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Biology LibreTexts

21: Capsule Stain

Objectives

  • Identify capsules around bacteria.

The capsule is a thick polysaccharide layer around the outside of the cell. It is nonionic,so the dyes that we commonly use will not bind to it. Two dyes, one acidic and one basic, are used to stain the background and the cell wall, respectively. The area between the 2 dyes is the halo of the capsule surrounding the cell.

The capsule gives added protection to the bacteria, making it virtually impossible to be phagocytosed by white blood cells. In addition, the capsule protects cells against dessication and enhances the attachment of the cells to surfaces (e.g. biofilms).  The thickness of the capsule can vary (depending on the species of bacterium, its age, and the medium in which the bacterium is growing).

 

        

Capsulated bacterium                                                                  Noncapsulated bacterium

 

MATERIALS NEEDED

  • prepared slides of encapsulated bacteria

THE PROCEDURES

  1. These stains are bought and ready to use.
  2. Use brightfield microscopy.
  3. Use oil when on 100X magnification.
  4. Be sure to remove the oil with the lens paper.

QUESTIONS

  1. Why does the capsule NOT take in any dye?
  2. What is the function of the capsule?

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