The principle of grams staining method as differential staining lies in the following structure of bacterial cell.

In gram staining the bacterial cells are stained with a basic stain crystal violet and later iodine solution is added which enhances the binding of crystal violet. When alcohol treatment is given the thick cell wall of Gram +ve bacteria becomes shrunken and the pores in the cell wall become very narrow that crystal violet iodine complex can not be diffused out and the bacteria retain the purple coloration. In Gram -ve bacteria, the cell wall is thin and outer layer is rich in lipids which are dissolved during alcohol treatment and large holes are formed in the cell wall due to dissolution of lipids of cell wall. The stain comes out and hence gram -ve bacteria cannot retain the purple color. To see the gram -ve bacteria as colored objects the secondary stain safranin is added which impart red or pink color to Gram -ve bacteria.