If you forget to use the decolorizer during a Gram stain, which cell type(s) will appear purple under the microscope?

• A. Neither Gram-positive nor Gram-negative bacteria
• B. Both Gram-positive and Gram-negative bacteria
• C. Only Gram-negative bacteria
• D. Only Gram-positive bacteria

Answer: (B)

Verified step by step guidance

1. Understand the purpose of the decolorizer in the Gram staining process: it differentiates between Gram-positive and Gram-negative bacteria by removing the crystal violet-iodine complex from Gram-negative cells but not from Gram-positive cells.
2. Recall that Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain even after decolorization, appearing purple under the microscope.
3. Recall that Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, so the decolorizer removes the crystal violet stain, allowing them to take up the counterstain (usually safranin) and appear pink/red.
4. If the decolorizer step is skipped, the crystal violet stain is not removed from any cells, meaning both Gram-positive and Gram-negative bacteria will retain the purple color of the crystal violet stain.
5. Therefore, without the decolorizer, both types of bacteria will appear purple under the microscope, making it impossible to distinguish between them based on color.