Gram-negative bacteria doesn’t take gram stain because it is made of:

The exact mechanism of gram reaction is not known. The possible reason are : Gram positive bacteria have a more acidic protoplasm, which account for their retaining the basic primary dye more strongly. Dye lodine complex is retained in gram positive cells by thick peptide glycan mesh and is readily wash through the very thin peptidoglycan layer of gram – ve cells.