A sputum smear from a suspected TB patient is reported as 3+ AFB positive on Ziehl-Neelsen staining. In Ziehl-Neelsen staining, the primary stain used and the reason for acid-fastness of mycobacteria is:
- A Crystal violet; retained due to thick peptidoglycan layer
- B Methylene blue; retained due to lipopolysaccharide in outer membrane
- C Carbol fuchsin; retained due to high mycolic acid content in the cell wall ✓
- D Safranin; retained due to waxy coat preventing decolorization
Explanation
Ziehl-Neelsen staining uses carbol fuchsin as the primary stain, with heat application (hot method) to drive the stain into the bacteria. Mycobacteria are acid-fast because their cell walls contain large amounts of mycolic acids (long-chain branched fatty acids) combined with arabinogalactan and peptidoglycan. This waxy lipid-rich wall resists decolorization by 3% hydrochloric acid in alcohol (acid-alcohol decolorizer), and the bacteria retain the red carbol fuchsin dye against the blue methylene blue counterstain. Fluorescent staining with auramine-rhodamine has higher sensitivity for screening.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.