What color do Gram-negative organisms retain when exposed to Gram stain?

• A. Red to pink
• B. Blue to purple
• C. Gray
• D. Gram-negative organisms do not stain.

Answer: (A)

Gram staining distinguishes bacteria by cell wall structure. Gram-negative organisms have a thin peptidoglycan layer and an outer membrane, so during the alcohol decolorization step the crystal violet-iodine complex is washed out. When the counterstain (safranin) is applied, these cells take up the pink/red color, making them appear red to pink under the microscope. This contrasts with Gram-positive bacteria, which retain the purple dye due to their thick peptidoglycan layer. So, Gram-negative organisms retain red to pink after Gram staining.