Ziehl-Neelsen (ZN) staining for acid-fast bacilli uses hot carbol-fuchsin as the primary stain. The acid-fastness of mycobacteria is due to:
- A High mycolic acid content in the cell wall that resists decolourisation by acid-alcohol ✓
- B Peptidoglycan layer cross-linking that traps the fuchsin dye intracellularly
- C Waxy lipoarabinomannan capsule that repels aqueous decolourising solutions
- D Efflux pumps that concentrate carbol-fuchsin within the bacterial cytoplasm
Explanation
Acid-fastness of mycobacteria results from the exceptionally high content of long-chain mycolic acids (C70–C90) in the cell wall, which are covalently attached to arabinogalactan esterified to peptidoglycan. These lipid-rich, hydrophobic structures bind carbol-fuchsin during hot staining and resist decolourisation by 3% hydrochloric acid-alcohol (acid-alcohol), unlike non-acid-fast organisms. After decolourisation, non-acid-fast organisms take up the counterstain (methylene blue or malachite green) and appear blue/green, while acid-fast bacilli retain the red fuchsin. Lipoarabinomannan contributes to virulence and immune evasion but is not primarily responsible for acid-fastness.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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