Why do Gram-positive bacteria stain purple in Gram staining?

Why do Gram-positive bacteria stain purple in Gram staining?

• A. They have an outer membrane
• B. They have a thick peptidoglycan layer that traps crystal violet-iodine
• C. They produce LPS
• D. They have porins

Answer: (B)

Gram staining separates bacteria by how their cell walls interact with the dye, especially the thickness of the peptidoglycan layer. The crystal violet is applied and forms a complex with iodine inside all cells. During the decolorization step with alcohol, the outer membrane and thin peptidoglycan of other bacteria are disrupted, so the dye-laden complex can be washed out. In bacteria with a thick, dense peptidoglycan layer, the crystal violet–iodine complex is trapped inside the cell wall and stays put, so the cells remain purple. The other features mentioned—an outer membrane, LPS, and porins—are associated with different bacteria that lose the dye during decolorization and do not explain the purple color in these organisms. Nanosecond takeaway: thick peptidoglycan holds the dye, preserving the purple stain.

Gram staining separates bacteria by how their cell walls interact with the dye, especially the thickness of the peptidoglycan layer. The crystal violet is applied and forms a complex with iodine inside all cells. During the decolorization step with alcohol, the outer membrane and thin peptidoglycan of other bacteria are disrupted, so the dye-laden complex can be washed out. In bacteria with a thick, dense peptidoglycan layer, the crystal violet–iodine complex is trapped inside the cell wall and stays put, so the cells remain purple. The other features mentioned—an outer membrane, LPS, and porins—are associated with different bacteria that lose the dye during decolorization and do not explain the purple color in these organisms. Nanosecond takeaway: thick peptidoglycan holds the dye, preserving the purple stain.