Most bacteria can be categorized under Gram-positive and Gram-negative bacteria. The categories can be differentiated based on the cell wall composition and the reaction with chemicals using the Gram Staining method. This method is introduced by Hans Christian Gram which is very popular among microbiologists as a primary test to the sample. The cell wall between these two groups of bacteria varies based on the thickness of peptidoglycan and the presence of the outer membrane. Generally, the presence of a thick layer of peptidoglycan in Gram-positive bacteria will stain violet or purple color on the cell, while thinner layers of peptidoglycan in Gram-negative bacteria stain red or pink color on the cell wall.