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Gram-negative
 
 
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Gram-negative bacteria are those bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
 that do not retain crystal violet dye in the Gram staining
Gram staining

Gram staining is an empiricism method of differentiating bacterium species into two large groups based on the chemical and physical properties of their cell walls....
 protocol. In a Gram stain test, a counterstain
Counterstain

A counterstain is a staining with color contrasting to the principal stain, making the stained structure more easily visible.An example is the malachite green counterstain to the fuchsine stain in the Gimenez stain....
 (commonly safranin
Safranin

Safranin is a staining used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring all cell nucleus red....
) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell wall
Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cell . It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism....
s.On the other hand, Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution.

Many species of Gram-negative bacteria are pathogen
Pathogen

A pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its Host .There are several substrates and pathways whereby pathogens can invade a host; the principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring...
ic, meaning that they can cause disease in a host organism.






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Encyclopedia


Gram-negative bacteria are those bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
 that do not retain crystal violet dye in the Gram staining
Gram staining

Gram staining is an empiricism method of differentiating bacterium species into two large groups based on the chemical and physical properties of their cell walls....
 protocol. In a Gram stain test, a counterstain
Counterstain

A counterstain is a staining with color contrasting to the principal stain, making the stained structure more easily visible.An example is the malachite green counterstain to the fuchsine stain in the Gimenez stain....
 (commonly safranin
Safranin

Safranin is a staining used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring all cell nucleus red....
) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell wall
Cell wall

A cell wall is a tough, flexible and sometimes fairly rigid layer that surrounds some types of cell . It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism....
s.On the other hand, Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution.

Many species of Gram-negative bacteria are pathogen
Pathogen

A pathogen , infectious agent, or germ, is a biological agent that causes disease or illness to its Host .There are several substrates and pathways whereby pathogens can invade a host; the principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring...
ic, meaning that 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
Lipopolysaccharide

Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals....
 (also known as LPS or endotoxin
Endotoxin

Endotoxins are toxins associated with certain bacteria. Classically, an "endotoxin" is a toxin which, unlike an "exotoxin", is not secreted in soluble form by live bacteria, but is a structural component in the bacteria which is released mainly when bacteria are lysis....
) layer. In humans, LPS triggers an innate immune response characterized by cytokine
Cytokine

Cytokines are a category of signaling molecules that, like hormones and neurotransmitters, are used extensively in cell communication. They are proteins, peptides or glycoproteins....
 production and immune system
Immune system

An immune system is a collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells....
 activation. Inflammation
Inflammation

Inflammation is the complex biological response of Blood vessel tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. It is a protective attempt by the organism to remove the injurious stimuli as well as initiate the healing process for the tissue....
 is a common result of cytokine(from the greek cyto=cell,kinesis=movement) production, which can also produce host toxicity.

Characteristics



The following characteristics are displayed by Gram-negative bacteria:

  1. Cytoplasmic membrane
  2. Thin peptidoglycan
    Peptidoglycan

    Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall....
     layer (which is present in much higher levels in Gram-positive bacteria)
  3. Outer membrane
    Outer membrane

    The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane include a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's peptidoglycan by Braun's lipoprotein....
     containing lipopolysaccharide
    Lipopolysaccharide

    Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals....
     (LPS, which consists of lipid A
    Lipid A

    Lipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide molecule, and its hydrophobe nature allows it to anchor the LPS to the outer membrane....
    , core polysaccharide, and O antigen) outside the peptidoglycan layer
  4. Porin
    Porin (protein)

    Porins are beta barrel proteins that cross a cellular cell membrane and act as a pore through which molecules can diffusion. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as Ion channel that are specific to different types of molecules....
    s exist in the outer membrane, which act like pores for particular molecules
  5. There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space
    Periplasmic space

    The periplasmic space or periplasm is a space between the inner cytoplasmic membrane and external outer membrane of Gram-negative bacterium or the equivalent space between the cell membrane and cell wall in Gram-positive bacteria....
  6. The S-layer
    S-layer

    An S-layer is a part of the cell envelope commonly found in bacteria, as well as among archaea. It consists of a monomolecular layer composed of identical proteins or glycoproteins....
     is directly attached to the outer membrane, rather than the peptidoglycan
  7. If present, flagella have four supporting rings instead of two
  8. No teichoic acid
    Teichoic acid

    Teichoic acids are polysaccharides of glycerol phosphate or ribitol phospate linked via phosphodiester bonds....
    s or lipoteichoic acid
    Lipoteichoic acid

    Lipoteichoic acid is a surface-associated adhesion amphiphile from Gram-positive bacteria. These organisms have an inner membrane and, external to it, a thick peptidoglycan layer.It contains wall teichoic acids, long chains of sugars made by ribitol or glycerol and phosphate embedded in the peptidoglycan layer....
    s are present
  9. Lipoprotein
    Lipoprotein

    A lipoprotein is a biochemistry assembly that contains both proteins and lipids. The lipids or their derivatives may be covalently or non-covalently bound to the proteins....
    s are attached to the polysaccharide backbone.
  10. Most do not sporulate (Coxiella burnetti, which produces spore-like structures, is a notable exception)


Example species


The proteobacteria
Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....
 are a major group of Gram-negative bacteria, including Escherichia coli
Escherichia coli

'Escherichia coli' , is a Gram negative bacterium that is commonly found in the lower gastrointestinal tract of warm-blooded animals. Most E....
, Salmonella
Salmonella

Salmonella is a genus of rod-shaped Gram-negative enterobacteriaceae that causes typhoid fever, paratyphoid fever, and the foodborne illness salmonellosis....
, Shigella
Shigella

Shigella is a genus of Gram-negative, Endospore rod-shaped bacterium closely related to Escherichia coli and Salmonella. The causative agent of human shigellosis, Shigella cause disease in primates, but not in other mammals....
,and other Enterobacteriaceae
Enterobacteriaceae

The Enterobacteriaceae are a large family of bacterium, including many of the more familiar pathogens, such as Salmonella and Escherichia coli....
, Pseudomonas
Pseudomonas

Pseudomonas is a genus of gamma proteobacteria, belonging to the larger family of pseudomonads.Recently, 16S ribosomal RNA sequence analysis has redefined the taxonomy of many bacterial species....
, Moraxella
Moraxella catarrhalis

Moraxella catarrhalis is a Gram-negative, Aerobic_organism, Oxidase test diplococcus which may both colonize and cause respiratory tract-associated infection in humans....
, Helicobacter
Helicobacter

Helicobacter is a genus of Gram-negative bacterium possessing a characteristic helix shape. They were initially considered to be members of the Campylobacter genus, but since 1989 they have been grouped in their own genus....
, Stenotrophomonas, Bdellovibrio
Bdellovibrio

Bdellovibrio is a genus of Gram-negative, obligate aerobe bacterium.One of the more notable characteristics of this genus is that members parasitism other Gram-negative bacteria by entering into their periplasmic space and feeding on the biopolymers, e.g....
, acetic acid bacteria
Acetic acid bacteria

Acetic acid bacteria are bacterium that derive their energy from the oxidation of ethanol to acetic acid during Cellular respiration. They are Gram-negative, Obligate aerobe, rod-shaped bacterium....
, Legionella
Legionella

Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably Legionella pneumophila....
 and alpha-proteobacteria as Wolbachia
Wolbachia

Wolbachia is a genus of inherited bacterium which infects arthropod species, including a high proportion of insects. It is one of the world's most common parasitic microbes and is potentially the most common reproductive parasite in the biosphere....
 and many others. Other notable groups of Gram-negative bacteria include the cyanobacteria
Cyanobacteria

Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis....
, spirochaete
Spirochaete

Spirochaetes is a phylum of distinctive Gram-negative bacterium, which have long, helix coiled cells. Spirochetes are chemoheterotroph in nature, with lengths between 5 and 250 ?m and diameters around 0.1-0.6 ?m....
s, green sulfur
Green sulfur bacteria

The green sulfur bacteria are a family of obligately anaerobic organism photoautotrophic bacterium. Most closely related to the nonetheless distant Bacteroidetes, they are accordingly assigned their own phylum....
 and green non-sulfur bacteria.

Medically relevant Gram-negative cocci
Coccus

Cocci are any microorganism whose overall shape is sphere or nearly spherical. Describing a bacterium as a coccus, or sphere, distinguishes it from Bacillus , or rod....
 include three organisms, which cause a sexually transmitted disease
Sexually transmitted disease

A sexually transmitted disease , also known as sexually transmitted infection or venereal disease , is an illness that has a significant probability of transmission between humans or animals by means of sexual contact, including sexual intercourse, oral sex, and anal sex....
 (Neisseria gonorrhoeae
Neisseria gonorrhoeae

Neisseria gonorrhoeae, also known as Gonococci , or Gonococcus , is a species of Gram-negative kidney bean-shaped diplococci bacteria responsible for the sexually transmitted disease gonorrhoea....
), a meningitis
Meningitis

Meningitis is a medical condition caused by inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges....
 (Neisseria meningitidis
Neisseria meningitidis

Neisseria meningitidis, also known as meningococcus, is the bacterium that causes meningitis, an infection of the membrane that covers the brain....
), and respiratory symptoms (Moraxella catarrhalis
Moraxella catarrhalis

Moraxella catarrhalis is a Gram-negative, Aerobic_organism, Oxidase test diplococcus which may both colonize and cause respiratory tract-associated infection in humans....
).

Medically relevant Gram-negative bacilli
Bacillus (shape)

Although Bacillus, capitalized and Italics, specifically refers to the genus, the word bacillus may also be used to describe any rod-shaped bacterium, and in this sense, bacilli are found in many different taxonomic groups of bacteria....
 include a multitude of species. Some of them primarily cause respiratory problems (Hemophilus influenzae, Klebsiella pneumoniae
Klebsiella pneumoniae

Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermentation , facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....
, Legionella pneumophila
Legionella pneumophila

Legionella pneumophila is a thin, Wiktionary:pleomorphism, flagellatedGram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogen in this group and is the causative agent of legionellosis or Legionnaires' disease....
, Pseudomonas aeruginosa
Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common bacterium which can cause disease in animals and humans. It is found in soil, water, and most man-made environments throughout the world....
), primarily urinary problems (Escherichia coli
Escherichia coli

'Escherichia coli' , is a Gram negative bacterium that is commonly found in the lower gastrointestinal tract of warm-blooded animals. Most E....
, Proteus mirabilis
Proteus mirabilis

Proteus mirabilis is a Gram-negative, facultatively Anaerobic organism bacterium. It shows swarming motility, and urease activity. P. mirabilis causes 90% of all 'Proteus' infections in humans....
, Enterobacter
Enterobacter

Enterobacter is a genus of common Gram-negative, Facultative anaerobic organism, bacillus bacteria of the family Enterobacteriaceae. Several strains of the these bacteria are pathogen and cause opportunistic infections in immunocompromised hosts....
 cloacae, Serratia marcescens
Serratia marcescens

Serratia marcescens is a species of Gram-negative, bacillus bacteria in the family Enterobacteriaceae. A human pathogen, S. marcescens is involved in nosocomial infections, particularly Central venous catheter#Infection bacteremia, urinary tract infections and wound infections, and is responsible for 1.4% of nosocomial bacteremi...
), and primarily gastrointestinal problems (Helicobacter pylori
Helicobacter pylori

Helicobacter pylori is a Gram-negative, microaerophile bacterium that inhabits various areas of the stomach and duodenum. It causes a chronic low-level inflammation of the stomach lining and is strongly linked to the development of duodenal and gastric peptic ulcers and stomach cancer bacteria....
, Salmonella enteritidis, Salmonella typhi).

Gram negative bacteria associated with nosocomial infection
Nosocomial infection

Nosocomial infections are infections which are a result of treatment in a hospital or a healthcare service unit, but secondary to the patient's original condition....
s include Acinetobacter baumannii
Acinetobacter baumannii

Acinetobacter baumannii is a species of pathogenic bacteria called aerobic gram-negative bacillus and is naturally sensitive to relatively few antibiotics....
, 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
Outer membrane

The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane include a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's peptidoglycan by Braun's lipoprotein....
. The outer leaflet of the membrane comprises a complex lipopolysaccharide
Lipopolysaccharide

Lipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals....
 whose lipid
Lipid

Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
 portion acts as an endotoxin
Endotoxin

Endotoxins are toxins associated with certain bacteria. Classically, an "endotoxin" is a toxin which, unlike an "exotoxin", is not secreted in soluble form by live bacteria, but is a structural component in the bacteria which is released mainly when bacteria are lysis....
. 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
Antibiotic

In common usage, an antibiotic is a substance or compound that kills or inhibits the growth of bacteria. Antibiotics belong to the group of antimicrobial compounds used to treat infections caused by microorganisms, including fungus and protozoa....
s, dyes, and detergent
Detergent

A detergent is a material intended to assist cleaning. The term is sometimes used to differentiate between soap and other surfactants used for cleaning....
s which would normally damage the inner membrane or cell wall (peptidoglycan
Peptidoglycan

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall....
). The outer membrane provides these bacteria with resistance to lysozyme
Lysozyme

Lysozymes, also known as muramidase or N-acetylmuramide glycanhydrolase, are a family of enzymes which damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins....
 and penicillin
Penicillin

Penicillin is a group of antibiotics derived from Penicillium fungi. They are Beta-lactam antibiotics used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms....
. Fortunately, alternative medicinal treatments such as lysozyme with EDTA
EDTA

EDTA is a widely used acronym for the chemical compound ethylenediaminetetraacetic acid . EDTA is a polyamino carboxylic acid with the chemical formula [CH2N2]2....
, and the antibiotic ampicillin
Ampicillin

Ampicillin is a beta-lactam antibiotic antibiotic that has been used extensively to treat bacterium infections since 1961. It is considered part of the aminopenicillin family and is roughly equivalent to amoxicillin in terms of spectrum and level of activity....
 have been developed to combat the protective outer membrane of some pathogenic Gram-negative organisms. Other drugs can be used, namely chloramphenicol
Chloramphenicol

Chloramphenicol is a bacteriostatic antimicrobial originally derived from the bacterium Streptomyces venezuelae, isolated by David Gottlieb, and introduced into clinical practice in 1949....
, streptomycin
Streptomycin

Streptomycin is an antibiotic drug, the first of a class of drugs called aminoglycosides to be discovered, and was the first antibiotic remedy for tuberculosis....
, and nalidixic acid
Nalidixic acid

Nalidixic acid is the basis for quinolone antibiotics.Nalidixic acid is effective against both gram-positive and gram-negative bacterium. In lower concentrations, it acts in a bacteriostatic manner; that is, it inhibits growth and reproduction....
.

See also


  • Gram-positive bacteria
  • Braun's lipoprotein
    Braun's lipoprotein

    Braun's lipoprotein , found in gram-negative cell walls, is one of the most abundant membrane proteins; its molecular weight is about 7.2 kDa. It is bound at its C-terminal end by a covalent bond to the peptidoglycan layer and is embedded in the outer membrane by its hydrophobic head ....
Category:Gram negative bacteria
  • Gram-indeterminate bacteria


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