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Baro a panid: thumb|right|300px|Gram-negative ''[[Pseudomonas aeruginosa'' bacteria (pink-red rods).]] '''Gram-negative bacteria''' are those bacteria ...
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Many species of Gram-negative bacteria are [[pathogen]]ic, meaning they can cause disease in a host organism. This pathogenic capability is usually associated with certain components of Gram-negative cell walls, in particular the [[lipopolysaccharide]] (also known as LPS or [[endotoxin]]) layer.<ref name=Baron /> In humans, LPS triggers an [[innate immune response]] characterized by [[cytokine]] production and [[immune system]] activation. [[Inflammation]] is a common result of cytokine production, which can also produce host toxicity.
[[Image:Gram_Stain_Anthrax.jpg|thumb|right|300px|Gram-positive [[Bacillus anthracis]] bacteria (purple rods) in [[cerebrospinal fluid]] sample. The other cells are [[white blood cell]]s]]
'''Gram-positive''' [[Bacteria|bacteria]] are those that are stained dark blue or violet by [[Gram staining]]. This is in contrast to [[Gram-negative]] bacteria, which cannot retain the crystal violet stain, instead taking up the [[counterstain]] ([[safranin]]) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of [[peptidoglycan]] in the [[cell wall]]. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.
[[Image:Gram-Cell-wall.svg|thumb|right|300px|Gram-positive and -negative cell wall structure]]
[[Image:Gram-positive cellwall-schematic.png|thumb|right|Structure of Gram-positive cell wall]]
The following characteristics are generally present in a Gram-positive bacterium:<ref name=Brock>{{cite book | author = Madigan M; Martinko J (editors). | title = Brock Biology of Microorganisms | edition = 11th ed. | publisher = Prentice Hall | year = 2005 | id = ISBN 0131443291 }}</ref>
#Cytoplasmic lipid membrane
#Thick [[peptidoglycan]] layer
#*[[Teichoic acids]] and [[lipoteichoic acid]]s are present, which serve to act as [[Chelation|chelating]] agents, and also for certain types of adherence.
#Capsule [[polysaccharides]] (only in some species)
#[[Flagellum]] (only in some species)
#*If present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.
In the original bacterial phyla, the Gram-positive organisms made up the [[phylum (biology)|phylum]] [[Firmicutes]], a name now used for the largest group. It includes many well-known genera such as ''[[Bacillus]]'', ''[[Listeria]]'', ''[[Staphylococcus]]'', ''[[Streptococcus]]'', ''[[Enterococcus]]'', and ''[[Clostridium]]''. It has also been expanded to include the Mollicutes, bacteria like ''[[Mycoplasma]]'' that lack cell walls and cannot be Gram stained, but are derived from such forms. [[Actinobacteria]] are the other major group of Gram-positive bacteria, which have a high [[guanosine]] and [[cytosine]] content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content.
Both Gram-positive and Gram-negative bacteria may have a membrane called an [[S-layer]]. In Gram-negative bacteria, the S-layer is directly attached to the [[outer membrane]]. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of [[teichoic acid]]s in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.
=== Exceptions ===
The [[Deinococcus-Thermus]] bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.
== Pathogenesis ==
Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive organisms are typically pathogenic in humans. Two of these, ''Streptococcus'' and ''Staphylococcus'', are [[cocci]] (round bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are ''Corynebacterium'' and ''Listeria'', while ''Bacillus'' and ''Clostridium'' produce spores.<ref>{{cite book |title=Clinical Microbiology made ridiculously simple |last=Gladwin |first=Mark |coauthors=Bill Trattler |year=2007 |publisher=MedMaster, Inc |location=Miami, FL |isbn=978-0-940780-81-1 |pages=4-5 }}</ref> The spore-forming bacteria can again be divided based on their [[respiration]]: ''Bacillus'' is a facultative anaerobe, while ''Clostridium'' is an obligate anaerobe.
==See also==
* [[Gram-negative]]
* [[Carboxydothermus hydrogenoformans]]
* [[:Category:Gram positive bacteria]]

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