Editing Large intestine (section)

===Gut flora===
{{main|Gut microbiota}}

The large intestine houses over 700 species of [[bacteria]] that perform a variety of functions, as well as [[fungi]], [[protozoa]], and [[archaea]]. Species diversity varies by geography and diet.<ref>{{cite journal | last1 = Yatsunenko | first1 = Tanya | display-authors = etal | year = 2012 | title = Human gut microbiome viewed across age and geography | journal = Nature | volume = 486 | issue = 7402| pages = 222–227 | pmc = 3376388 | pmid = 22699611 | doi = 10.1038/nature11053 | bibcode = 2012Natur.486..222Y }}</ref> The microbes in a human distal gut often number in the vicinity of 100 trillion, and can weigh around 200 grams (0.44 pounds). This mass of mostly symbiotic microbes has recently been called the latest human organ to be "discovered" or in other words, the "forgotten organ".<ref>{{cite journal | last1 = O'Hara | first1 = Ann M. | last2 = Shanahan | first2 = Fergus | year = 2006 | title = The gut flora as a forgotten organ | journal = EMBO Reports | volume = 7 | issue = 7| pages = 688–693 | doi=10.1038/sj.embor.7400731| pmid = 16819463 | pmc = 1500832 }}</ref>

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 that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.<ref>{{Cite journal|title = The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism|journal = Journal of Lipid Research|date = 2013-09-01|issn = 0022-2275|pmc = 3735932|pmid = 23821742|pages = 2325–2340|volume = 54|issue = 9|doi = 10.1194/jlr.R036012 |doi-access=free |first1 = Gijs|last1 = den Besten|first2 = Karen|last2 = van Eunen|first3 = Albert K.|last3 = Groen|first4 = Koen|last4 = Venema|first5 = Dirk-Jan|last5 = Reijngoud|first6 = Barbara M.|last6 = Bakker}}</ref>

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 who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with [[antibiotics]] that inhibit the vitamin producing species of bacteria as well as the intended disease-causing bacteria.<ref>{{Cite journal|title = Time to Recognize Our Fellow Travellers|journal = Journal of General Internal Medicine|date = 2012-12-01|issn = 0884-8734|pmc = 3509308|pmid = 22588826|pages = 1704–1706|volume = 27|issue = 12|doi = 10.1007/s11606-012-2105-6|first1 = Travis B.|last1 = Murdoch|first2 = Allan S.|last2 = Detsky}}</ref>

Other bacterial products include gas ([[flatulence|flatus]]), which is a mixture of [[nitrogen]] and [[carbon dioxide]], with small amounts of the gases [[hydrogen]], [[methane]], and [[hydrogen sulfide]]. Bacterial [[Fermentation (biochemistry)|fermentation]] of undigested [[polysaccharides]] produces these. Some of the fecal odor is due to [[indole]]s, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and [[lymphatics]].{{Citation needed|date=March 2014}}

They are 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 two most prevalent phyla of the colon are [[Bacillota]] and [[Bacteroidota]]. The ratio between the two seems to vary widely as reported by the Human Microbiome Project.<ref name="Structure, function and diversity of the healthy human microbiome">{{cite journal|last1=Human Microbiome Project Consortium|title=Structure, function and diversity of the healthy human microbiome|journal=Nature|date=Jun 14, 2012|volume=486|issue=7402|pages=207–214|doi=10.1038/nature11234|pmid=22699609|pmc=3564958|bibcode=2012Natur.486..207T}}</ref> [[Bacteroides]] are implicated in the initiation of [[colitis]] and [[colon cancer]]. [[Bifidobacteria]] are also abundant, and are often described as 'friendly bacteria'.<ref>{{Cite journal|title = Commensal Bacteroides species induce colitis in host-genotype-specific fashion in a mouse model of inflammatory bowel disease|journal = Cell Host & Microbe|date = 2011-05-19|issn = 1931-3128|pmc = 3241010|pmid = 21575910|pages = 390–403|volume = 9|issue = 5|doi = 10.1016/j.chom.2011.04.009|first1 = Seth M.|last1 = Bloom|first2 = Vinieth N.|last2 = Bijanki|first3 = Gerardo M.|last3 = Nava|first4 = Lulu|last4 = Sun|first5 = Nicole P.|last5 = Malvin|first6 = David L.|last6 = Donermeyer|first7 = W. Michael|last7 = Dunne|first8 = Paul M.|last8 = Allen|first9 = Thaddeus S.|last9 = Stappenbeck}}</ref><ref>{{Cite journal|title = Diversity, ecology and intestinal function of bifidobacteria|journal = Microbial Cell Factories|date = 2014-08-29|issn = 1475-2859|pmc = 4155821|pmid = 25186128|pages = S4|volume = 13|issue = Suppl 1|doi = 10.1186/1475-2859-13-S1-S4|first1 = Francesca|last1 = Bottacini|first2 = Marco|last2 = Ventura|first3 = Douwe|last3 = van Sinderen|first4 = Mary|last4 = O'Connell Motherway | doi-access=free }}</ref>

A [[mucus]] layer protects the large intestine from attacks from colonic [[commensal bacteria]].<ref>{{Cite journal|title = The gastrointestinal mucus system in health and disease|journal = Nature Reviews. Gastroenterology & Hepatology|date = 2013-06-01|issn = 1759-5045|pmc = 3758667|pmid = 23478383|pages = 352–361|volume = 10|issue = 6|doi = 10.1038/nrgastro.2013.35|first1 = Malin E.V.|last1 = Johansson|first2 = Henrik|last2 = Sjövall|first3 = Gunnar C.|last3 = Hansson}}</ref>