Question

In Gram staining, if some bacteria retain the crystal violet stain after alcohol treatment, then the bacteria is

• A. Incomplete experiment
• B. Gram positive
• C. Gram negative
• D. None of these

Answer 1

The Correct Option is B

Gram staining is a differential staining technique that categorizes bacteria based on their cell wall composition. 

Step 1: Understanding the Gram Staining Process - The primary stain, crystal violet, stains all bacteria. - Iodine acts as a mordant, forming a crystal violet-iodine complex. - Alcohol treatment (decolorization step) dissolves the outer membrane of Gram-negative bacteria, allowing the crystal violet stain to be washed out. - Gram-positive bacteria have a thick peptidoglycan layer, which retains the crystal violet stain even after alcohol treatment, appearing purple under a microscope. 

Step 2: Evaluating the Options - Option (A) - Incorrect: If crystal violet is retained, it is not due to an incomplete experiment, but due to the bacterial cell wall structure. - Option (B) - Correct: Gram-positive bacteria retain the crystal violet stain after alcohol treatment due to their thick peptidoglycan layer. - Option (C) - Incorrect: Gram-negative bacteria do not retain crystal violet; they take up the safranin counterstain, appearing pink. - Option (D) - Incorrect: The correct classification is Gram-positive. 

Step 3: Conclusion Since Gram-positive bacteria retain the crystal violet stain after alcohol treatment, the correct answer is option (B).