Which of the following is an example of a cationic dye?

Methylene blue is classified as a cationic dye because it has a positive charge at a neutral pH. Cationic dyes are positively charged and typically bind to negatively charged components of cells and tissues, such as nucleic acids and certain proteins, allowing for effective staining of these structures during microscopy. This property makes methylene blue particularly useful for staining bacteria, which often have negatively charged cell walls.

In contrast, other dyes that were listed, such as acid fuchsin and Congo red, are considered anionic dyes. Anionic dyes have a negative charge, which leads to different staining properties and applications. Nigrosin, on the other hand, is a negatively charged dye that is commonly used for negative staining techniques, where it does not penetrate the cells but instead stains the background, allowing the cells to be visualized against a dark background. Understanding the charge and behavior of these dyes in relation to the cellular structures they target is crucial for effective staining techniques in microbiology.