Human flora

The human flora is the microrganisms that constantly inhabit the human body. They include 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 known beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, are termed "normal flora", 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 1000 species of bacteria live in the human body.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] Bacterial cells are much smaller than human cells, and there are at least ten times as many bacteria as human cells in the body (approximately 10¹⁴ versus 10¹³).citation
last = Savage | first = D. C.
year = 1977
title = Microbial Ecology of the Gastrointestinal Tract
journal = Annual Review of Microbiology
volume = 31
pages = 107
doi = 10.1146/annurev.mi.31.100177.000543] citation
last = Berg | first = R.
year = 1996
title = The indigenous gastrointestinal microflora
journal = Trends in Microbiology
volume = 4
pages = 430
doi = 10.1016/0966-842X(96)10057-3] 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), 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 carbohydrates that humans otherwise could not digest. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen. Bacteria of the normal flora can act as opportunistic pathogens at times of lowered immunity.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]

"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. [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] Certain mutated strains of these gut bacteria do cause disease; an example is "".

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 plaque. If this is not removed by brushing, it hardens into 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 "L. iners" followed by "L. crispatus". Other lactobacilli found in the vagina are "L. delbruekii" and "L. gasseri". Disturbance of the vaginal flora can lead to bacterial vaginosis.

Human bacterial flora and human health

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 vitamins such as folic acid, vitamin K and biotin, and they ferment complex indigestible carbohydrates. [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] [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] Other beneficial bacteria in the normal flora include "Lactobacillus" species, which convert lactose and other sugars to lactic acid in the gut. [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] 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 supplements. [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]

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. [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] The dominant group is the methanogens, particularly "Methanobrevibacter smithii" and "Methanosphaera stadtmanae". [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] However, colonization by methanogens is variable and only about 50% of humans have easily-detectable populations or these organisms. [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]

Archaea and human health

As of 2007, no clear examples of archaeal pathogens are known, [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] [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] although a relationship has been proposed between the presence of some methanogens and human periodontal disease. [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]

Fungal flora

Fungi, particularly yeasts are present in the human gut. The best studied of these are "Candida" species, due to their ability to become pathogenic in immunocompromised hosts. [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] Yeasts are also present on the skin, particularly "Malassezia" species, where they consume oils secreted from the sebaceous glands. [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] [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]

References

ee also

*Microorganism
*Human microbiome project