Capsular staining

Certain strains of bacteria and yeasts have a protective outer covering called capsule, protecting them from host immune system. Presence of capsule increases the pathogenicity of the organism. Capsule does not absorb most dyes and negative stain is used to stain background.

Two procedures of capsular staining:

1. Negative only: Nigrosin or Indian ink is used to stain background.
2. Negative and positive both: Crystal violet is used first and then washed off with CuSO4. Both cells and background are stained. Clear halo around bacteria indicates capsule.