Staining means coloring of microbes with a dye that colors the microbe to emphasize or elucidate different important structures like cell wall or other cellular organelles of microorganisms including bacteria, virus, protozoa and etc.

Staining is done to:

  1. Highlight the different parts of microorganisms to enable it to be seen under microscopes.
  2. Differentiate various types of microorganisms.
  3. Highlight the microorganism from the background
  4. Study the morphology of the microorganisms.
  5. Cells are stained to study morphology of microorganisms.
  6. The cells are stained to enhance visualization of the cell and cell organelles under a microscope.
  7. Cells may also be stained to highlight and study their metabolic process.
  8. To differentiate between viable and non-viable cells in a sample.
  9. Cells are stained to demonstrate the presence of internal and external structures or organelles.
  10. Cells are stained to distinguish between different types of organisms.

The Different types of Staining:

  1. Simple Stains.
  2. Differential Stains.
  3. Special Stains.

A simple staining method is the one in which total microbes are stained to highlight to be seen under a microscope. They are also referred as monochrome stains because only one dye is used for the coloration of bacterial smear. The surface of a bacterial cell has an overall acidic characteristic because of large amount of carboxyl groups located on the cell surface due to acidic amino acids. A positively charged dye like (methylene blue) attaches to the negatively charged surface of the bacteria and gives it a colored appearance.
Various types of dyes that are used, for example, Methylene Blue, Crystal Violet, Carbol Fuschin etc.

Differential staining is the process in which staining is done to differentiate different types of microbes.

Examples of this type includes:

  1. Gram’s Staining: in this process Gram negative bacteria are differentiated from Gram positive bacteria, as on the basis of their cell wall composition they take up different dyes.
  2. Acid fast stain: This staining process is employed to identify the bacteria having cell wall of waxy material. Mycobacterium tuberculosis Mycobacterium leprae.
    Acid fast staining is another widely used differential stain­ing procedure in bacteriology. This stain was developed by Paul Ehrlich in 1882, during his work on etiology of tuberculosis. Some bacteria resist decolorization by both acid and alcohol and hence they are referred as acid-fast organisms. Acid alcohol is very intensive decolorizer.
    This staining technique divides bacteria into two groups:
    (i) acid-fast
    (ii) non-acid-fast.
    This procedure is extensively used in the diagnosis of tuberculosis and leprosy.
  3. Special Stains: this method is used when we want to stain special part of the microorganism. For example, staining of flagella, staining of endospores, Staining of capsule.
  4. Iron-Heamatoxylin Stain: it is used to stain tissue components. For example, myelin, muscle striations etc.
  5. Wheatley Trachoma Stain: it is the easiest & quickest method to stain protozoa.
  6. Periodic acid-Schiff (PAS) Staining: This technique is used to look for fungi in tissues and in cytology specimens. For example, PAS staining of Cryptococcus.

What is the difference between stain and dye?

  1. A dye is a coloring agent used for general purposes and a stain is used for any biological specimen staining. Also, a dye is crude and a stain is purified.
  2. A dye is the coloring agent that has been prepared with lesser specifications and it may contain impurities. Stains are the biological coloring agents that are purer and prepared with greater care and specification.