Question

The gram’s stain used to stain bacteria are
A. Safranin and iodine
B. Cotton blue and iodine
C. Crystal violet and safranin
D. Crystal violet and cotton blue

Answer 1

Hint: Gram staining is a highly popular technique to distinguish between gram positive and gram negative bacteria. Staining is a technique that helps to visualize bacteria under the microscope by increasing the contrast.

Complete Answer:
- Gram staining was first developed by the bacteriologist Hans Christian Gram. It is known to distinguish between cells on the basis of their chemical and physical properties of their cell wall.
- Gram positive bacteria have a thicker cell wall consisting of cross linking of peptidoglycan whereas gram negative bacteria do not have such a thick layer of peptidoglycan instead have an outer membrane rich in lipids and thus differs in gram staining property.
- Gram staining mechanism involves the following steps- firstly crystal violet which is the primary stain is applied to heat fixed smear of bacterial culture and kept for a minute, next iodine is added that binds to crystal violet and fixes it to the cell wall further, next ethanol wash is done to wash out the excess stain and finally safranin which is the counterstain is added.
- At the end of the process the gram positive cells appear bluish violet as they remain stained with crystal violet whereas gram negative cells appear red due to safranin staining.

So the correct answer is C) Crystal violet and safranin.

Note: The difference in colour between gram positive and gram negative cells is due to the difference in their cell wall composition. The alcohol washing step dissolves most of the lipid in the outer membrane of gram negative bacteria due to which it loses crystal violet and takes up the counterstain. But gram positive bacteria do not contain lipids and thus retain crystal violet.