Difference Between Gram-positive and Gram-negative Bacteria

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Bacteria are a large group of minute, unicellular, microscopic organisms, which have been classified as prokaryotic cells, as they lack a true nucleus. These microscopic organisms comprise a simple physical structure, including cell wall, capsule, DNA, pili, flagellum, cytoplasm and ribosomes.

Bacteria can be gram-positive or gram-negative depending upon the staining methods. Let us have a detailed look at the difference between the two types of bacteria.

Gram Staining

This technique was proposed by Christian Gram to distinguish the two types of bacteria based on the difference in their cell wall structures. The gram-positive bacteria retain the crystal violet dye, which is because of their thick layer of peptidoglycan in the cell wall.

This process distinguishes bacteria by identifying peptidoglycan that is found in the cell wall of the gram-positive bacteria. A very small layer of peptidoglycan is dissolved in gram-negative bacteria when alcohol is added.

Gram-Positive and Gram-Negative Bacteria – Overview

The gram-positive bacteria retain the crystal violet colour and stain purple whereas the gram-negative bacteria lose crystal violet and stain red. Thus, the two types of bacteria are distinguished by gram staining.

Gram-negative bacteria are more resistant to antibodies because their cell wall is impenetrable.

Gram-positive and gram-negative bacteria are classified based on their ability to hold the gram stain. The gram-negative bacteria are stained by a counterstain such as safranin, and they are de-stained because of the alcohol wash. Hence under a microscope, they are noticeably pink in colour. Gram-positive bacteria, on the other hand, retains the gram stain and show a visible violet colour upon the application of mordant (iodine) and ethanol (alcohol).

Gram-positive bacteria constitute a cell wall, which is mainly composed of multiple layers of peptidoglycan that forms a rigid and thick structure. Its cell wall additionally has teichoic acids and phosphate.  The teichoic acids present in the gram-positive bacteria are of two types – the lipoteichoic acid and the teichoic wall acid.

In gram-negative bacteria, the cell wall is made up of an outer membrane and several layers of peptidoglycan. The outer membrane is composed of lipoproteins, phospholipids, and LPS. The peptidoglycan stays intact to lipoproteins of the outer membrane that is located in the fluid-like periplasm between the plasma membrane and the outer membrane. The periplasm is contained with proteins and degrading enzymes which assist in transporting molecules.

The cell walls of the gram-negative bacteria, unlike the gram-positive, lacks the teichoic acid. Due to the presence of porins, the outer membrane is permeable to nutrition, water, food, iron, etc.

Difference between Gram-Positive and Gram-Negative Bacteria – Key Points

  • The cell wall of gram-positive bacteria is composed of thick layers peptidoglycan.
  • The cell wall of gram-negative bacteria is composed of thin layers of peptidoglycan.
  • In the gram staining procedure, gram-positive cells retain the purple coloured stain.
  • In the gram staining procedure, gram-negative cells do not retain the purple coloured stain.
  • Gram-positive bacteria produce exotoxins.
  • Gram-negative bacteria produce endotoxins.

Difference between Gram-Positive and Gram-Negative Bacteria

Following are the important differences between gram-positive and gram-negative bacteria:

Gram-positive and Gram-negative Bacteria

Difference between Gram-Positive and Gram-Negative Bacteria


Gram-Positive bacteria Gram-Negative bacteria
Cell Wall
A single-layered, smooth cell wall  A double-layered, wavy cell-wall
Cell Wall thickness
The thickness of the cell wall is 20 to 80 nanometres The thickness of the cell wall is 8 to 10 nanometres
Peptidoglycan Layer
It is a thick layer/ also can be multilayered It is a thin layer/ often single-layered.
Teichoic acids
Presence of teichoic acids Absence of teichoic acids
Outer membrane
The outer membrane is absent The outer membrane is present (mostly)
Absent Occurs in Outer Membrane
It is more prominent. It is less prominent.
Cocci or spore-forming rods Non-spore forming rods.
Flagella Structure
Two rings in basal body Four rings in basal body
Lipid content
Very low 20 to 30%
Absent Present
Toxin Produced
Exotoxins Endotoxins or Exotoxins
Resistance to Antibiotic
More susceptible More resistant
Staphylococcus, Streptococcus, etc. Escherichia, Salmonella, etc.
Gram Staining 
These bacteria retain the crystal violet colour even after they are washed with acetone or alcohol and appear as purple-coloured when examined under the microscope after gram staining. These bacteria do not retain the stain colour even after they are washed with acetone or alcohol and appear as pink-coloured when examined under the microscope after gram staining.


For more information on the differences between gram-positive and gram-negative bacteria, keep visiting BYJU’S website or download the BYJU’S app for further reference.

Further Reading:

Frequently Asked Questions


Give a few examples of gram-positive bacteria.

Gram-positive bacteria include the bacteria of genre Staphylococcus, Streptococcus, Enterococcus. These bacteria are the most common cause of clinical infections.


Which is more harmful- gram-positive bacteria or gram-negative bacteria?

Gram-negative bacteria are more harmful and cause certain diseases. Their outer membranes are hidden by a slime layer that hides the antigens present in the cell.


Is it easier to kill gram-positive bacteria?

The cell wall of the gram-positive bacteria absorbs antibiotics and cleaning products. Because of the outer peptidoglycan layer, they are easier to kill. Gram-negative bacteria cannot be killed easily.


What infections are caused by gram-positive bacteria?

Gram-positive bacteria usually cause Urinary Tract Infections. These are caused commonly in people who are more prone to urinary tract infections or are elderly or pregnant.


Which infections are caused by gram-negative bacteria?

The gram-negative bacteria cause various infections in humans such as indigestion, food poisoning,  pneumonia, meningitis and other bacterial infections in the blood cells,  bloodstream, wound infections, etc. The infections are caused by Acinetobacter, Pseudomonas aeruginosa and E.coli.

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