Gram-negative: Difference between revisions

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Gram-negative refers to a classification of bacteria based upon their cell wall structure. Gram-negative bacteria appear red as a result of [[Gram staining|Gram testing]], whereas&nbsp;[[Gram-positive]]&nbsp;stain purple.<br>
[[Image:Pseudomonas aeruginosa Gram negative bacteria.jpg|thumb|right|400x275px]]The gram-negative [[Bacteria|bacterial]] [[Cell wall|cell walls]] are one of the two types of bacterial cell walls which can be identified by [[Gram staining|the Gram stain&nbsp;test]]. After the Gram stain test Gram-negative bacteria appear red or pink when observed under the [[Microscope|microscope]]&nbsp;as they do not retain the [[Crystal violet|crystal violet dye]] after rinsing and red safranin dye enters the cell wall when counterstaining all due to thin [[Peptidoglycan|peptidoglycan]] layer.  


<u></u>The structure of the Gram-negative bacterial cell wall is what distinguishes it from Gram-positive bacteria. Gram-negative bacteria contain a much thinner layer of [[Peptidoglycan]]&nbsp;in comparison to a Gram-positive bacterial cell wall. The Gram-negative cell wall consists of a unique outer membrane, containing lipopolysaccharides, murein lipoproteins and porin channels. There is also a periplasmic space between the peptidoglycan cell wall, and the cell membrane.  
Gram-negative bacteria include&nbsp;''[[Hemophilus influenzae|Haemophilus influenzae]]'', ''[[E.Coli|Escherichia coli]]<ref>Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003</ref>'' and other bacteria.  


Gram-negative bacteria exhibit stronger resistance to [[Antibiotics]]&nbsp;such as [[Lysozyme]]&nbsp;and [[pennicillin|pennicillin]] G, as well as greater resistance to dyes and detergents.&nbsp; The lipopolysaccharide consists of a core [[polysaccharide|polysaccharide]], Lipid A and O-antigen. This lipolysaccharide layer is important in excluding large hydrophobic susbstances from interacting with the cell. Lipid A attaches to the outer membrane ensuring that the [[lipopolysaccharide|lipopolysaccharide]] remains attached to the cell&nbsp;<ref>Oh S., Antibiotics Attack (http://www.hhmi.org/biointeractive/Antibiotics_Attack/bb_2.html), July 1999, Montgomery Blair</ref>.<br>  
=== Cell wall structure of the Gram-negative bacteria ===
 
The cell envelope of Gram-negative bacteria contains two membranes (outer membrane and inner (cell) membrane). In-between these membranes lies a thin [[Peptidoglycan|peptidoglycan]] layer and&nbsp;the aqueous periplasm where&nbsp;various nutrient-transporting [[Enzyme|enzymes]] and transporters are found. The outer membrane of Gram-negative cell walls is porous to ions, contains toxic lipid parts of lipopolysaccharides<ref>Reece JB et al. Campbell Biology. 9th ed. San Francisco: Pearson Education Inc. 2011</ref> and has defensive properties<ref>Slonczewski J, Foster JW. Microbiology: an evolving science. 4th ed. New York: W. W. Norton &amp;amp; Company. 2016</ref>.
 
=== Characteristics  ===
 
*Two membranes (inner and outer) in the cell envelope
*A thin layer of peptidoglycan in-between the two membranes
*<span style="font-size: 13.28px;">Porin protein</span>s in the outer membrane limiting the size of molecules entering the cell
*Do not retain crystal violet dye after the Gram stain test
 
=== Infections caused by Gram-negative bacteria and treatment implications  ===
 
Gram-negative bacteria cause such infections as [[Pneumonia|pneumonia]] and [[Septicaemia|septicaemia]]<ref>Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003</ref>. Gram-negative bacterial cell walls are more complex than [[Gram positive|Gram-positive]] bacterial cell walls as the former possess two membranes and their outer membrane contains porin proteins that limit the size of molecules entering the cell, therefore, preventing large quantities of antibiotics from entering the cell wall where they would take effect<ref>Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003</ref>. As many [[Antibiotics|antibiotics]] inhibit [[Enzyme|enzymes]] important in the assembly of [[Peptidoglycan|peptidoglycan]] layer which in Gram-negative bacteria lies between two membranes, many antibiotics that are effective against Gram-positive bacteria are ineffective against Gram-negative bacteria as they cannot go through the outer membrane and access the peptidoglycan layer inside the cell envelope<ref>Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003</ref>.<br>  


=== References  ===
=== References  ===


<references />
<references /><br>

Latest revision as of 17:58, 9 December 2018

The gram-negative bacterial cell walls are one of the two types of bacterial cell walls which can be identified by the Gram stain test. After the Gram stain test Gram-negative bacteria appear red or pink when observed under the microscope as they do not retain the crystal violet dye after rinsing and red safranin dye enters the cell wall when counterstaining all due to thin peptidoglycan layer.

Gram-negative bacteria include Haemophilus influenzae, Escherichia coli[1] and other bacteria.

Cell wall structure of the Gram-negative bacteria

The cell envelope of Gram-negative bacteria contains two membranes (outer membrane and inner (cell) membrane). In-between these membranes lies a thin peptidoglycan layer and the aqueous periplasm where various nutrient-transporting enzymes and transporters are found. The outer membrane of Gram-negative cell walls is porous to ions, contains toxic lipid parts of lipopolysaccharides[2] and has defensive properties[3].

Characteristics

  • Two membranes (inner and outer) in the cell envelope
  • A thin layer of peptidoglycan in-between the two membranes
  • Porin proteins in the outer membrane limiting the size of molecules entering the cell
  • Do not retain crystal violet dye after the Gram stain test

Infections caused by Gram-negative bacteria and treatment implications

Gram-negative bacteria cause such infections as pneumonia and septicaemia[4]. Gram-negative bacterial cell walls are more complex than Gram-positive bacterial cell walls as the former possess two membranes and their outer membrane contains porin proteins that limit the size of molecules entering the cell, therefore, preventing large quantities of antibiotics from entering the cell wall where they would take effect[5]. As many antibiotics inhibit enzymes important in the assembly of peptidoglycan layer which in Gram-negative bacteria lies between two membranes, many antibiotics that are effective against Gram-positive bacteria are ineffective against Gram-negative bacteria as they cannot go through the outer membrane and access the peptidoglycan layer inside the cell envelope[6].

References

  1. ↑ Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003
  2. ↑ Reece JB et al. Campbell Biology. 9th ed. San Francisco: Pearson Education Inc. 2011
  3. ↑ Slonczewski J, Foster JW. Microbiology: an evolving science. 4th ed. New York: W. W. Norton &amp; Company. 2016
  4. ↑ Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003
  5. ↑ Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003
  6. ↑ Walsh C. Antibiotics: actions, origins, resistance. Washington: ASM Press. 2003


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