The Gram stain is used to classify cells based on differences in their ________.

• A. Nucleic acids
• B. Cell membrane
• C. Capsule
• D. Peptidoglycan layer

Answer: (D)

Gram staining differentiates bacteria by the structure of their cell walls, especially the thickness and composition of the peptidoglycan layer. The dye complex is retained differently after the decolorization step: a thick peptidoglycan layer traps the dye and the cells stay purple (Gram-positive), while a thinner peptidoglycan layer with an outer membrane loses the dye and take up the counterstain, appearing pink (Gram-negative). Nucleic acids, the cell membrane itself, or capsules do not determine the differential outcome. This staining pattern reflects cell wall architecture and helps guide choices that target cell wall synthesis.