Which statement best describes Gram-negative bacteria?

• A. Have a thick peptidoglycan layer and stain purple
• B. Have less peptidoglycan and will stain pink
• C. Have no cell wall and stain colorless
• D. Are always pathogenic

Answer: (B)

Gram staining distinguishes bacteria by how their cell walls interact with dyes. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, so the crystal violet-iodine complex is not retained after the decolorization step. They then take up the pink counterstain and appear pink under the microscope. This is in contrast to Gram-positive bacteria, which have a thicker peptidoglycan layer that traps the dye and remains purple. The statement that Gram-negative bacteria have less peptidoglycan and stain pink captures this behavior. Note that being Gram-negative does not mean a bacterium is pathogenic; many are harmless or beneficial, while some Gram-positive bacteria can cause disease.