Capsule Stain

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:









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