Bacteria

[[Image:Bacterial cell.png|thumb|right|165px|A bacterium with its [[Irreducible_complexity|irreducibly complex]] flagellum which allegedly proves the existence of [[Intelligent Design]]]]{{biology}}
A '''bacterium''' ({{hint|plural|pl}}. '''bacteria''') is a single-celled organism lacking the more complex organelles such as [[Mitochondrion|mitochondria]] and [[chloroplast]]s, which are themselves thought to be symbiotic bacteria (see [[endosymbiosis]]). They reproduce by binary fission{{efn|'Binary Fission': Asexual reproduction by a separation of the body into two new bodies.}} and share genes through transfer of plasmids, small ring-shaped chains of [[DNA]].

Bacteria are sometimes thought to be the "oldest" domain (or "superkingdom") of living organisms, in the sense that they probably appeared first in the history of life on [[Earth]], before the also single-celled domain Archaea (and ''long'' before [[eukaryote]] domain). Whether this specific statement is true or not, microfossils going back hundreds of millions of years before the origin of eukaryotic life show that they are true evolutionary heavyweights, collectively robust against every major climatic change or other environmental disruption in their very long history. Their endurance and [[Biodiversity|diversity]], with more than a dozen phyla represented comprising millions of species, and a truly massive array of different [[Biochemistry|biochemistries]] across a staggering variety of environments, arguably make them the most successful form of [[life]] that has ever existed.

==Classification==
As a taxonomic domain, bacteria are immensely varied, so a proper taxonomy involves subdividing them in a variety of ways. One of the most basic of these is the use of the Gram stain, a violet dye which is absorbed into the cell walls of some bacteria but not others, followed by a counterstain. Bacteria that take up the first stain are termed ''Gram-positive'', and develop a deep violet color as a result of the procedure. Bacteria that reject the stain are given a red or pink color by the counterstain, and are termed ''Gram-negative''. The results of this procedure, while not perfect (Gram-variable and Gram-indeterminate bacteria exist), provide taxonomically valuable information about the physical structure of the organism(s).<ref>[http://www.life.umd.edu/classroom/bsci424/LabMaterialsMethods/GramStain.htm The Gram Stain, online at the University of Maryland]</ref> While it is hardly a be-all and end-all, it remains critically important to bacteriology long after its discovery.

Today, with the advent of [[molecular biology]] and genetic analysis, it is possible to classify bacteria much more specifically than traditional methods such as the Gram stain allowed for. Molecular studies have allowed for the creation of detailed bacterial [[phylogeny|phylogenies]].

==Microbiomes==
Bacteria interact with more specialized forms of life in a variety of ways. The [[human]] body is home to hundreds of distinct [[species]] of bacteria living in the mouth, gut, on the skin and elsewhere. Most of these are harmless passengers and some, such as ''E. coli'' are actually very important in keeping the body running properly. (The amount of ''Escherichia coli'' present is often used to indicate how many pathogens there are in [[water]], but they themselves are actually quite harmless.) Some of these bacteria can become pathogenic to humans under certain circumstances, such as when a person's immune system is compromised. Other species of bacteria, such as ''Neisseria meningitidis'' (also known as meningococcus), [[MRSA|''Staphylococcus aureus'']], ''Yersinia pestis'' (the cause of the [[plague|Black Death]]) and ''Mycobacterium tuberculosis'' (which causes tuberculosis) can cause illnesses ranging from trivial to severe. In [[plants]], [[Nitrogen fixation|nitrogen-fixing]] bacteria such as those found on the root nodules of legumes (e.g. peas, beans, [[soy]], lentils and peanuts) can be very important to maintaining healthy soil.

Bacteria also interact with [[Virus|viruses]], for example by serving as "prey" for a group of viruses called bacteriophages. Because of their extreme selectivity in the cells that they attack (sometimes restricted to specific species of bacteria, and generally ignoring eukaryotic cells), these viruses have been studied for use as medical treatments in a set of techniques called phage therapy.

===Importance of microbiomes===
It was once thought that the microbiome primarily or even exclusively existed in the mouth, gut, [[vagina]] and skin, but new research has found microbes in healthy people in the uterus, [[breast]] [[milk]],<ref>[http://www.the-scientist.com/?articles.view/articleNo/40038/title/The-Maternal-Microbiome/ The Maternal Microbiome: Moms bombard their babies with bugs both before and after they’re born],</ref> urine,<ref>[http://www.asmscience.org/content/journal/microbe/10.1128/microbe.8.388.2 Urine, Once Thought Sterile, Has Its Own Microbiome]</ref> and blood.<ref>[http://www.ncbi.nlm.nih.gov/pubmed/23372728 Blood microbiota dysbiosis is associated with the onset of cardiovascular events in a large general population: the D.E.S.I.R. study]</ref> The gut microbiome also has an effect on [[brain]] functioning,<ref>[http://www.nature.com/nrn/journal/v15/n2/full/nrn3669.html The gut–microbiome–brain connection]</ref><ref name="bloodbrain">[http://news.sciencemag.org/biology/2014/11/bodys-bacteria-may-keep-our-brains-healthy Body's bacteria may keep our brains healthy]</ref> and mice that have had their gut microbiomes removed by chronic antibiotics have impaired [[memory]] and reduced nerve cell growth in the hippocampus.<ref>[https://www.sciencenews.org/article/wiping-out-gut-bacteria-impairs-brain: Wiping out gut bacteria impairs brain Nerve cell production, memory affected in mice treated with antibiotics] by Laura Sanders (12:00pm, May 19, 2016) ''Science News''.</ref><ref>[http://www.cell.com/cell-reports/abstract/S2211-1247(16)30518-6?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS2211124716305186%3Fshowall%3Dtrue Ly6C<sup>hi</sup> Monocytes Provide a Link between Antibiotic-Induced Changes in Gut Microbiota and Adult Hippocampal Neurogenesis] by Luisas Möhle et al. ''Cell Reports'' Volume 15, Issue 9, p. 1945–1956, 31 May 2016. DOI: http://dx.doi.org/10.1016/j.celrep.2016.04.074.</ref>

The estimates of how many microbial cells that humans carry around has varied rather wildly in recent years, ranging from 100 microbes to 1 human cell down to 1-to-1, the latter being the most recent estimate for bacteria-only to human cells as of 2016.<ref>[https://www.sciencenews.org/article/body%E2%80%99s-bacteria-don%E2%80%99t-outnumber-human-cells-so-much-after-all?mode=magazine&context=191406&tgt=nr Body’s bacteria don’t outnumber human cells so much after all: New calculations suggest roughly equal populations, not 10-to-1 ratio] by  Tina Hesman Saey (5:47pm, January 8, 2016) ''Science News''.</ref><ref>[https://www.sciencenews.org/article/year-review-your-body-mostly-microbes Microbiome results argue for new view of animals as superorganisms]</ref> 

Germ-free mice (that is mice that are free of bacteria, [[fungi]] and [[parasite]]s) have inherited viral diseases that regular mice do not.<ref>[http://www.nature.com/nature/journal/v222/n5188/abs/222092b0.html Spontaneous "Secondary" Disease in Germfree AKR Mice]</ref> Germ-free mice are also subject to inflammation of the lungs and colon (comparable to asthma and colitis) that regular mice do not normally experience.<ref>[http://www.sciencedaily.com/releases/2012/03/120322142157.htm Getting the dirt on immunity: Scientists show evidence for hygiene hypothesis]</ref> And germ-free mice appear to have a compromised blood-brain barrier; a compromised blood-brain barrier is associated with several diseases.<ref name="bloodbrain"/> Germ-free mice also suffer from behavioral problems including abnormal movement, memory problems, risk-taking, and stress.<ref name=games>[https://www.sciencenews.org/article/microbes-can-play-games-mind?mode=magazine&context=191647 Microbes can play games with the mind: The bacteria in our guts may help decide who gets anxiety and depression] by Laura Sanders (9:30am, March 23, 2016) ''Science News''.</ref><ref>[http://www.ncbi.nlm.nih.gov/pubmed/23000955 The interplay between the intestinal microbiota and the brain] by S. M. Collins et al. ''Nat. Rev. Microbiol.'' 2012 Nov;10(11):735-42. doi: 10.1038/nrmicro2876. Epub 2012 Sep 24.</ref> An explanation for this is that several genera of bacteria have been shown to release neural-messaging chemicals: ''Bacillus'' (dopamine, norepinephrine), ''Bifido-bacterium'' (γ-aminobutyric acid or GABA), ''Enterococcus'' (serotonin), ''Escherichia'' (norepinephrine, serotonin), ''Lactobacillus'' (acetylcholine, GABA) and ''Streptococcus'' (serotonin).<ref name=games/><ref>[http://www.journalofpsychiatricresearch.com/article/S0022-3956%2815%2900065-5/abstract?cc=y= Collective unconscious: How gut microbes shape human behavior] T. G. Dinan et al. ''J. Psych. Res.'' April 2015 Volume 63, Pages 1–9 DOI: http://dx.doi.org/10.1016/j.jpsychires.2015.02.021.</ref>

==Research==
Due to their simplicity, small [[genome]], ease of culture and not being a major focus of any [[animal rights | animal-rights]] groups, bacteria are used in biological research, such as in studies of [[genetic engineering]] and of [[evolution]]. The most well-known example of the latter is the use of ''E. coli'' in [[Richard Lenski#Lenski's long-term E. coli evolution experiment and intelligent design| Lenski's famous experiment]].

==Advertising==
{{main|Advertising}}
The common association of bacteria with disease leads many people to think of all bacteria as "bad." Of course there are non-pathogenic bacteria but so strong is the popular link that adverts for [[probiotic]] drinks steer clear of the B-word: 

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==See also==
*[[GFAJ-1]], little beasties that eat [[arsenic]] (supposedly).
*[[Lenski affair]]

==Notes==
{{notelist}}

==References==
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[[Category:Organisms]]
[[Category:Evolution]]

Categories: Organisms, Evolution