Question
Acid fast staining
Answer
Acid-fast staining demonstrates bacteria whose cell wall, rich in mycolic acid (a waxy lipid), resists decolorization by acid-alcohol after being stained with a strong dye — the defining property of the Mycobacterium genus (and, to a lesser extent, Nocardia species, which are “partially acid-fast”).
Ziehl-Neelsen (ZN) method (the standard technique):
- The smear is flooded with carbol fuchsin (a strong red dye combined with phenol, which aids penetration of the lipid-rich cell wall) and heated (steaming, not boiling) to further drive the dye through the waxy wall.
- The slide is decolorized with acid-alcohol (typically 20% sulphuric acid in ethanol) — acid-fast organisms retain the red dye despite this treatment, since their mycolic-acid-rich wall prevents the dye from being washed out, while non-acid-fast organisms and background material are decolorized.
- Counterstained with methylene blue — non-acid-fast material takes up this blue counterstain, providing background contrast.
Result: acid-fast bacilli (e.g., Mycobacterium tuberculosis, M. leprae) appear as pink/red rods against a blue background under oil-immersion microscopy.
Fluorescence method (auramine-rhodamine): an alternative, more sensitive screening method using fluorescent dyes, examined under a fluorescence microscope — acid-fast bacilli appear as bright yellow-orange rods against a dark background; faster to screen at lower magnification, making it preferred for high-throughput programmatic use, though ZN staining remains the more widely available, low-cost standard method, especially in resource-limited settings.
Clinical significance: the primary diagnostic screening tool for pulmonary tuberculosis (sputum smear microscopy) and for leprosy (skin/nasal smear).