What color does a Gram-positive stain appear?

What color does a Gram-positive stain appear?

• A. Red.
• B. Blue/purple.
• C. Green.
• D. Colorless.

Answer: (B)

The main idea is how Gram staining differentiates bacteria by cell wall structure. Gram-positive bacteria appear blue or purple because their thick peptidoglycan layer traps the crystal violet-iodine complex and remains after the decolorization step. The thick wall holds onto the dye, so the cells stay purple. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; during decolorization, the dye is washed out and these cells take up the counterstain, turning red or pink. Green isn’t produced by this method, and colorless would mean staining didn’t work properly. So the observed color for Gram-positive organisms is blue/purple.

The main idea is how Gram staining differentiates bacteria by cell wall structure. Gram-positive bacteria appear blue or purple because their thick peptidoglycan layer traps the crystal violet-iodine complex and remains after the decolorization step. The thick wall holds onto the dye, so the cells stay purple. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; during decolorization, the dye is washed out and these cells take up the counterstain, turning red or pink. Green isn’t produced by this method, and colorless would mean staining didn’t work properly. So the observed color for Gram-positive organisms is blue/purple.