Which statement best describes Gram staining?

• A. It uses crystal violet to stain all bacteria blue
• B. It cannot differentiate bacteria
• C. It differentiates bacteria into Gram-positive (purple) and Gram-negative (pink) based on cell wall properties
• D. It only works on Gram-negative bacteria

Answer: (C)

Gram staining is a differential staining method used to classify bacteria into two groups based on cell wall structure. The process uses crystal violet as the primary stain, followed by iodine as a mordant, then a decolorizing step, and finally a counterstain. Bacteria with thick peptidoglycan cell walls retain the crystal violet-iodine complex and appear purple (Gram-positive). Bacteria with thinner peptidoglycan and an outer membrane do not retain it after decolorization and take up the counterstain, appearing pink (Gram-negative). This contrast helps guide initial decisions about antibiotic therapy since the Gram type generally correlates with certain cell wall targets and drug classes. Stating that it stains all bacteria blue is incorrect, as not all bacteria retain the violet stain; Gram-negative end up pink after counterstaining. It also does not work only on Gram-negative bacteria; it differentiates both groups.