Why capsule stain

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Principle of Capsule Staining Capsules stain very poorly with reagents used in simple staining and a capsule stain can be, depending on the method, a misnomer because the capsule may or may not be stained. Congo Red is easier to see, but it does not work well with some strains. India Ink generally works, but it has tiny particles that display Brownian motion that must be differentiated from your bacteria. Nigrosin may need to be kept very thin or diluted. Using sterile technique, add a loopful of bacterial culture to slide, smearing it in the dye.

Use the other slide to drag the ink-cell mixture into a thin film along the first slide and let stand for minutes. Allow to air dry do not heat fix. Capsule is synthesized in the cytoplasm and secreted to the outside of the cell where it surrounds the bacterium. Most of the capsulated bacteria have a capsule made up of a polysaccharide layer but some bacteria have capsule made up of polypeptide, or glycoprotein. Capsules are associated with virulence in several microorganisms, including Streptococcus pneumoniae and Neisseria meningitides , because capsules resist phagocytosis thus evading the host immune system.

Bacterial capsules are non-ionic, so neither acidic nor basic stains will adhere to their surfaces. Therefore, the best way to visualize them is to stain the background using an acidic stain e. Various types of methods are available for the demonstration of the presence of a capsule. The results stain of the cells, background, and capsule depend on the type of method used. Two commonly used methods are discussed here:.

In this method, two dyes, crystal violet, and India ink are used. The capsule is seen as a clear halo around the microorganism against the black background. On the other hand, crystal violet is used for number of reasons including:. When viewed under the microscope, the background will be dark as a result of India ink, bacteria cells will be purple having taken the crystal violet dye while the capsule will be clear against a dark background given that it takes no stain.

Some of the other requirements include:. This is a positive staining method used in capsule staining. After the slide is air dried, it becomes possible to observe the stain that remained on the capsular layer. Here, one will see a dark violet color of the cell and a light violet color of the capsule. Unlike endospores, capsules are more delicate. For this reason, using heat during the staining process should be avoided given that capsules would be easily destroyed.

Rinsing the slide with water is also avoided in capsule staining because it would dislodge the capsule. A drop of serum may be used on the smear to enhance the size of the capsule. This helps make it more visible to the technician under the typical light microscope. Given that this is a sensitive procedure, sterile technique should be used when obtaining the sample and introducing it onto the slide.

This prevents any contamination and helps determine whether the sample really has any bacteria capsules. As previously mentioned, capsules are considered to be the virulence factor which means that they enhance the ability of a bacterial to cause a disease.

With a capsule in place, the bacteria is protected from such body defense cells as macrophages. Therefore, it becomes easier for the bacteria to thrive and cause diseases. For this reason, capsule stain is of great benefit in that it helps identify the presence of capsules and thus allowing healthcare professionals to recommend the right treatment. Cell Staining in Microscopy. Gram Staining - Purpose, Procedure and Preparation.