After completing a Gram stain, a culture appears purple under oil immersion. What is its Gram reaction?

After completing a Gram stain, a culture appears purple under oil immersion. What is its Gram reaction?

• A. Gram-positive
• B. Gram-negative
• C. Gram-variable
• D. Not determinate

Answer: (D)

Interpreting Gram stain results hinges on which stain color is retained after the decolorization step. A purple appearance means the organism kept the crystal violet-iodine complex, which is typical of Gram-positive bacteria. This retention occurs because Gram-positive cells have a thick peptidoglycan layer that traps the dye and they lack an outer membrane that would let the dye escape during decolorization. In contrast, Gram-negative cells have a thinner peptidoglycan layer plus an outer membrane, so they lose the purple dye during decolorization and take up the pink counterstain. So seeing purple under oil immersion indicates a Gram-positive reaction, not determinate only if staining or reading the slide was compromised.

Interpreting Gram stain results hinges on which stain color is retained after the decolorization step. A purple appearance means the organism kept the crystal violet-iodine complex, which is typical of Gram-positive bacteria. This retention occurs because Gram-positive cells have a thick peptidoglycan layer that traps the dye and they lack an outer membrane that would let the dye escape during decolorization. In contrast, Gram-negative cells have a thinner peptidoglycan layer plus an outer membrane, so they lose the purple dye during decolorization and take up the pink counterstain. So seeing purple under oil immersion indicates a Gram-positive reaction, not determinate only if staining or reading the slide was compromised.