Which statement correctly describes Gram-positive bacteria?

Which statement correctly describes Gram-positive bacteria?

• A. Stain red due to a thin cell wall with high lipid content
• B. Stain dark blue/violet due to a thick cell wall
• C. Stain green due to mycolic acids
• D. Stain colorless in Gram stain

Answer: (B)

Gram-positive bacteria are defined by a thick peptidoglycan layer in their cell wall that traps the crystal violet-iodine complex during the decolorization step of the Gram stain. This retention makes them appear purple or violet under the microscope after the counterstain is applied. The thick wall and absence of an outer membrane prevent the stain from washing out, which is why they stay darkly colored. The statement describing them staining dark blue or violet because of a thick cell wall captures this key idea, so it’s the best fit. In contrast, a red color after staining points to Gram-negative bacteria, a green color would be associated with staining targets like mycolic acid-containing organisms in acid-fast stains, and a colorless result would indicate an issue with the procedure rather than a feature of Gram-positive cells.

Gram-positive bacteria are defined by a thick peptidoglycan layer in their cell wall that traps the crystal violet-iodine complex during the decolorization step of the Gram stain. This retention makes them appear purple or violet under the microscope after the counterstain is applied. The thick wall and absence of an outer membrane prevent the stain from washing out, which is why they stay darkly colored.

The statement describing them staining dark blue or violet because of a thick cell wall captures this key idea, so it’s the best fit. In contrast, a red color after staining points to Gram-negative bacteria, a green color would be associated with staining targets like mycolic acid-containing organisms in acid-fast stains, and a colorless result would indicate an issue with the procedure rather than a feature of Gram-positive cells.