In microbiology, the Gram stain is primarily used to distinguish bacteria by their

• A. DNA sequence
• B. Growth rate
• C. Antibody binding
• D. Cell wall composition

Answer: (D)

Gram staining distinguishes bacteria based on differences in their cell wall composition. The process uses crystal violet and iodine to color all cells, but subsequent alcohol decolorization affects them differently: Gram-positive cells have a thick peptidoglycan layer that traps the dye and stays purple, while Gram-negative cells have a thinner peptidoglycan layer with an outer membrane that is disrupted by the alcohol, allowing the dye to wash out and the counterstain to color them pink. Because the key difference being detected is the structure and makeup of the cell wall, this staining method is about cell wall composition. It’s not about DNA sequence, growth rate, or antibody binding, which are assessed by other methods.