Difference Between Gram Positive and Gram Negative Bacteria

Gram staining is the first stage in identification of the bacteria. It differentiates the bacteria on the basis of chemical properties of their Cell Wall. Hans Christian Gram was the inventor of Gram staining. Please note that NOT all bacteria can be classified by this technique and only those bacteria which can be classified by using this technique are called Gram variable. Otherwise they are called Gram indeterminate.

How does Gram Staining work?

The staining distinct two types of bacteria viz. Gram positive and Gram negative denoted by G+ and G-. The primary stain used in the technique is crystal violet. Crystal violet is followed by use of a trapping agent (Gram Iodine) and after that alcohol is used to decolorize and finally Safranin / Basic fushcin is used to counter stain. The crystal violet gets dissociated in CV+ and Cl- ions in water and these ions penetrate the cell walls.

The cell wall which is made up of Peptidoglycan as well as lipids gets violet due to the reaction of the CV+. After the decolorization with alcohol, the lipids gets dissolved and the bacteria with higher Peptidoglycan remain violet. These are called Gram Positive bacteria. The bacteria which lose the violet color are called Gram negative bacteria.

Properties of Gram negative bacteria
  • The cell wall is heterogeneous.
  • Thickness of the wall is in the range of 10-15 nanometer.
  • The cell wall is 10-20% of the cell’s dry weight
  • Composition: Peptidoglycan (5-15%), Phospholipids 35% and proteins 15%. 50 % is lipopolysachharides.
  • Teichoic Acid , which provides the rigidity to the cell walls is absent in gram negative bacteria.
Properties of Gram Positive Bacteria
  • Cell wall is homogenous.
  • Thickness of the Cell wall is around 25-30 nanometer
  • 20-40% of dry weight is made by Cell wall.
  • 20 to 80% of the cell wall is made up of Peptidoglycan.
  • The teichoic acid is present in the cell walls of Gram Positive bacteria.


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