What is Proteobacteria?
{{Taxobox | name = Proteobacteria | image = E. coli Bacteria (7316101966).jpg | image_width = 230px | image_caption = ''[[Escherichia coli]]'' | domain = [[Bacteria]] | phylum = '''Proteobacteria''' | phylum_authority = Stackebrandt et al., 1988 }} [[Image:Gram negative cell wall.svg|thumb|250px|right|Structure of gram-negative cell wall]] [[Image:Gram-Cell-wall.svg|thumb|250px|Gram-positive- and negative bacteria are chiefly differentiated by their [[cell wall]] structure.]] The '''Proteobacteria''' are a major [[phylum]] of [[bacteria]].<ref>It is named after the Greek god [[Proteus]], who could change his shape, because of the great diversity of forms found in this group. </ref> They are [[gram-negative]] bacteria. This means they do not retain the [[Violet (color)|violet]] dye in the Gram [[stain]]ing protocol.<ref name=Baron>{{cite book | author = Salton MJR, Kim KS | title = ''Structure. ''in:'' Baron's Medical Microbiology ''(Baron S ''et al.'', eds.)| edition = 4th | publisher = Univ of Texas Medical Branch | year = 1996 | url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.289 | isbn = 0-9631172-1-1 }}</ref> In a Gram stain test, a counterstain (commonly [[safranin]]) is added after the crystal violet, colouring all gram-negative bacteria with a pink colour. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their [[cell wall]]s. Proteobacteria include a wide variety of [[pathogen]]s, such as ''[[Escherichia coli]]'', ''[[Salmonella]]'', ''[[Vibrio]]'', ''[[Helicobacter]]'', and many other notable genera.<ref name=Tree_of_Life>{{cite web | title=Proteobacteria | work=Discover Life: Tree of Life | url=http://stri.discoverlife.org/mp/20m?tree=Proteobacteria&res=800 | accessdate=2007-02-09}}</ref> Others are free-living, and include many of the bacteria responsible for [[nitrogen fixation]]. The group is defined primarily in terms of ribosomal RNA (rRNA) sequences. Most members are facultatively or obligately [[anaerobic]], [[