Gram-negative bacteria have a cell wall that is thin and does not accept the stain, making it appear red or pink.
Gram staining is used for differentiation gram-positive from gram-negative bacteria based on the characteristic of their cell wall. Gram-negative bacteria have a thinner cell wall than gram-positive bacteria. In this staining, bacteria are first stained with crystal violet dye. Only gram-positive bacteria will be violet colored because they can retain the dye thanks to their cell wall. The second staining with fuchsine or safranin will give the red or pink color to the gram-negative bacteria.
