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Fite stain

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Fite stain
SynonymsFite-Faraco stain, Wade-Fite stain
PurposeDetection 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

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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

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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

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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

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  • Acid-fast bacilli: Bright red
  • Background tissue: Pale blue
  • Non-acid-fast organisms: Blue

Applications

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Primary Clinical Use

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  • 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

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Comparison with Other Stains

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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

[11]

Variations

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  • 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

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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

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  • Perform in fume hoods (xylene/phenol exposure)
  • Wear nitrile gloves and eye protection
  • Use mineral oil in allergy-prevalent settings

See also

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References

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  1. ^ 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.
  2. ^ 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)
  3. ^ 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.
  4. ^ 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.
  5. ^ 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.
  6. ^ 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.
  7. ^ 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.
  8. ^ Ridley, D.S.; Jopling, W.H. (1966). Classification of Leprosy According to Immunity. International Journal of Leprosy. pp. 255–273.
  9. ^ World Health Organization (2018). Guidelines for the Diagnosis, Treatment and Prevention of Leprosy (Report). WHO. ISBN 978-92-9022-638-3.
  10. ^ 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.
  11. ^ 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.
  12. ^ 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.
  13. ^ 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)
  14. ^ 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)
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