What color do Gram-negative bacteria appear after Gram staining?

After Gram staining, Gram-negative bacteria appear pink due to the structure of their cell walls. The Gram staining process differentiates bacteria based on the composition of their cell walls.

In Gram-negative bacteria, the outer membrane contains a thin layer of peptidoglycan surrounded by an outer membrane composed of lipopolysaccharides. During the staining process, the crystal violet stain is first applied, which initially stains all bacteria purple. Following this, the bacterial cells are treated with iodine, which forms a complex with the crystal violet.

The crucial step for Gram-negative bacteria comes when alcohol or acetone is used as a decolorizer. This treatment dissolves the outer membrane and allows the violet stain to leak out due to the thin peptidoglycan layer. As a result, Gram-negative bacteria lose the initial purple color. When a counterstain, usually safranin, is added afterward, it stains the decolorized cells resulting in a pink appearance. This characteristic pink coloration is specific to Gram-negative bacteria, making it a key identifying feature in microbiology.