The cell wall is the layer that lies outside the plasma membrane. The chemical difference between the cell walls of gram positive and gram negative bacteria are as follows-
Gram positive bacteria
- The cell wall of gram positive bacteria is thick as compared to the gram negative bacteria. It consist of a single 20-80nm thick peptidoglycan layer.
Gram negative bacteria
- In gram negative bacteria the cell wall comprises of peptidoglycan layer and outer membrane. The peptidoglycan layer is 2 to 7 nm thick and it is covered by a 7 to 8 nm thick outer membrane.
During staining procedure the bacteria are first stained with crystal violet and then treated with the iodine to promote dye retention. After this When gram positive bacteria are treated with alcohol (ethanol) for decolorization, the alcohol shrinks the pores of the thick peptidoglycan cell wall, which retains the crystal violet dye-iodine complex and the gram positive bacteria remains purple. However, the peptidoglycan layer of gram-negative bacteria is thin and has large pores because of which alcohol easily removes the crystal violet dye-iodine complex from the cell wall and gets easily counterstain by safranin.
So, the rate of decolorization in gram negative bacteria is faster than the rate of decolorization in gram positive bacteria due to the presence of thin peptidoglycan layer in gram negative bacteria.
Learn more about cell wall of gram-positive and gram-negative bacteria.
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