Purpose: The capsule stain is a differential stain which selectively stains external capsules surrounding bacterial cells.
How it works: Capsules are highly ordered polymers of sugars and proteins that surround some bacterial cells, and can be easily dislodged by heat or water. Accordingly, capsule stains are not heat-fixed, and water is never used to rinse. The primary stain applied is crystal violet, which stains both the bacterial cell and the surrounding capsule. A 20% copper sulfate solution is then applied, which serves a dual function as both decolorizer and counterstain. It removes and replaces the crystal violet in the capsule only. At the end of the staining procedure, the capsule appears as a faint blue or white halo around a purple cell.
Overview of capsule-staining process:
Back to Tutorial