Principle of Staining
In positive staining, the surface of the bacterial cell takes on the colour of the stain. When basic stain is applied, there is an attraction between the negatively charged cell surface and positively charged chromophore, which leads to staining of the cell (Figure 3.2).
In negative staining, the background is coloured and bacteria remains colourless. It is because the acidic dyes are repelled by the negatively charged bacterial surface. The background gets stained and the cell remains colourless. This technique is useful for revealing the cell shape, size and demonstrating capsule (Figure 3.3).