What does a Gram stain differentiate, and what colors indicate Gram-positive vs Gram-negative organisms?

What does a Gram stain differentiate, and what colors indicate Gram-positive vs Gram-negative organisms?

• A. Differentiates metabolic activity; Gram-positive green, Gram-negative yellow
• B. Differentiates cell wall structure; Gram-positive appear purple, Gram-negative pink/red after counterstain.
• C. Differentiates capsule; Gram-positive red, Gram-negative blue
• D. Differentiates spore formation; Gram-positive orange, Gram-negative violet

Answer: (B)

Gram staining differentiates bacteria by cell wall structure. The thick peptidoglycan layer in Gram-positive organisms traps the crystal violet–iodine complex so they stay purple after rinsing and the counterstain is applied. Gram-negative organisms have a thinner peptidoglycan layer and an outer membrane that is disrupted by alcohol, causing them to lose the crystal violet-iodine complex; they then take up the counterstain and appear pink or red. So you see purple for Gram-positive and pink/red for Gram-negative. This stain is about wall architecture, not metabolic activity, capsules, or spores.

Gram staining differentiates bacteria by cell wall structure. The thick peptidoglycan layer in Gram-positive organisms traps the crystal violet–iodine complex so they stay purple after rinsing and the counterstain is applied. Gram-negative organisms have a thinner peptidoglycan layer and an outer membrane that is disrupted by alcohol, causing them to lose the crystal violet-iodine complex; they then take up the counterstain and appear pink or red. So you see purple for Gram-positive and pink/red for Gram-negative. This stain is about wall architecture, not metabolic activity, capsules, or spores.