Gram-Positive and Gram-Negative Bacteria

Gram-Positive and Gram-Negative Bacteria

Bacteria are a large group of microscopic, unicellular organisms and have been grouped under prokaryotic cells, that means they lack a true nucleus. They have a simple internal structure, including capsule, cell wall, DNA, flagellum, pili, 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 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
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.

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 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 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.

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

• Gram-positive bacteria have cell wall composed of thick layers peptidoglycan.
• Gram-Negative bacteria have cell wall with thin layers of peptidoglycan.
• Gram-positive cells stain purple during gram staining.
• Gram-negative cells stain pink during gram staining.
• Both gram-positive and gram-negative bacteria produce exotoxins whereas only gram-negative bacteria produce endotoxins.