Question

In Gram staining technique, Gram-positive bacteria

• A. do not get stained by crystal violet
• B. appear violet even after the addition of counterstain
• C. take up the counterstain and appear orange-red
• D. lose crystal violet stain on washing with alcohol.

Answer 1

The correct option is B appear violet even after the addition of counterstain

Christain Gram invented the gram staining technique.


Gram-positive and Gram-negative bacteria take up the crystal violet stain and appear purple. Gram-positive bacteria do not lose stain after washed by alcohol, but Gram-negative bacteria lose stain and get colourless after washed by alcohol. This is because of the difference in the thickness and cell wall composition of Gram-positive and Gram-negative bacteria.
Gram-positive bacteria have a thick layer of peptidoglycan in the cell wall because of which they retain the crystal violet stain. Gram-negative bacteria have a thin layer of peptidoglycan and lipopolysaccharides in their cell wall due to which they lose crystal violet stain after washed by alcohol. Gram-negative bacteria lose stain and become colourless, they take up counterstain safranin and appear orange-red (or pink). Gram-positive bacteria do not lose crystal violet stain on the addition of alcohol and appear violet (or purple) even after stained with safranin.