Difference Between Gram Positive And Gram Negative Bacteria

Gram Positive Vs Gram Negative Bacteria

Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the Gram-negative.

The gram-positive bacteria retain the crystal violet color 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 color. Gram-positive bacteria, on the other hand, retain the gram stain and show a visible violet color 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 form a rigid and thick structure.

Its cell wall additionally has teichoic acids and phosphate.  The teichoic acids present in gram-positive bacteria are of two types – lipoteichoic acid and 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 that assist in transporting molecules.

The cell walls of the gram-negative bacteria, unlike the gram-positive, lacks 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

What is Gram Positive Bacteria?

The cell walls of Gram-positive bacteria differ structurally from the cell walls of Gram-negative bacteria. The primary component of bacterial cell walls is peptidoglycan.

Peptidoglycan is a macromolecule composed of sugars and amino acids that are assembled structurally like woven material. The amino sugar component consists of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

These molecules are crosslinked together by short peptides which help give peptidoglycan strength and structure. Peptidoglycan provides protection for bacteria and defines their shape.

The Gram-positive cell wall has several layers of peptidoglycan. The thick layers of peptidoglycan help to support the cell membrane and provide a place of attachment for other molecules.

The thick layers also enable Gram-positive bacteria to retain most of the crystal violet dye during Gram staining causing them to appear purple.

Gram-positive cell walls also contain chains of teichoic acid that extend from the plasma membrane through the peptidoglycan cell wall. These sugar-containing polymers assist in maintaining cell shape and play a role in proper cell division. Teichoic acid helps some Gram-positive bacteria to infect cells and cause disease.

Some Gram-positive bacteria have an additional component, mycolic acid, in their cell walls. Mycolic acids produce a waxy outer layer that provides additional protection for mycobacteria, such as Mycobacterium tuberculosis.

Gram-positive bacteria with mycolic acid are also called acid-fast bacteria because they require a special staining method, known as acid-fast staining, for microscope observation.

Pathogenic Gram-positive bacteria cause disease by the secretion of toxic proteins known as exotoxins. Exotoxins are synthesized within the prokaryotic cell and released into the exterior of the cell.

They are specific to certain bacterial strains and can cause serious damage to body organs and tissues. Some Gram-negative bacteria also produce exotoxins.

What are Gram Negative Bacteria?

Like Gram-positive bacteria, the Gram-negative bacterial cell wall is composed of peptidoglycan. However, the peptidoglycan is a single thin layer compared to the thick layers in Gram-positive cells.

This thin layer does not retain the initial crystal violet dye but picks up the pink color of the counterstain during Gram staining. The cell wall structure of Gram-negative bacteria is more complex than that of Gram-positive bacteria.

Located between the plasma membrane and the thin peptidoglycan layer is a gel-like matrix called periplasmic space.

Unlike Gram-positive bacteria, Gram-negative bacteria have an outer membrane layer that is external to the peptidoglycan cell wall. Membrane proteins, murein lipoproteins, attach the outer membrane to the cell wall.

Another unique characteristic of Gram-negative bacteria is the presence of lipopolysaccharide (LPS) molecules on the outer membrane. LPS is a large glycolipid complex that protects bacteria from harmful substances in their environment.

It is also a bacterial toxin (endotoxin) that can cause inflammation and septic shock in humans if it enters the blood.4 There are three components of the LPS: Lipid A, a core polysaccharide, and an O antigen.

The lipid A component attaches the LPS to the outer membrane. Attached to lipid A is the core polysaccharide. It is located between the lipid A component and the O antigen.

The O antigen component is attached to the core polysaccharide and differs between bacterial species. It can be used to identify specific strains of harmful bacteria.

Main Difference Between Gram Positive and Gram Negative Bacteria?

  • Gram positive bacteria have a thick peptidoglycan layer in their cell wall while gram negative bacteria have a thin peptidoglycan layer in their cell wall.
  • Gram negative bacteria possess an outer membrane and it is absent in gram positive bacteria.
  • Gram negative bacteria have a periplasmic space and two layers in the cell wall while gram positive bacteria lack a periplasmic space and they have a single layered rigid and even cell wall.

What are the Similarities Between Gram Positive and Gram Negative Bacteria?

  • Gram Positive and Gram Negative Bacteria have a similar cellular organization.
  • Also, both are prokaryotic unicellular microorganisms which possess capsules.
  • And, they both possess a single chromosome and are ubiquitous.
  • Furthermore, both may contain plasmids as their extrachromosomal DNA.
  • Besides, both reproduce asexually by binary fission.
  • Similarly, both undergo a transformation, transduction and conjugation.
  • Moreover, Gram Positive and Gram Negative Bacteria are inhibited by antibiotics.
  • Their cell walls contain peptidoglycan.
  • And also both types of bacteria have a surface layer (S layer).
  • They respond to grams staining procedure.
  • Furthermore, they cause diseases to humans, plants and animals.

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