View source for Bacteria

{{Automatic taxobox
| name = Bacteria
| fossil_range = {{Long fossil range|4200|0|[[Archean]] or earlier&nbsp;– [[Holocene|present]]}}
| image = EscherichiaColi NIAID.jpg
| image_caption = [[Scanning electron microscope|Scanning electron micrograph]] of ''[[Escherichia coli]]'' [[bacillus (shape)|rods]]
| taxon = Bacteria
| authority = [[Carl Woese|Woese]], [[Otto Kandler|Kandler]] & [[Mark Wheelis|Wheelis]], 1990
| subdivision_ranks = Phyla
| subdivision =
*'''[[Gram positive]] / no [[Bacterial outer membrane|outer membrane]]'''
[[Actinobacteria]] (high-[[GC-content|G+C]])<br />
[[Firmicutes]] (low-[[GC-content|G+C]])<br />
[[Tenericutes]] (no [[Cell wall|wall]])
*'''[[Gram negative]] / [[Bacterial outer membrane|outer membrane]] present'''
[[Aquificae]]<br/>
[[Bacteroidetes]]/[[Fibrobacteres]]–[[Chlorobi]] ([[Sphingobacteria (phylum)|FCB&nbsp;group]])<br/>
[[Chlamydiae]]<br/>
[[Deinococcus-Thermus]]<br/>
[[Fusobacteria]]<br />
[[Gemmatimonadetes]]<br />
[[Nitrospirae]]<br />
[[Planctomycetes]]–[[Verrucomicrobia]]/[[Chlamydiae]] ([[Planctobacteria|PVC&nbsp;group]])<br/>
[[Proteobacteria]]<br />
[[Spirochaete]]s<br />
[[Synergistetes]]
*'''Unknown / ungrouped'''
[[Acidobacteria]]<br />
[[Chloroflexi (phylum)|Chloroflexi]]<br />
[[Chrysiogenetes]]<br />
[[Cyanobacteria]]<br />
[[Deferribacteraceae|Deferribacteres]]<br />
[[Dictyoglomi]]<br />
[[Thermodesulfobacteria]]<br />
[[Thermotogae]]
| synonyms = Eubacteria <small>Woese & Fox, 1977</small>
}}

'''Bacteria''' (sing. '''bacterium''') are very small [[organism]]s. They are [[prokaryotic]] [[microorganisms]]. Bacterial cells do not have a [[nucleus]], and most have no [[organelles]] with membranes around them. Most have a [[cell wall]]. They do have [[DNA]], and their [[biochemistry]] is basically the same as other living things.  
They are amongst the simplest and the oldest organisms.   They function as independent [[Organism|organisms]].  

Almost all bacteria are so tiny they can only be seen through a [[microscope]]. Bacteria are made up of one [[cell]], so they are a kind of ''[[unicellular organism]]''. They are among the simplest single-celled organisms on [[Earth]], and were one of the earliest forms of life. They include a number of [[extremophile]]s which live in extreme [[habitats]].

There are probably more individual bacteria than any other sort of organism on the planet.<ref name=lots/> Most bacteria live in the [[ground]] or in [[water]], but many live inside or on the skin of other organisms, including humans. There are about ten times as many bacterial cells as human cells in each of our bodies. Some bacteria can cause [[disease]]s, but others help us in everyday activities like digesting food ([[gut flora]]). Some even work for us in factories, producing [[cheese]] and [[yogurt]].

The founder of [[bacteriology]] was a [[Germans|German]] [[biologist]] called [[Ferdinand Cohn]] (1828{{ndash}}1898). He published the first [[biological classification]] of bacteria, based on their appearance.<ref>[http://www.britannica.com/EBchecked/topic/48314/bacteriology Encyclopedia Britanniaca: Bacteriology]</ref>

== Reproduction and gene transfer ==
A bacterium [[reproduction|reproduces]] (creates more bacteria) by [[Cell division|dividing in half]] and creating two "daughter" cells. Each daughter is identical in shape to the parent, but is smaller.

Bacteria do not have sexes, but they do transmit [[DNA]] by several kinds of [[horizontal gene transfer]].  This is how they share resistance to [[antibiotics]] from one strain to another.  The complete [[DNA sequence]] is known for many bacterial strains.

== Shape ==
Bacteria vary widely in size and shape, but in general they are at least ten times larger than viruses. A typical bacterium is about 1&nbsp;µm (one [[micrometer]]) in diameter, so a thousand bacteria lined up would be one millimeter long. There are about five nonillion (5×10<sup>30</sup>) bacteria on Earth.<ref name=lots>{{cite journal |author=Whitman W, Coleman D, Wiebe W |title=Prokaryotes: the unseen majority | url=http://www.pnas.org/cgi/content/full/95/12/6578 |journal=Proc Natl Acad Sci USA |volume=95 |issue=12 |pages=6578–83 |year=1998|pmid = 9618454 |doi=10.1073/pnas.95.12.6578}}</ref>

Bacteria are identified and grouped by their shapes. Bacilli are rod-shaped, cocci are ball-shaped, spirilla are spiral-shaped, and vibrio are shaped like a comma or a boomerang.
[[File:Bacterial morphology diagram.svg|thumb|Different shapes of bacteria]]

== Pathogens ==
[[Pathogen]]ic bacteria, the harmful kind, enter the human body from the air, water or food. Once inside, these bacteria attach themselves to or invade specific cells in our respiratory system, digestive tract or in any open wound. There they begin to reproduce and spread while using your body's food and nutrients to give them energy to help them reproduce.

== Extremophiles ==
Some bacteria are [[extremophile|extremophiles]]<nowiki/>.  Some microbes thrive inside rocks up to 580 meters below the sea floor under 2.6 kilometers of ocean off the [[Pacific Northwest]] of the United States.<ref name="LS-20130317">{{cite web |last=Choi |first=Charles Q. |title=Microbes thrive in deepest spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date= 2013 |publisher=LiveScience |accessdate=17 March 2013 }}</ref><ref name="LS-20130314">{{cite web |last=Oskin |first=Becky |title=Intraterrestrials: Life thrives in ocean floor |url=http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=2013 |publisher=LiveScience |accessdate=17 March 2013 }}</ref> According to one of the researchers, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."<ref name="LS-20130317" /> 

== History of their classification ==
All modern ideas start with the [[sequence analysis]] of DNA and RNA. In 1987, [[Carl Woese]], the forerunner of the [[molecular phylogeny]] revolution, divided bacteria into 11 divisions based on 16S [[ribosomal RNA]] (SSU) sequences:<ref name="woese87">Woese C.R. 1987. Bacterial evolution. ''Microbiological reviews'' '''51''' (2): 221–71. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC373105/]</ref><ref>{{cite journal
|author=Holland L.| title=Woese, Carl in the forefront of bacterial evolution revolution| journal      = Scientist| volume       =3| issue        =10| date = 1990}}</ref>

* '''[[Proteobacteria]]''': [[Purple bacteria]] and their relatives
** [[Alphaproteobacteria|alpha subdivision]] ([[purple non-sulfur bacteria]], [[rhizobacteria]], ''[[Agrobacterium]]'', ''[[Bartonella]]'', ''[[Rickettsiae]]'', ''[[Nitrobacter]]'')
** [[Betaproteobacteria|beta subdivision]] (''[[Rhodocyclus]]'', (some) ''[[Thiobacillus]]'', ''[[Alcaligenes]]'', ''[[Spirillum]]'', ''[[Nitrosovibrio]]'')
** [[Gammaproteobacteria|gamma subdivision]] (enterics, [[Pseudomonadaceae|fluorescent pseudomonads]], [[purple sulfur bacteria]], ''[[Legionella]]'', (some) ''[[Beggiatoa]]'')
** [[Deltaproteobacteria|delta subdivision]] (Sulfur and sulfate reducers (''[[Desulfovibrio]]''), [[Myxobacteria]], ''[[Bdellovibrio]]'')
* '''[[Gram-positive]] Eubacteria'''
** [[GC-content|High-G+C species]] - [[Actinobacteria]] (''[[Actinomyces]]'', ''[[Streptomyces]]'', ''[[Arthrobacter]]'', ''[[Micrococcus]]'', ''[[Bifidobacterium]]'')
** [[Low-G+C species]] - [[Firmicutes]] (''[[Clostridium]]'', ''[[Peptococcus]]'', ''[[Bacillus]]'', ''[[Mycoplasma]]'')
** Photosynthetic species (''[[Heliobacterium]]'')
** Species with gram-negative walls (''[[Megasphaera]]'', ''[[Sporomusa]]'')
* '''[[Cyanobacteria]]''' and chloroplasts (''[[Aphanocapsa]]'', ''[[Oscillatoria]]'', ''[[Nostoc]]'', ''[[Synechococcus]]'', ''[[Gleoebacter]]'', ''[[Prochloron]]'')
* '''[[Spirochaete]]s''' and relatives
** Spirochetes (''[[Spirochaeta]]'', ''[[Treponema]]'', ''[[Borrelia]]'')
** Leptospiras (''[[Leptospira]]'', ''[[Leptonema]]'')
* '''[[Green sulfur bacteria]]''' (''[[Chlorobium]]'', ''[[Chloroherpeton]]'')
* '''[[Bacteroides]], [[Flavobacteria]]''' and relatives
** Bacteroides (''[[Bacteroides]]'', ''[[Fusobacterium]]'')
** Flavobacterium group (''[[Flavobacterium]]'', ''[[Cytophaga]]'', ''[[Saprospira]]'', ''[[Flexibacter]]'')
* '''[[Planctomycetes|Planctomyces]]''' and relatives
** Planctomyces group (''[[Planctomyces]]'', ''[[Pasteuria]]'')
** Thermophiles (''[[Isocystis pallida]]'')
* '''[[Chlamydiae]]''' (''[[Chlamydia psittaci]]'', ''[[Chlamydia trachomatis]]'')
* '''Radioresistant micrococci''' and relatives
** Deinococcus group (''[[Deinococcus radiodurans]]'')
** Thermophiles (''[[Thermus aquaticus]]'')
* '''Green non-sulfur bacteria''' and relatives
** Chloroflexus group (''[[Chloroflexus]]'', ''[[Herpetosiphon]]'')
** Thermomicrobium group (''[[Thermomicrobium roseum]]'')
* '''[[Thermotogae]]'''

== Related pages ==
{{wiktionary}}
* [[Enterococcus]]
* [[Mycoplasma]]
* [[Microorganism]]
* [[Archaea]]
* [[Virus]]

== References ==
{{reflist}}

[[Category:Bacteria| ]]
[[Category:Microbiology]]
[[Category:Prokaryotes]]
[[Category:Horizontal gene transfer]]