Difference Between Gram-positive and Gram-negative Bacteria


Bacteria are a large group of microscopic, unicellular organisms and have been grouped into prokaryotic cells, which means the absence of the true nucleus. They have a simple internal structure, including capsule, cell wall, DNA, Flagellum, Pilli, Cytoplasm, and Ribosomes.

The study of bacteria is known as bacteriology. They exist in every place around us and are present in communities of millions. In length, the complete bacteria measure only a few micrometres.

Types Of Bacteria

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.

Gram-Positive and Gram-Negative Bacteria

The gram-positive bacteria retain the crystal violet colour and stains 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 against antibodies because their cell wall is impenetrable.

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 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 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 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 because of a thick layer of peptidoglycan.

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.

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

Difference 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 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 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 difference between gram-positive and gram-negative bacteria, keep visiting BYJU’S website or download 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 are an important cause of 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 causes infections such as pneumonia, bloodstream infections, wound infections and meningitis. The infections are caused by Acinetobacter, Pseudomonas aeruginosa and E.coli.

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