Negative Stains

Negative stains are simple stains applied to a non-heat fixed bacterial sample
Purpose: Negative stains are used to view cell morphology and arrangement of microorganisms and selected because of the minimal damage and distortion of the bacterial structures.

Acidic stains such as Nigrosin are used in the staining process. The bacterial sample is mixed with the dye on the slide. A second slide is used as a spreader to create a film over the entire slide surface. The sample is then allowed to air dry.

Acidic stains are anionic, that is, they are negatively charged since they give up a H+.
Bacteria also have a negatively charged surface, so the two negative charges repell one another.

The bacteria will appear clear against a colored background.

Negative stain of cocci shown as clear spheres against a black background.