Ziehl–Neelsen stain

Mycobacterium tuberculosis visualization using the Ziehl–Neelsen stain.

The Ziehl–Neelsen stain, also known as the acid-fast stain, was first described by two German doctors: the bacteriologist Franz Ziehl (1859–1926) and the pathologist Friedrich Neelsen (1854–1898). It is a special bacteriological stain used to identify acid-fast organisms, mainly Mycobacteria. Mycobacterium tuberculosis is the most important of this group because it is responsible for tuberculosis (TB). Other important Mycobacterium species involved in human disease are Mycobacterium leprae, Mycobacterium kansasii, Mycobacterium marinum, and members of the Mycobacterium avium complex. Acid fast organisms like Mycobacterium contain large amounts of lipid substances within their cell walls called mycolic acids. These acids resist staining by ordinary methods such as a Gram stain.^[1] It can also be used to stain a few other bacteria, such as Nocardia. The reagents used are Ziehl–Neelsen carbol fuchsin, acid alcohol, and methylene blue. Acid-fast bacilli will be bright red after staining.

A variation on this staining method is used in mycology to differentially stain acid-fast incrustations in the cuticular hyphae of certain species of fungi in the genus Russula.^[2][3] It is also useful in the identification of some protozoa, namely Cryptosporidium and Isospora. The Ziehl–Neelsen stain can also hinder diagnosis in the case of paragonimiasis because the eggs in an ovum and parasite sputum sample (OnP) can be dissolved by the stain, and is often used in this clinical setting because signs and symptoms of paragonimiasis closely resemble those of TB.

Procedure

1. Drop suspension onto slide
2. Air dry slide 10 minutes at 60 °C, heat-fix slide 10 minutes at 90 °C
3. Flood slide with Carbol Fuchsin
4. Hold a flame beneath the slide until steam appears but do not allow it to boil
5. Allow hot slide to sit for 3 to 5 minutes, rinse with tap water
6. Flood slide with 3% hydrochloric acid in isopropyl alcohol
7. Allow to sit 1 minute, rinse with tap water
8. Flood slide with Methylene Blue
9. Allow to sit 1 minute, rinse with tap water
10. Blot dry
11. View under oil immersion lens

Studies have shown that an AFB stain without a culture has a poor negative predictive value. An AFB Culture should be performed along with an AFB stain; this has a much higher negative predictive value.

Mechanism explanation

• Initially, Carbol Fuchsin stains every cell
• When they are destained with acid-alcohol, only non-acid-fast bacteria get destained since they don't have a thick, waxy lipid layer like acid-fast bacteria.
• When counter stain is applied, non-acid-fast bacteria pick it up and become blue when viewed under the microscope. Acid-fast bacteria retains Carbol Fuchsin so they appear red.

Modifications

• 5% sulfuric acid is used for destaining Mycobacterium leprae instead of the 20% used for Mycobacterium tuberculosis.
• 1% sulfuric acid for actinomycetes, nocardia.
• 0.5-1% sulfuric acid for oocysts ofisospora, cyclospora.
• 0.25-0.5% sulfuric acid for bacterial endospores.
• Brucella differential stain - glacial acetic acid used, no heat applied, secondary stain is loeffler's methylene blue.
• Kinyoun modification (or cold Ziehl–Neelsen technique) is also available.
• A protocol in which a detergent is substituted for the highly toxic phenol in the fuchsin staining solution.^[4]

See also

• Kinyoun stain
• Lowenstein-Jensen medium

References

• "Microbiology with Diseases by Body System", Robert W. Bauman, 2009, Pearson Education, Inc.
• Morello, Josephine A., Paul A. Granato, Marion E. Wilson, and Verna Morton. Laboratory Manual and Workbook in Microbiology: Applications to Patient Car. 10th ed. Boston: McGraw-Hill Higher Education, 2006. Print.

Online protocol examples

• Ziehl–Neelsen protocol (PDF format).

References

External links

• Media related to Ziehl-Neelsen stain at Wikimedia Commons