Positive stains for capsule
Because of the non-ionic nature of capsule, it has got a very low affinity for dyes. In this technique, crystal violet is applied which stains bacterial cell. On the application of 20% CuSO4 solution, osmotic difference is created which causes diffusion of stain towards the outer surface of the cell.
On removal of copper sulphate solution and drying the slide, some of the stain which has not crossed capsular surrounding during the course of diffusion will be retained in the capsular layer. This results into the appearance of capsular layer as light violet against the deep violet cell.
