Gentian violet colouration of gram-positive bacteria is due to -

GRAM STAINING The gram stain differentiates bacteria into two broad groups 1. Gram-positive → Resist decolourization and retain primary stain, appear violet. 2. Gram-negative → Decolourised by organic solvents, and therefore, take the counterstain, appearing red. Smear stained with gentian violet (primary stain) ↓ Application of dilute solution of iodine ↓ Decolourisation with an organic solvent, e.g., ethanol ↓ Counterstaining by carbol fuchsin ↓ ↓ ↓ Retain primary stain (violet) ↓ Gram-positive Take colour of counterstain (red) ↓ Gram-negative  Why do gram-positive organisms retain primary stain, but not gram-negative organisms.  In gram-positive bacteria, there is a thick layer of peptidoglycan (cell wall) just outside the bacterial cytoplasmic membrane.  This thick wall traps the primary stain (crystal violet) which is not decolorized by ethanol.  In gram-negative bacteria this peptidoglycan (cell wall) layer is thin and just outside this peptidoglycan layer is outer cell membrane (which is not present in gram-positive bacteria).  This outer cell membrane is partially dissolved by ethanol, thus washing out the crystal violet and allowing the counterstain to take