How do gram-negative bacteria appear under a Gram stain?

• A. They stain dark purple
• B. They stain pink
• C. They remain clear
• D. They stain green

Answer: (B)

Gram-negative bacteria appear pink under a Gram stain due to their unique cell wall structure. The Gram staining procedure involves a series of steps that differentiate bacteria based on the composition of their cell walls. In this process, crystal violet is initially applied, which stains all bacteria dark purple.

After this, a iodine solution is added to form a complex with the crystal violet. The next step involves the decolorization process using alcohol or acetone, which has a significant impact on Gram-negative bacteria. Unlike Gram-positive bacteria, which have a thick peptidoglycan layer that retains the crystal violet stain even after decolorization, Gram-negative bacteria have a much thinner peptidoglycan layer surrounded by an outer membrane. When decolorized, the crystal violet is washed out of the Gram-negative bacteria.

Finally, a counterstain, typically safranin, is applied which stains the now-colorless Gram-negative bacteria pink. This clear distinction in appearance is crucial for microbiologists when identifying bacteria, as the staining properties help determine their classification and guide treatment strategies.