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[[Image:Pseudomonas aeruginosa Gram.jpg|thumb|right|300px|Gram-negative ''[[Pseudomonas aeruginosa]]'' bacteria (pink-red rods).]]
'''Gram-negative bacteria''' are those [[bacteria]] that do not retain [[crystal violet]] dye in the [[Gram staining]] 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 ed. | publisher = Univ of Texas Medical Branch | year = 1996 | url = | isbn = 0-9631172-1-1 }}</ref> [[Gram-positive bacteria]] will retain the crystal violet dye when washed in a decolorizing solution. In a Gram stain test, a [[counterstain]] (commonly [[safranin]]) is added after the crystal violet, coloring all Gram-negative bacteria a red or pink color. The test itself is useful in classifying two distinct types of bacteria based on structural differences in their [[cell wall]]s.
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]]
[[Image:Gram negative cell wall.svg|thumb|350px|right|Structure of gram-negative cell wall]]
[[Image:Gram-Cell-wall.svg|thumb|300px|Gram-positive and -negative bacteria are chiefly differentiated by their [[cell wall]] structure.]]
The following characteristics are displayed by Gram-negative bacteria:
#Cytoplasmic membrane
#Thin [[peptidoglycan]] layer (which is present in much higher levels in Gram-positive bacteria)
#[[Outer membrane]] containing [[lipopolysaccharide]] (LPS, which consists of [[lipid A]], core polysaccharide, and [[O antigen]]) outside the peptidoglycan layer
#[[Porin (protein)|Porin]]s exist in the outer membrane, which act like pores for particular molecules
#There is a space between the layers of peptidoglycan and the secondary cell membrane called the [[periplasmic space]]
#The [[S-layer]] is directly attached to the outer membrane, rather than the peptidoglycan
#If present, [[flagella]] have four supporting rings instead of two
#No [[teichoic acid]]s or [[lipoteichoic acid]]s are present
#[[Lipoprotein]]s are attached to the polysaccharide backbone whereas in Gram-positive bacteria no lipoproteins are present
#Most do not sporulate (''Coxiella burnetti'', which produces spore-like structures, is a notable exception)
==Example species==
The [[proteobacteria]] are a major group of Gram-negative bacteria, including ''[[Escherichia coli]]'', ''[[Salmonella]]'', and other [[Enterobacteriaceae]], ''[[Pseudomonas]]'', ''[[Moraxella catarrhalis|Moraxella]]'', ''[[Helicobacter]]'', ''Stenotrophomonas'', ''[[Bdellovibrio]]'', [[acetic acid bacteria]], ''[[Legionella]]'' and alpha-proteobacteria as ''[[Wolbachia]]'' and many others. Other notable groups of Gram-negative bacteria include the [[cyanobacteria]], [[spirochaete]]s, [[green sulfur bacteria|green sulfur]] and [[green non-sulfur bacteria|green non-sulfur]] bacteria.
Medically relevant Gram-negative [[Coccus|cocci]] include three organisms, which cause a [[sexually transmitted disease]] (''[[Neisseria gonorrhoeae]]''), a [[meningitis]] (''[[Neisseria meningitidis]]''), and respiratory symptoms (''[[Moraxella catarrhalis]]'').
Medically relevant Gram-negative [[bacilli]] include a multitude of species. Some of them primarily cause respiratory problems (''[[Hemophilus influenzae]]'', ''[[Klebsiella pneumoniae]]'', ''[[Legionella pneumophila]]'', ''[[Pseudomonas aeruginosa]]''), primarily urinary problems (''[[Escherichia coli]]'', ''[[Proteus mirabilis]]'', ''[[Enterobacter]] cloacae'', ''[[Serratia marcescens]]''), and primarily gastrointestinal problems (''[[Helicobacter pylori]]'', ''[[Salmonella enteritidis]]'', ''[[Salmonella typhi]]'').
Gram negative bacteria associated with [[nosocomial infection]]s include ''[[Acinetobacter baumanii]]'', which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in intensive care units of hospital establishments.
==Medical treatment==
One of the several unique characteristics of Gram-negative bacteria is the structure of the [[outer membrane]]. The outer leaflet of the membrane comprises a complex [[lipopolysaccharide]] whose [[lipid]] portion acts as an [[endotoxin]]. If endotoxin enters the circulatory system it causes a toxic reaction with the sufferer having a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal.
This outer membrane protects the bacteria from several [[antibiotic]]s, dyes, and [[detergent]]s which would normally damage the inner membrane or cell wall ([[peptidoglycan]]). The outer membrane provides these bacteria with resistance to [[lysozyme]] and [[penicillin]]. Fortunately, alternative medicinal treatments such as lysozyme with [[EDTA]], and the antibiotic [[ampicillin]] have been developed to combat the protective outer membrane of some pathogenic Gram-negative organisms. Other drugs can be used, namely [[chloramphenicol]], [[streptomycin]], and [[nalidixic acid]].
==See also==
* [[Gram-positive bacteria]]
* [[Braun's lipoprotein]]
* [[:Category:Gram negative bacteria]]
<div class="references-small">
* {{NCBI-scienceprimer}}
==External links==
*[ 3D structures of proteins from inner membranes of Gram-negative bacteria]
[[Category:Gram-negativo a bacteria|*]]
[[Category:Gram-positivo a bacteria|*]]

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