Gram Staining MCQ - Practice Questions with Answers

• The Gram staining is a microbiological differential staining technique. 
• It was developed by Dr Christian Gram in 1884. 
• It categorizes bacteria according to the chemical composition of their cell wall. 

Principle of Gram staining:

• The structure of a bacterium's cell wall determines whether it is gram positive or negative. 
• When stained with a primary stain and fixed by a mordant, some bacteria are able to retain the primary stain by resisting decolourization.
• While others get decolourized by a decolorizing agent. 
• Those bacteria which retain the primary stain are called Gram-positive and those bacteria which gets decolourized and then get counterstained are called Gram-negative.

Staining Procedure:

Types of bacteria based on Gram staining:

Gram-positive bacteria

• These bacteria retain the blue or purple colour even after washing with alcohol.
• The cell wall of Gram-positive bacteria is single-layered.
• The thickness of the cell is 20-80nm.
• Lipid content of the wall is very low.
• Mucopeptide is high i.e. 70-80%.
• Mesosomes are prominent.
• These bacteria are susceptible to antibiotics.
• Cell wall contains teichoic acid. 

Gram-negative bacteria 

• Bacteria which do not retain any colour and become colourless after Gram staining are called Gram-negative bacteria.
• The wall is two layered and it is 8-12 nm thick.
• The lipid content of the cell wall is on a higher side. It is at 20-30 %.
• The content of mucopeptide is low i.e. 10-20%.
• Mesosomes are less prominent.
• These bacteria are resistant to antibiotics. 
• Teichoic acids are absent in Gram-negative bacteria.