Fite stain
 |
| Fite stain | |
|---|---|
| Synonyms | Fite-Faraco stain, Wade-Fite stain |
| Purpose | Detection of acid-fast bacteria, particularly Mycobacterium leprae |
The Fite stain is a histological staining technique primarily used to detect acid-fast bacteria, especially Mycobacterium leprae (the causative agent of leprosy). Named after its developer G. L. Fite (1938), this method modifies traditional acid-fast staining by incorporating an oil-based dewaxing step that preserves the delicate mycolic acid layers in weakly acid-fast organisms. The technique demonstrates superior sensitivity for M. leprae detection in paraffin-embedded tissues compared to conventional methods.[1]
History and Development
[edit]The technique originated from Fite's 1938 observation that standard xylene dewaxing compromised the acid-fast property of M. leprae. Key developments include:
- Fite-Faraco stain (1939): Brazilian pathologist V. Faraco formalized the oil-xylene dewaxing procedure[2]
- Wade-Fite stain (1957): H. Wade developed the "protective principle" by mixing peanut oil directly with xylene[3]
- Job-Chacko modification (1960s): Standardized decolorization with 1% sulfuric acid instead of acid-alcohol, improving detection of Nocardia species[4]
Principle
[edit]The stain exploits acid-fastness - the resistance of certain bacteria to acid-alcohol decolorization due to high mycolic acid content in cell walls. M. leprae has thinner mycolic acid layers than M. tuberculosis, making it "weakly acid-fast" and vulnerable to degradation during standard processing.[5]
The critical innovation involves substituting pure xylene with a 2:1 xylene-peanut oil mixture during dewaxing. Peanut oil's long-chain triglycerides form a protective coating around bacilli, preventing mycolic acid dissolution. This allows organisms to retain carbol fuchsin during acid decolorization, appearing bright red against a methylene blue counterstain.[6]
Staining Procedure
[edit]The standard protocol for formalin-fixed, paraffin-embedded tissue sections (4-5 µm thick) involves:[7]
| Step | Reagent | Time |
|---|---|---|
| 1. Dewaxing | Xylene-peanut oil (2:1) | 20 min (3 changes) |
| 2. Oil removal | Blotting + tap water rinse | Until uniformly wetted |
| 3. Primary stain | Carbol fuchsin with heat | 5-10 min |
| 4. Rinsing | Cold tap water | - |
| 5. Decolorization | 1% acid-alcohol* or 1% H₂SO₄ | 1-2 min |
| 6. Counterstain | 0.1% methylene blue | 10-15 sec |
| 7. Dehydration & mounting | Ethanol → xylene → resin | - |
*Job-Chacko modification uses 1% H₂SO₄
Results
[edit]- Acid-fast bacilli: Bright red
- Background tissue: Pale blue
- Non-acid-fast organisms: Blue
Applications
[edit]Primary Clinical Use
[edit]- Gold standard for detecting M. leprae in skin/nerve biopsies (sensitivity: 70-90% in multibacillary cases)[8]
- Bacillary Index (BI) quantification for WHO leprosy classification:[9]
| BI Score | Bacilli per Oil-Immersion Field |
|---|---|
| 0 | None |
| 1+ | 1-10 |
| 2+ | 10-100 |
| 3+ | 100-1000 |
| 4+ | >1000 |
| 5+ | >10,000 |
| 6+ | >100,000 |
Additional Diagnostic Uses
[edit]- Nocardia identification in immunocompromised patients[10]
- Cryptosporidium, Isospora, and Cyclospora oocyst detection
- Buruli ulcer diagnosis
Comparison with Other Stains
[edit]| Technique | M. leprae Sensitivity | Advantages | Limitations |
|---|---|---|---|
| Fite stain | 80-90% | Best for tissue sections | Labor-intensive |
| Ziehl-Neelsen | 20-40% | Rapid | Degrades weak acid-fastness |
| Kinyoun | 50-70% | No heating required | High background |
| Auramine-rhodamine | 90-95% | Screening efficiency | Requires fluorescence microscopy |
Variations
[edit]- Fite-Faraco: Standard oil-xylene dewaxing
- Wade-Fite: Explicit 2:1 xylene-peanut oil ratio
- Job-Chacko: 1% sulfuric acid decolorization (enhances Nocardia detection)[12]
- Mineral oil substitution: Alternative for peanut allergy settings[13]
Advantages and Limitations
[edit]Advantages:
- Superior sensitivity for M. leprae
- Adaptable to routine laboratories
- Mineral oil eliminates allergen concerns
- Job-Chacko improves Nocardia detection
Limitations:
- Reduced sensitivity in paucibacillary leprosy (≤50%)
- Requires meticulous technique
- Less effective for smears than tissue sections[14]
Safety Considerations
[edit]- Perform in fume hoods (xylene/phenol exposure)
- Wear nitrile gloves and eye protection
- Use mineral oil in allergy-prevalent settings
See also
[edit]- Ziehl–Neelsen stain
- Kinyoun stain
- Acid-fastness
- Leprosy
- Mycobacterium leprae
- Nocardia
- Histopathology
References
[edit]- ^ Fite, G.L. (1938). "A new method for demonstrating acid-fast bacilli in paraffin sections". American Journal of Pathology. 14 (4): 491–492. PMC 1965017. PMID 19970496.
- ^ Faraco, V. (1939). "Uma modificação do método de Fite para coloração do bacilo de Hansen em cortes de parafina". Memórias do Instituto Oswaldo Cruz. 34 (1): 119–120. doi:10.1590/S0074-02761939000100007 (inactive 19 June 2025).
{{cite journal}}: CS1 maint: DOI inactive as of June 2025 (link) - ^ Wade, H.W. (1957). "A note on the Fite stain". International Journal of Leprosy. 25 (4): 452–454. doi:10.1111/j.1471-0528.1958.tb06213.x. PMID 13514553.
- ^ Job, C.K.; Chacko, C.J.G. (1986). "A modification of Fite's stain for demonstration of M. leprae in tissue sections". Indian Journal of Leprosy. 58 (1): 17–19. PMID 2427624.
- ^ Brennan, P.J. (2003). "Structure, function, and biogenesis of the cell wall of Mycobacterium tuberculosis". Tuberculosis. 83 (1–3): 91–97. doi:10.1016/S1472-9792(02)00089-6. PMID 12758196.
- ^ Shetty, V.P.; Suchitra, K.; Uplekar, M.W.; Antia, N.H. (1997). "The efficacy of modified Fite's stain in the demonstration of M. leprae in tissue sections". Indian Journal of Leprosy. 69 (2): 179–186. doi:10.5694/j.1326-5377.1997.tb138802.x. PMID 9251705.
- ^ Bancroft, J.D.; Gamble, M. (2008). Theory and Practice of Histological Techniques (6th ed.). Churchill Livingstone. pp. 173–174. ISBN 978-0-443-10279-0.
- ^ Ridley, D.S.; Jopling, W.H. (1966). Classification of Leprosy According to Immunity. International Journal of Leprosy. pp. 255–273.
- ^ World Health Organization (2018). Guidelines for the Diagnosis, Treatment and Prevention of Leprosy (Report). WHO. ISBN 978-92-9022-638-3.
- ^ McTaggart, L.R.; Richardson, S.E. (2011). "Modified Fite staining for detection of Nocardia species". Journal of Clinical Microbiology. 49 (12): 4297–4299. doi:10.1128/JCM.05213-11. PMC 3232983. PMID 21998419.
- ^ Shetty, V.P.; Antia, N.H. (1986). "A comparative evaluation of skin and nerve histology in leprosy". International Journal of Leprosy. 54 (1): 11–16. PMID 3700925.
- ^ Job, C.K.; Chacko, C.J.G. (1986). "A modification of Fite's stain for demonstration of M. leprae in tissue sections". Indian Journal of Leprosy. 58 (1): 17–19. PMID 2427624.
- ^ Hansen, G.H.; Francis, D. (1995). "Mineral oil as substitute for peanut oil in Fite's stain". Journal of Histotechnology. 18 (2): 143–145. doi:10.1179/his.1995.18.2.143 (inactive 19 June 2025).
{{cite journal}}: CS1 maint: DOI inactive as of June 2025 (link) - ^ Kumar, B.; Yadav, S. (2015). "Comparison of bacillary index on slit skin smear with biopsy in leprosy". Indian Journal of Dermatology. 60 (2): 211. doi:10.4103/0019-5154.152564 (inactive 19 June 2025). PMC 4372934. PMID 25814737.
{{cite journal}}: CS1 maint: DOI inactive as of June 2025 (link)