After Gram's iodine is added during the Gram stain procedure, what color do bacterial cells typically appear under a light microscope?

• A. Purple
• B. Red
• C. Green
• D. Colorless

Answer: (A)

Verified step by step guidance

1. Understand the role of Gram's iodine in the Gram stain procedure: Gram's iodine acts as a mordant, which means it forms a complex with the crystal violet dye previously applied to the bacterial cells.
2. Recall that after the crystal violet stain and Gram's iodine treatment, the cells are subjected to a decolorization step with alcohol or acetone. However, this step has not yet occurred at the point when Gram's iodine is added.
3. Recognize that the crystal violet-iodine complex is large and insoluble, so it gets trapped inside the thick peptidoglycan layer of Gram-positive bacteria, causing these cells to retain the purple color.
4. Note that Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, so the complex is not retained as strongly, but immediately after Gram's iodine is added (before decolorization), both Gram-positive and Gram-negative cells appear purple.
5. Therefore, under a light microscope immediately after Gram's iodine is added, bacterial cells typically appear purple because the crystal violet-iodine complex is still present in the cells.