Difference Between Gram-positive and Gram-negative bacteria

The difference between Gram-positive and Gram-negative bacteria is mainly due to the following factors like:

  • Difference in the cell wall
  • Staining difference.

Cell wall difference:
Gram-positive bacteria contain high peptidoglycan content (70-80%) and low lipid content (1-4%). It lacks lipopolysaccharide, porins that are present on the outer membrane and having less periplasmic space.

Gram-negative bacteria contain low peptidoglycan content (10-20%) and high lipid content (20-30%). The outer membrane is present which contains lipopolysaccharide, porins and having more periplasmic space.

cell wall difference between Gram positive and Gram negative bacteria

Staining or colour difference:
Gram-positive bacteria retain the colour of crystal violet (Primary stain) and appear violet in colour due to the thick peptidoglycan cross-linkages.

Gram-negative bacteria lose the colour of crystal violet and appear pink in colour after counterstaining with safranin due to the thin peptidoglycan layer.

gram positive Vs gram negative

Content: Gram-positive Vs Gram-negative bacteria

  1. Comparison Chart
  2. Definition
  3. Difference in cell wall
  4. Colour difference
  5. Key Differences
  6. Similarities
  7. Conclusion

Comparison Chart

PropertiesGram positiveGram negative
ColourAppears violet in colourAppears pink in colour
Cell wall thickness20-80nm thick8-12nm thick
Cell wall compositionLow lipid content and high peptidoglycan content (70-80%)High lipid content (20-30%) and low peptidoglycan content (10-20%)
Outer membraneAbsentPresent
Teichoic acidPresentAbsent
Peptidoglycan layerThickThin
LipopolysaccharideAbsentPresent
MesosomeMore prominentLess prominent
Flagellar structure2 rings (S,M) in basal body4 rings (L, P, S and M) in basal body
Toxins producedMajority of them produces exotoxinsMajority of them produces endotoxins
Porins in outer membraneAbsentPresent
Periplasmic spaceComparatively lessMore
Resistance to physical disruptionHighLow
Resistance to cell wall disruption by lysozymeHighLow
Resistance to dryingHighLow
Resistance to sodium-azideHighLow
Inhibition by basic dyesHighLow
Susceptibility to penicillin and sulphonamideHighLow
Susceptibility to streptomycin, chloramphenicol and tetracyclineLowHigh
Susceptibility to anionic detergentsHighLow
PathogenicityLess bacteria are pathogenicMore bacteria are pathogenic
ExamplesBacillus, Streptococcus, Clostridium, Corynebacterium etc.Enterobacter, Acetobacter, Bortadella etc.

Definition

Gram-positive bacteria: These are the type of bacteria which appears “Violet” in colour by giving a positive reaction on gram staining i.e. retain the colour of primary stain (Crystal violet).

Gram-negative bacteria: These are the type of bacteria which appears “Pink” in colour by giving a negative reaction on gram staining i.e. not retain the colour of primary stain and take the colour of counterstain (Safranin).

Difference in cell wall

The cell wall of Gram-positive bacteria

The cell wall of gram-positive bacteria is made of “Sacculus” which are the continuous cells that are why the surface of gram-positive bacteria is even and smooth.

cell wall of gram positive bacteria

Peptidoglycan constitutes about 70-80% of the cell weight and lipid content is about 1-4% in the cell wall of gram-positive bacteria. The cell wall is thick due to high peptidoglycan content which is usually multilayered and having dense cross-linkages.

In some bacteria, on the peptidoglycan layer teichoic acid is found which is negatively charged. The distinguishing feature of gram-positive bacteria is that it lacks an outer membrane, Porins and lipopolysaccharide in its cell structure.

The cell wall of Gram-negative bacteria

Gram-negative bacteria are having continuous or smooth outer membrane as it consists of lipopolysaccharide and Porins in its structure. It consists of an outer membrane that surrounds the peptidoglycan layer which is absent in gram-positive bacteria.

cell wall of gram negative bacteria

Gram-negative bacteria are having high lipid content about 20-30% and low murein (polysaccharide) content about 10-20%. The cell wall consists of a  thin peptidoglycan layer.

Colour difference

The colour or staining difference between the gram-positive and gram-negative bacteria is due to the “Retaining ability of the Crystal violet-Iodine complex” by the cell wall. This CV-I complex is formed when the Gram’s iodine binds to the crystal violet. The retaining property of the CV-I complex depends upon the following properties:

  • Lipid concentration
  • Peptidoglycan thickness
  • Presence of magnesium ribonuclease

Reason for the violet colour of Gram-positive bacteria

The colour of gram-positive bacteria is violet in colour is due to the Crystal violet-Iodine complex which does not come out of the cell for the following reasons:

  1. The gram-positive bacteria contain 1-4% lipid concentration and by the addition of decolourizing agent (Ethanol), the lipid gets dehydrated that decreases the pore size in the cell wall. This is the first reason by which the CV-I complex cannot come out of the gram-positive bacterial cell.
  2. The second reason is the presence of high peptidoglycan content which comprises of dense cross-linkages that strongly holds the CV-I complex.
  3. Gram-positive bacteria contain magnesium ribonuclease which covalently binds to the CV-I complex that resists the violet colour.

Reason for the pink colour of Gram-negative bacteria

The colour of gram-negative bacteria is pink in colour is due to the Crystal violet-Iodine complex which easily comes out of the cell for the following reasons:

  1. The gram-negative bacteria contain high lipid content (11-20%) and by the addition of decolourizing agent (Ethanol), the lipid will dehydrate that increases the pore size in the cell wall. This is the first reason by which the CV-I complex comes out of the gram-negative bacterial cell.
  2. The second reason for the elimination of the CV-I complex is due to the thin peptidoglycan layer which comprises 5-10% of peptidoglycans cross-linkages.
  3. It lacks magnesium ribonuclease by which it cannot covalently link to the CV-I complex and cause the release of CV-I complex.

Key Differences Between Gram-positive and Gram-negative bacteria

  1. The colour of gram-positive bacteria is violet whereas the colour of gram-negative bacteria is pink.
  2. The cell wall of gram-positive bacteria is thick about 20-80nm whereas thin in a gram-negative cell which is about 8-12nm.
  3. The gram-positive bacteria contain 70-80% murein and 1-4% lipid whereas gram-negative bacteria contain 20-30% lipid content and 10-20% murein.
  4. Another identifying feature is that the outer membrane, lipopolysaccharide, Porins are absent in gram-positive bacteria whereas present in gram-negative.
  5. Another main feature is the flagellar structure, where S and M rings are present in gram-positive bacteria. L, P, S and M rings are present in the gram-negative group.
  6. Majority of the exotoxins are produced by the gram-positive bacteria and endotoxins by gram-negative bacteria.
  7. There is less periplasmic space in the gram-positive group of bacteria whereas more in the gram-negative group.
  8. Gram-positive bacteria are more resistant to physical dryness, cell disruption, drying and sodium-azide whereas gram-negative bacteria are less resistant.

Similarities Between Gram-positive and Gram-negative bacteria

  • Both gram-positive and gram-negative bacteria are Prokaryotic organisms.
  • Membrane-bound organelles are absent in both the groups of bacteria.
  • The genome is covalently closed circular DNA in both the types.
  • Both the gram-positive and gram-negative groups of bacteria possess the outermost covering i.e. capsule.
  • Peptidoglycan is present in both groups of bacteria.
  • The cytoplasm is composed of the lipid bilayer in both the gram-positive and gram-negative group.
  • Both the gram-positive and gram-negative bacteria reproduce asexually by the means of Binary fission.
  • Gram-positive and gram-negative bacteria reproduce sexually through transformation, conjugation and transduction.

Conclusion

In this context, we have discussed the many differences in the properties like staining and structural properties between the gram-positive and gram-negative bacteria.

This content also gives the idea of the difference in staining properties and the reasons for the appearance of violet and pink colour in gram-positive and gram-negative bacteria.

Therefore we can conclude that both the groups of bacteria are structurally and functionally different due to their cell wall difference but also have some similarities as they in the group of bacteria.

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