What type of staining method uses an anionic dye?

A negative stain is a technique in microbiology where an anionic dye, usually a negatively charged dye like India ink or nigrosin, is used to color the background while leaving the cells themselves unstained or only lightly stained. This methodology allows for the visualization of bacterial cells in their natural shape and size without the distortion that can occur with heat-fixing or with positive stains that directly color the cells.

The use of an anionic dye is particularly advantageous because it repels the negatively charged components of bacterial cell walls, leading to a clear contrast between the dark background and the transparent cells. This is especially useful for observing morphological features and capsule structures in certain bacteria.

In contrast, other staining methods such as simple, Gram, and acid-fast stains utilize cationic dyes that bind to negatively charged components of the bacterial cells. These techniques highlight the cells rather than their surroundings, which is the opposite of what occurs in a negative stain.