Brown-Hopps Modification of the Gram Stain demonstrates Gram-positive and Gram-negative bacteria in tissues. In this method, Gram-positive bacteria appear as what color?

• A. Blue
• B. Red
• C. Green
• D. Yellow

Answer: (A)

In this staining approach, the key idea is that cell wall structure determines how bacteria hold onto the stain. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex during the staining process, so they retain the primary stain and appear blue (or blue-purple) in Brown-Hopps. Gram-negative bacteria, with a thinner peptidoglycan layer and an outer membrane, lose the initial stain during decolorization and take up the counterstain, ending up red. The Brown-Hopps modification is designed to highlight bacteria in tissue sections this way, so Gram-positive organisms show blue and Gram-negative organisms show red. Therefore, Gram-positive bacteria appear blue.