Question

Why do we want to avoid mixing the cells with water and heat fixing during the capsule stain?

Answer 1

A capsule in a bacterium is a layer of polysaccharides or polypeptides present outside the cell envelope. It provides the organism with virulence as it enhances its ability to cause diseases by avoiding phagocytosis by the macrophages. Capsules also protect the bacterium by preventing desiccation and help to fix the bacterium to the host. So the presence of the capsule helps to identify the species.  

In capsule staining the capsule has to be seen to identify the disease causing bacterium. Capsules are soluble in water and hence are difficult to be stained. So, in capsule staining the background is stained and when viewed under the microscope the capsule appears as a pale or colourless halo/ring around the cell. This method of staining is called negative staining. Since the most important function of capsule staining is to differentiate the capsule from the bacterium, the capsule has to be retained around the bacterium.

Since the capsule is soluble in water washing the specimen with water can remove the capsule. Also the capsule is very delicate and heating the slide to fix the specimen shrinks the capsule and destroys it. Due to the above mentioned reasons, mixing the cells with water and heat fixing should be avoided during capsule staining.