Questions: 27. What bacterial structure is stained in a Gram stain?

The answer is the cell wall.

Explanation:

• The Gram stain is a common technique used to differentiate bacterial species into two groups based on the structural differences in their cell walls.
• Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain, appearing purple under a microscope.
• Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which does not retain the crystal violet stain after the alcohol wash. Instead, they take up the counterstain (usually safranin) and appear pink or red.
• Therefore, the structure that is primarily stained and differentiated in a Gram stain is the bacterial cell wall.