**Question:** Gentian violet colouration of gram-positive bacteria is due to -

**Core Concept:** Gram staining is a technique used in microbiology to differentiate between Gram-negative and Gram-positive bacteria. Gentian violet is a vital component of the Gram stain process, which binds to specific components of the bacterial cell wall, resulting in coloration.

**Why the Correct Answer is Right:** Gentian violet is a fat-soluble dye that binds to the peptidoglycan layer of the Gram-positive bacteria cell wall. The binding of gentian violet to Gram-positive bacteria results in a purple coloration due to the presence of the diphosphodiesterase enzyme, which cleaves the dye molecule, allowing the dye to be retained within the cell wall.

**Why Each Wrong Option is Incorrect:**

A. Gram-negative bacteria do not have a peptidoglycan layer in their cell wall, which is why they do not stain purple with gentian violet.

B. Diphosphodiesterase enzyme activity is not responsible for the colour change in Gram-positive bacteria, as it is involved in cleaving the dye molecule in Gram-negative bacteria.

C. The binding of gentian violet to Gram-positive bacteria does not require the presence of diphosphodiesterase enzyme, unlike in Gram-negative bacteria.

D. Gentian violet coloration is not specific to Gram-positive bacteria and does not distinguish them from Gram-negative bacteria, which is incorrect.

**Gentian Violet Colouration of Gram-positive Bacteria:**

Gentian violet, in combination with other reagents, makes up the Gram stain technique. This technique is used to differentiate between Gram-negative and Gram-positive bacteria based on the structure of their cell walls. In the case of Gram-positive bacteria, the presence of diphosphodiesterase enzyme allows the binding of gentian violet to the peptidoglycan layer, resulting in the characteristic purple coloration. This process is crucial for distinguishing Gram-positive bacteria from Gram-negative bacteria and is essential in microbiology for clinical diagnosis and treatment decisions.