Here's why:

* Acidic dyes are negatively charged molecules. This is because they contain acidic groups (like carboxyl groups, -COOH) that release hydrogen ions (H+) in solution, leaving the dye molecule negatively charged.

* Bacterial cells have a slightly negatively charged cell surface due to the presence of negatively charged molecules like phospholipids and proteins in their cell membrane.

* Opposite charges attract. Because of this, the negatively charged acidic dyes are repelled by the negatively charged bacterial cell surface.

Therefore, acidic dyes are not typically used for staining bacterial cells. They are more commonly used for staining backgrounds in microscopy, creating a contrasting color that makes the bacteria easier to see.

In contrast, basic dyes (like methylene blue and crystal violet) carry a positive charge and are readily attracted to the negatively charged bacterial cell surface, allowing them to stain the cells effectively.