Differential staining of bacteria: capsule stain.

Bacterial capsules are composed of high-molecular-weight polysaccharides and/or polypeptides, and are associated with virulence and biofilm formation. Unfortunately, capsules do not stain well with crystal violet, methylene blue, or other simple stains. This unit describes two methods of capsule staining. The first is a wet-mount method using india ink; the capsule is visualized as a refractile zone surrounding a cell. The second is a direct-staining dry-mount method that precipitates copper sulfate and leaves the capsule as a pale blue zone. Both methods are easily performed within approximately 5 min.