Ziehl-Neelsen (ZN) stain uses carbol fuchsin with heat as the primary stain for acid-fast organisms. Which component of the mycobacterial cell wall confers acid fastness and retains the red dye against decolourisation with acid-alcohol?
- A Mycolic acids (long-chain hydroxylated fatty acids) in the cell wall ✓
- B Peptidoglycan layer with D-amino acid cross-links
- C Arabinogalactan polymer forming a hydrophobic barrier
- D Outer lipopolysaccharide (LPS) layer
Explanation
Acid fastness is conferred by the unique mycobacterial cell wall architecture: mycolic acids are very long chain (C60–C90), α-branched, β-hydroxylated fatty acids that are covalently linked to arabinogalactan and form a highly hydrophobic, waxy outer layer. Once carbol fuchsin penetrates this wax layer with heat (hot ZN stain), the strong hydrophobic interaction prevents penetration of the decolouriser (3% HCl in 95% ethanol) and the pink colour is retained. While arabinogalactan is a scaffold, the mycolic acids themselves are the key acid-fastness determinants. Peptidoglycan is present in all bacteria. LPS is a Gram-negative outer membrane component absent in mycobacteria.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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