Here's why:

* Bacteria have a net negative charge. This is due to the presence of negatively charged molecules like phospholipids in their cell membranes and nucleic acids within their cells.

* Like charges repel. A negatively charged dye would be repelled by the negatively charged surface of the bacteria, making it difficult for the dye to adhere and provide a clear stain.

So, how are bacteria stained?

* Basic dyes (positively charged) are commonly used to stain bacteria. These dyes are attracted to the negatively charged bacterial surface and readily adhere. Examples include methylene blue, crystal violet, and safranin.

* Negative staining is a technique used to visualize the background around the bacteria, not the bacteria themselves. In this technique, a negatively charged dye is used, which repels the bacteria and stains the surrounding medium. This technique is helpful for viewing bacterial capsules, which are often not stained by basic dyes.

Let me know if you have any more questions about bacterial staining!