Human flora
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The '''human flora''' is the [[microrganism]]s that constantly inhabit the [[human body]]. They include [[bacterium|bacteria]], [[fungi]] and [[archaea]]. Some of these organisms are known to perform tasks that are useful for the human host, while the majority have no beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, are termed ''normal flora'',<ref name="texas"/> or ''microbiota''. An effort to better describe the [[microflora]] of humans has been initiated; see [[Human microbiome project]].
==Bacterial flora==
It is estimated that 500 to 100,000 [[species (biology)|species]] of bacteria live in the human body <ref name="sears">{{cite journal|title=A dynamic partnership: celebrating our gut flora |url=http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=16701579&cmd=showdetailview&indexed=google |author=Sears CL |date=2005 Oct;11|volume=(5):247-51|work=Anaerobe| publisher=Academic Press|pages=247–251}}</ref>. Bacterial cells are much smaller than human [[cell (biology)|cells]], and there are about ten times as many bacteria as human cells in the body (1000 trillion or 1 quadrillion (10<sup>15</sup>) versus 100 trillion (10<sup>14</sup>)).<ref name="sears"/> Though normal flora are found on all surfaces exposed to the environment (on the skin and eyes, in the mouth, nose, [[small intestine]], and [[colon (anatomy)|colon]]), the vast majority of bacteria live in the [[large intestine]].
Many of the bacteria in the [[digestive tract]], collectively referred to as [[gut flora]], are able to break down certain nutrients such as [[carbohydrate]]s that humans otherwise could not [[digestion|digest]]. The majority of these [[commensal bacteria]] are [[anaerobe]]s, meaning they survive in an environment with no [[oxygen]]. Bacteria of the normal flora can act as [[opportunistic pathogen]]s at times of [[immune system|lowered immunity]]. <ref name="texas">{{cite journal|title=Bacteriology|pages= Chapter 6. Normal Flora |date= 1996 | coauthors= Charles Patrick. Davis |publisher=University of Texas Medical Branch at Galveston| author=Samuel Baron MD|unused_data=|http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.chapter.500}}</ref>
''[[Escherichia coli]]'' (a.k.a. ''E. coli'') is a bacterium that lives in the colon; it is an extensively studied [[model organism]] and probably the best understood bacterium of all.<ref>{{cite journal | author = Lee PS, Lee KH | title = Escherichia coli--a model system that benefits from and contributes to the evolution of proteomics. | journal = Biotechnol Bioeng | volume = 84 | issue = 7 | pages = 801–14 | year = 2003 | pmid = 14708121 | doi = 10.1002/bit.10848}}</ref> Certain mutated strains of these gut bacteria do cause disease; an example is ''[[E. coli O157:H7]]''.
A number of types of bacteria, such as ''[[Actinomyces]] viscosus'' and ''A. naeslundii'', live in the mouth, where they are part of a sticky substance called [[Dental plaque|plaque]]. If this is not removed by brushing, it hardens into [[calculus (dental)|calculus]] (also called tartar). The same bacteria also secrete acids that dissolve [[tooth enamel]], causing [[tooth decay]].
The vaginal microflora consist mostly of various [[lactobacillus]] species. It was long thought that the most common of these species was ''[[Lactobacillus acidophilus]]'', but it has later been shown that the most common one is ''[[Lactobacillus iners|L. iners]]'' followed by ''[[Lactobacillus crispatus|L. crispatus]]''. Other lactobacilli found in the vagina are ''[[Lactobacillus delbruekii|L. delbruekii]]'' and ''[[Lactobacillus gasseri|L. gasseri]]''. Disturbance of the vaginal flora can lead to [[bacterial vaginosis]].
===Human bacterial flora and human health===
[[Image:SalmonellaNIAID.jpg|thumb|250px|right|Color-enhanced scanning electron micrograph showing ''[[Salmonella typhimurium]]'' (red) invading cultured human cells]]
[[Bacteria]] are vital for the maintenance of human health, but some [[pathogenic bacteria]] also pose a significant health threat by causing diseases. Large numbers of bacteria live on the skin and in the digestive tract. Their growth can be increased by warmth and sweat. Large populations of these organisms on humans are the cause of [[body odor]] and thought to play a part in [[acne]]. There are more than 500 bacterial species present in the normal human gut and are generally beneficial: they synthesize [[vitamin]]s such as [[folic acid]], [[vitamin K]] and [[biotin]], and they ferment complex indigestible [[carbohydrate]]s.<ref>{{cite journal | author = O'Hara A, Shanahan F | title = The gut flora as a forgotten organ | journal = EMBO Rep | volume = 7 | issue = 7 | pages = 688 – 93 | year = 2006 | pmid = 16819463 | doi = 10.1038/sj.embor.7400731}}</ref><ref>{{cite journal | author = Zoetendal E, Vaughan E, de Vos W | title = A microbial world within us | journal = Mol Microbiol | volume = 59 | issue = 6 | pages = 1639 – 50 | year = 2006 | pmid = 16553872 | doi = 10.1111/j.1365-2958.2006.05056.x}}</ref> Other beneficial bacteria in the normal flora include ''[[Lactobacillus]]'' species, which convert [[lactose]] and other sugars to lactic acid in the gut.<ref>{{cite journal | author = Gorbach S | title = Lactic acid bacteria and human health | journal = Ann Med | volume = 22 | issue = 1 | pages = 37 – 41 | year = 1990 | pmid = 2109988 | doi = 10.3109/07853899009147239}}</ref> The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria (usually through [[competitive exclusion]]) and some beneficial bacteria are consequently sold as [[probiotic]] [[dietary supplement]]s.<ref>{{cite journal | author = Salminen S, Gueimonde M, Isolauri E | title = Probiotics that modify disease risk | url=http://jn.nutrition.org/cgi/content/full/135/5/1294 | journal = J Nutr | volume = 135 | issue = 5 | pages = 1294 – 8 | year = 2005 | pmid = 15867327}}</ref>
==Archaean flora==
[[Archaea]] are present in the human gut, but in contrast to the enormous variety of [[bacteria]] in this organ, the number of archaeal species are much more limited.<ref>{{cite journal |author=Eckburg PB, Bik EM, Bernstein CN, ''et al'' |title=Diversity of the human intestinal microbial flora |journal=Science |volume=308 |issue=5728 |pages=1635–8 |year=2005 |pmid=15831718 |doi=10.1126/science.1110591}}</ref> The dominant group is the [[methanogen]]s, particularly ''[[Methanobrevibacter|Methanobrevibacter smithii]]'' and ''[[Methanosphaera|Methanosphaera stadtmanae]]''.<ref>{{cite journal |author=Duncan SH, Louis P, Flint HJ |title=Cultivable bacterial diversity from the human colon |journal=Lett. Appl. Microbiol. |volume=44 |issue=4 |pages=343–50 |year=2007 |pmid=17397470 |doi=10.1111/j.1472-765X.2007.02129.x}}</ref> However, colonization by methanogens is variable and only about 50% of humans have easily-detectable populations or these organisms.<ref>{{cite journal |author=Florin TH, Zhu G, Kirk KM, Martin NG |title=Shared and unique environmental factors determine the ecology of methanogens in humans and rats |journal=Am. J. Gastroenterol. |volume=95 |issue=10 |pages=2872–9 |year=2000 |pmid=11051362 |doi=10.1111/j.1572-0241.2000.02319.x}}</ref>
===Archaea and human health===
As of 2007, no clear examples of archaeal [[pathogen]]s are known,<ref>{{cite journal |author=Eckburg P, Lepp P, Relman D |title=Archaea and their potential role in human disease |journal=Infect Immun |volume=71 |issue=2 |pages=591–6 |year=2003 |pmid=12540534 |doi=10.1128/IAI.71.2.591-596.2003}}</ref><ref>{{cite journal |author=Cavicchioli R, Curmi P, Saunders N, Thomas T |title=Pathogenic archaea: do they exist? |journal=Bioessays |volume=25 |issue=11 |pages=1119–28 |year=2003 |pmid=14579252 |doi=10.1002/bies.10354}}</ref> although a relationship has been proposed between the presence of some methanogens and human [[periodontal disease]].<ref>{{cite journal |author=Lepp P, Brinig M, Ouverney C, Palm K, Armitage G, Relman D |title=Methanogenic Archaea and human periodontal disease |journal=Proc Natl Acad Sci U S A |volume=101 |issue=16 |pages=6176–81 |year=2004 |pmid=15067114 |doi=10.1073/pnas.0308766101}}</ref>
==Fungal flora==
Fungi, particularly [[yeast]]s are present in the human gut. The best studied of these are ''[[Candida (genus)|Candida]]'' species, due to their ability to become pathogenic in [[Immunodeficiency|immunocompromised hosts]].<ref>{{cite journal |author=Bernhardt H, Knoke M |title=Mycological aspects of gastrointestinal microflora |journal=Scand. J. Gastroenterol. Suppl. |volume=222 |issue= |pages=102–6 |year=1997 |pmid=9145460}}</ref> Yeasts are also present on the skin, particularly ''[[Malassezia]]'' species, where they consume oils secreted from the [[sebaceous gland]]s.<ref>{{cite journal |author=Marcon MJ, Powell DA |title=Human infections due to Malassezia spp |journal=Clin. Microbiol. Rev. |volume=5 |issue=2 |pages=101–19 |year=1992 |pmid=1576583 |url=http://cmr.asm.org/cgi/pmidlookup?view=long&pmid=1576583}}</ref><ref>{{cite journal |author=Roth RR, James WD |title=Microbial ecology of the skin |journal=Annu. Rev. Microbiol. |volume=42 |pages=441–64 |year=1988 |pmid=3144238 |doi=10.1146/annurev.mi.42.100188.002301}}</ref>
==References==
{{reflist|2}}
==See also==
*[[Microorganism]]
*[[Human microbiome project]]
[[Category:Bacteriology]]
[[Category:Bacteria]]
[[Category:Health]]
[[Category:Microbiology]]
[[pl:Flora fizjologiczna]]
[[sv:Normalflora]]
[[uk:Бактерії та здоров'я людини]]
[[zh:正常菌群]]