Question

Why is gram staining done?

Answer 1

This is basically an identification and staining technique which was developed and discovered by Christian Gram in the year 1884. In this technique a fixed bacterial smear is subjected to the following staining reagents in the order listed: crystal violet, iodine solution, alcohol (decolorizing agent), safranin (or any other suitable counterstain). Crystal violet is the main stain used here.

Complete answer:
The staining of microbes (bacteria) using Gram’s method divides the bacteria into two major groups: Gram-positive bacteria (The bacteria which retain the crystal violet and hence appear deep violet in color) and Gram-negative bacteria (the bacteria which fail to retain the color of crystal violet, are counterstained by safranin, and hence appear deep red in color).
The fact that these bacteria are differently stained is associated with the structure and composition of the cell wall. The cell wall of gram-negative bacteria is generally thicker than that of the gram-positive bacteria. Gram negative bacteria contain a higher percentage of lipid than the gram-positive bacteria. During staining, the gram-negative bacteria lose most of the lipid part during alcohol treatment, which results in increased porosity or permeability of the cell wall. Thus, the crystal violet-Iodine complex can be extracted and the gram-negative organism is decolorized. These cells subsequently take on the color of the safranin counterstain.
The cells of the gram-positive bacteria, because of their different composition (lower lipid content), become dehydrated during treatment with alcohol. The pore size decreases, permeability is reduced, and the CV-I complex cannot be extracted.
The Gram staining technique has its greatest use in characterizing bacteria. It forms the preliminary step in identification of bacteria. It may be useful in determination of the causative organism for a specific infection.

Note:
It should be noted that the gram staining method is not generally applicable for other groups of microorganisms such as protozoa and fungi. However, yeasts consistently stain gram-positive. Although gram-negative organisms consistently fail to retain the primary crystal violet stain, Gram-positive organisms may sometimes show variations in this respect, i.e., Gram-variable reactions. Like old cultures of gram positive bacteria may lose the ability to retain the stain and then they would be stained by safranin.