This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
[{'content': 'Gentian violet colouration of gram-positive bacteria is due to - \nSelect and explain. \nA. Peptidoglycan, B. Capsule, C. Cell membrane, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: GRAM STAINING \nThe gram stain differentiates bacteria into two broad groups\n1. Gram-positive → Resist decolourization and retain primary stain, appear violet.\n2. Gram-negative → Decolourised by organic solvents, and therefore, take the counterstain, appearing red. Smear stained with gentian violet (primary stain)\n↓ Application of dilute solution of iodine\n↓ Decolourisation with an organic solvent, e.g., ethanol\n↓ Counterstaining by carbol fuchsin\n↓ ↓ ↓ Retain primary stain (violet)\n↓ Gram-positive Take colour of counterstain (red)\n↓ Gram-negative\xa0\nWhy do gram-positive organisms retain primary stain, but not gram-negative organisms.\n\n\xa0In gram-positive bacteria, there is a thick layer of peptidoglycan (cell wall) just outside the bacterial cytoplasmic membrane.\n\xa0This thick wall traps the primary stain (crystal violet) which is not decolorized by ethanol.\n\xa0In 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).\n\xa0This outer cell membrane is partially dissolved by ethanol, thus washing out the crystal violet and allowing the counterstain to take', 'role': 'assistant'}]