Difference Between Gram-positive and Gram-negative Bacteria

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.

Difference between Gram-Positive and Gram-Negative Bacteria

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

Difference between Gram-Positive and Gram-Negative Bacteria

Gram-Positive and Gram-Negative Bacteria – Overview

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 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. 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, 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.

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.

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:

• Microorganisms – Useful Or Harmful
• Gram-Positive Bacteria