In the Gram stain procedure, which chemical step is responsible for the differential staining of gram-positive versus gram-negative bacteria, and what structural feature of gram-positive bacteria accounts for crystal violet retention?

• A Decolourisation with acetone-alcohol — extracts lipid from gram-negative outer membrane dissolving CV-iodine complex; thick peptidoglycan in gram-positives retains the complex when dehydrated ✓
• B Iodine (mordant) step — forms CV-iodine complex retained by gram-positive outer membrane
• C Safranin counter-stain step — preferentially binds to gram-positive cell wall
• D Crystal violet primary stain — forms covalent bonds only with gram-positive teichoic acids

Explanation

Gram staining mechanism: (1) Crystal violet (primary stain) penetrates all bacteria; (2) Iodine (mordant) forms an insoluble crystal violet-iodine (CVI) complex inside all cells; (3) Acetone-alcohol (decolouriser) — this is the critical differential step: gram-negative bacteria have a thin peptidoglycan layer and an outer lipopolysaccharide (LPS) membrane; alcohol dissolves the outer membrane lipids, creating channels through which CVI complex leaches out, resulting in decolouration. Gram-positive bacteria have a thick multilayered peptidoglycan (20–80 nm) with teichoic acids; alcohol dehydrates and shrinks the peptidoglycan pores, trapping the CVI complex, so they retain violet colour. Safranin counter-stains only the decolourised gram-negative bacteria pink.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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