Why Capsulate Staining is Difficult?
Capsule staining is difficult because capsular material is soluble in water and may be dislodged or removed during vigorous washing. Negative staining can be used to visualize the capsule. Here presence of capsule is shown by mixing the bacteria in a loopful solution containing tiny colloidal suspension of coloured particles e.g. India Ink or nigrosin which cannot penetrate the cell or its capsule. After spreading the mixture in a thin film and air-drying, counter staining of cells is done with simple stains such as safranin, crystal violet etc. Capsule does not accept these dyes so appear as a halo surrounding coloured cells in a dark blue-black background.