When do we use 1% h2so4 used as decolouriser
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We use 20% H2SO4 for mycobacterium tuberculosis, but where do we use 1% H2SO4?
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Here is a clear, complete answer:
1% H2SO4 as Decolouriser - Modified ZN Stain (Weak Acid-Fast Stain)
The Core Concept
The concentration of H2SO4 used reflects the strength of acid-fastness of the organism:
| Organism Type | Decolouriser Used | Stain |
|---|---|---|
| Strongly acid-fast (M. tuberculosis, M. leprae) | 20% H2SO4 (or 3% HCl-alcohol) | Standard ZN stain |
| Weakly acid-fast (partially acid-fast organisms) | 1% H2SO4 | Modified ZN stain |
Where 1% H2SO4 is Used - Weakly Acid-Fast Organisms
These organisms cannot withstand the harsh 20% H2SO4, but will retain carbol fuchsin against the weaker 1% H2SO4:
Bacteria:
- Nocardia species
- Rhodococcus equi
- Tsukamurella
- Gordonia
Parasites/Protozoa:
- Cryptosporidium parvum (oocysts)
- Cyclospora cayetanensis (oocysts)
- Cystoisospora (Isospora) belli (oocysts)
- Sarcocystis species
Other:
- Spines of Schistosoma eggs
- Hooklets of Echinococcus
Why the Difference?
All these organisms have some mycolic acid or lipid-rich cell wall content, but less than M. tuberculosis. If you used 20% H2SO4:
- The strong acid would strip the carbol fuchsin out completely
- They would appear falsely negative
Using 1% H2SO4 is gentle enough to leave the stain in these partially acid-fast structures.
Key Mnemonic
"Weakly acid-fast = Weak acid (1%) to decolourise"
- Nocardia, Rhodococcus, Cryptosporidium, Cyclospora, Isospora = all use 1% H2SO4
- Medical Microbiology 9e, Acid-Fast Stains table
- Quick Compendium of Clinical Pathology 5th ed., Acid Fast Stain section
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