In a negative stain, which of the following is stained?

• A. The cell wall only
• B. The bacterial cell
• C. The background
• D. Both the cell and the background

Answer: (C)

Verified step by step guidance

1. Understand the principle of a negative stain: it uses an acidic dye that carries a negative charge, which is repelled by the negatively charged bacterial cell surface, so the dye does not penetrate the cell.
2. Recognize that because the dye is repelled by the bacterial cell, the bacterial cells remain unstained and appear clear or transparent under the microscope.
3. Note that the acidic dye stains the background or the surrounding medium instead, creating a dark or colored background against which the clear cells can be easily observed.
4. Conclude that in a negative stain, it is the background that is stained, not the bacterial cell or the cell wall specifically.
5. Therefore, the correct understanding is that the background is stained in a negative stain, allowing visualization of the cells by contrast.