Microbiologists use the gram stain to aid in the identification of bacteria. What is the major difference between gram-positive and gram-negative bacteria?

Microbiologists use the gram stain technique to identify the gram-positive and gram-negative bacteria on the basis of the composition of their cell wall.

The major difference between the cell wall of gram-positive bacteria and gram-negative bacteria is that in gram-positive bacteria peptidoglycan is the major component which makes gram-positive cell wall whereas in gram-negative bacteria their cell wall has a thick outer layer of Lippo polysaccharide and a thin inner layer of peptidoglycan.

So in gram stain gram-positive cell wall retains the crystal violet because of the thick peptidoglycan layer and appears violet and gram-negative appears pink as it lacks the outer thick peptidoglycan layer.