What color does a Gram-negative stain appear?

What color does a Gram-negative stain appear?

• A. Blue/purple.
• B. Green.
• C. Colorless.
• D. Reddish/brown.

Answer: (D)

The key idea is that Gram staining distinguishes bacteria by cell wall structure. The thick peptidoglycan layer in Gram-positive traps the crystal violet-iodine complex, so these cells stay purple/blue. Gram-negative have a thinner peptidoglycan layer and an outer membrane, so after alcohol decolorization the crystal violet is washed away. They then take up the counterstain, typically safranin, which gives them a pink to red color. So Gram-negative organisms appear reddish-pink under the microscope. The option describing a reddish hue best matches this outcome.

The key idea is that Gram staining distinguishes bacteria by cell wall structure. The thick peptidoglycan layer in Gram-positive traps the crystal violet-iodine complex, so these cells stay purple/blue. Gram-negative have a thinner peptidoglycan layer and an outer membrane, so after alcohol decolorization the crystal violet is washed away. They then take up the counterstain, typically safranin, which gives them a pink to red color. So Gram-negative organisms appear reddish-pink under the microscope. The option describing a reddish hue best matches this outcome.