SSC Selection Post 2025 PYQ — Question 6

Correct Answer:

Gram positive bacteria have a thick peptidoglycan layer so they are unable to retain crystal violet.

Explanation:

This statement is incorrect. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which enables them to retain the crystal violet–iodine complex during Gram staining. As a result, they appear purple under the microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and cannot retain the crystal violet stain, so they appear pink or red after counterstaining.

Key Points:

• Gram-positive bacteria retain the crystal violet stain.
• Gram-positive bacteria have a thick peptidoglycan layer.
• Gram-negative bacteria have a thin peptidoglycan layer.
• Gram-negative bacteria appear pink or red under the microscope.

Additional Facts:

• The Gram staining technique was developed by Hans Christian Gram in 1884.
• Gram’s iodine acts as a mordant and forms the crystal violet–iodine complex.
• This technique is widely used for the identification and classification of bacteria.

Important Questions:

Q1: What color do Gram-positive bacteria appear under a microscope?
Ans: Purple.

Q2: What color do Gram-negative bacteria appear under a microscope?
Ans: Pink or red.

Q3: Who developed the Gram staining technique?
Ans: Hans Christian Gram.