What does a negative stain primarily stain?

A negative stain primarily stains the background rather than the cells themselves. This technique utilizes acidic dyes that are negatively charged, which are repelled by the negatively charged cell surfaces. This results in a clear silhouette of the cells against a dark background. Negative staining is particularly useful for observing the morphology and arrangement of cells without causing distortion, which can be a limitation with other staining methods that require fixation and heat, potentially altering the cells' shapes.

In this context, the focus is on how negative staining highlights the structure and size of cells without directly staining them, making the contrast between the cells and the background prominent. Therefore, it is an effective method for visualizing capsules around certain bacteria as well, which do not retain the stain and appear as a clear zone surrounding the stained cells.