A capsule is a gelatinous outer layer that is secreted by the cell and that surrounds and adheres to the cell wall. It is not common to all bacteria and may be helpful in the identification of the organism. Cells that have a heavy capsule are generally virulent and capable of producing disease. In some bacteria, the capsule protects them against the normal phagocytic activities of the host cell, antibiotics, and immunoglobulin. Chemically, the composition of the capsule is a polypeptide, glycoprotein, or a polysaccharide.
Capsules allow bacteria to adhere to other bacteria and to the surface of other cells, soil, sand, or surfaces. Capsules are not easily stained, because they are very fragile and are easily disrupted by water and heat. Capsular staining does not require heat-fixing a smear. The use of heat during fixation causes shrinkage, so the procedure is performed without heat.
Capsules cannot be stained so techniques, such as the Gin’s method, were developed to stain the background and leave the capsule clear. This is referred to as negative staining.
A negative stain dyes everything except the structure you wish to visualize. The acidic negative stain has a negative charge and will not penetrate the cell because it is repelled by the negative charge on the surface of the bacterial cell wall. The capsules appear as halos or rings around bacterial cells, and the background is dark.
India ink or nigrosin are used for negative staining. After drying, the slide preparation is stained with safranin, crystal violet, or methylene blue, dyes that do penetrate the cells and stains them.
When viewed under the microscope, the bacteria are pink, purple, or blue respectively; whereas the capsules appear clear, unstained zones that surround them, and their outlines are demarcated by the black background of the India ink or nigrosin.