Difference Between Gram-positive and Gram-negative Bacteria


Bacteria are a large group of microscopic, unicellular organisms and have been grouped into prokaryotic, which means the absence of the nucleus and they have a simple internal structure, including capsule, cell wall, DNA, Flagellum, Pilli, Cytoplasm, and Ribosomes. They exist in every place around us and are present in communities of millions. In length, the complete bacteria measure only a few micrometers.

A bacterium is a singular word for bacteria. The study of bacteria is known as bacteriology and it’s a branch of microbiology.

Types Of Bacteria

Types of Bacteria

There are several different types of bacteria and are mainly classified by their:

  • Shapes: coccus, Bacillus, Vibrio, spirillum or spirochete.
  • Cell wall composition: Gram-positive and Gram-negative.
  • Nutritional Patterns: Autotrophic or heterotrophic.
  • Mode of Respiration: Aerobic or Anaerobic.

Here, let us explore more information about the difference between Gram-positive and Gram-negative bacteria.


Difference between Gram-positive and Gram-negative Bacteria

Difference between Gram-positive and Gram-negative Bacteria

Gram-positive bacteria Gram-negative bacteria
Cell Wall Their cell wall is smooth and single layered They have a wavy and double layered 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 It is a thin layer
Teichoic acids Presence of teichoic acids Absence of teichoic acids
Outer membrane The outer membrane is absent The outer membrane is present
Porins Absent Occurs in Outer Membrane
Mesosome It is more prominent. It is less prominent.
Morphology Cocci or spore-forming rods Non-spore forming rods.
Flagella Structure 2 rings in basal body 4 rings in basal body
Lipid content Very low 20 to 30%
Lipopolysaccharide Absent Present
Toxin Produced Exotoxins Endotoxins or Exotoxins
Resistance to Antibiotic More susceptible More resistant
Examples- Staphylococcus, Streptococcus, etc. Escherichia, Salmonella, etc.
Gram staining characteristics

These bacteria retain the crystal violet color even after they are washed with acetone or alcohol and appear as purple colored when examined under the microscope after gram staining.

These bacteria do not retain the stain color even after they are washed with acetone or alcohol and appear as pink colored when examined under the microscope after gram staining.


Gram-positive And Gram-negative Bacteria

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

Gram-positive bacteria have a cell wall 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. The cell wall is known as murein.

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.

Gram Stain Test

Gram stain or Gram’s method is a method of staining that is used to discriminate and for the classification of bacterial species into two broad groups – the Gram-positive and Gram-negative. This process distinguishes bacteria on the basis of physical and chemical properties of their cell walls through the identification of 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.


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A farmer has a plot of land in which water is scarce. Which of the following methods should he adopt for irrigation?