ''microflora''. The gut microbiome refer to the genomes of the gut microbiota. Gut microorganisms benefit the host by gleaning the energy from the fermentation of undigested carbohydrates and the subsequent absorption of short-chain fatty acids. The most important of these fatty acids are butyrates, metabolised by the colonic epithelium; propionates by the liver; and acetates by the muscle tissue. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols and xenobiotics. The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than the total number of human cells in the body. The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a "forgotten" organ. It is estimated that these gut flora have around a hundred times as many genes in aggregate as there are in the human genome. ==Description== Vast majority of microbial content of colon consists of Bacteria. They also make up to 60% of the dry mass of feces. Somewhere between 300〔 and 1000 different species live in the gut,〔 with most estimates at about 500.〔 However, it is probable that 99% of the bacteria come from about 30 or 40 species. Fungi, protozoa, and archaea also make up a part of the gut flora, but little is known about their activities. Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship.〔 Though people can survive without gut flora,〔 the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful, pathogenic bacteria,〔 regulating the development of the gut, producing vitamins for the host, such as biotin〔http://www.pnas.org/content/28/7/285?ijkey=931998456351a78f5d633e70cbc46b08369e0414&keytype2=tf_ipsecsha〕 and vitamin K,〔https://www.clinicalkey.com/#!/ContentPlayerCtrl/doPlayContent/1-s2.0-S095816691200119X〕 and producing hormones to direct the host to store fats. In return, these microorganisms procure within the host a protected, nutrient-rich environment in which they can thrive. However, in certain conditions, some species are thought to be capable of causing disease by producing infection or increasing cancer risk for the host.〔 Over 99% of the bacteria in the gut are anaerobes,〔〔〔 but in the cecum, aerobic bacteria reach high densities.〔 The compositions of microbiota rely on several factors like host diet, colonization history, and immune status. Some microbes are better suited to complement specific metabolic enzymes over others. For example, ''Bacteroides'' change how they break down carbohydrates depending on if they received the right amount of certain substrates. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gut flora」の詳細全文を読む スポンサード リンク
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