Acid-Fast Staining Technique

September 25, 2026

 Aim

To demonstrate acid-fast bacteria by the Ziehl–Neelsen (ZN) staining technique.

Introduction

Acid-fast bacteria are bacteria that resist decolorization with acid-alcohol after being stained with carbol fuchsin. The most important acid-fast organism is Mycobacterium tuberculosis, the causative agent of tuberculosis (TB).

These bacteria have a lipid-rich cell wall containing a large amount of mycolic acid. This waxy layer makes them resistant to many disinfectants and desiccation and makes them difficult to stain by ordinary staining methods such as Gram staining.

Cell wall structure of Acid-fast bacteria



Principle

Ziehl–Neelsen staining is a differential staining technique used mainly for the detection of Mycobacteria.

  • Carbol fuchsin is used as the primary stain.

  • Phenol helps the stain penetrate the waxy, lipid-rich cell wall.

  • Acid-fast bacteria retain carbol fuchsin even after treatment with acid-alcohol.

  • Non-acid-fast organisms are decolorized by acid-alcohol.

  • Methylene blue is used as the counterstain.

Therefore: Acid-fast bacteria → Red/Pink

                   Non-acid-fast bacteria → Blue


Materials Required

  • Clean glass slide

  • Bacterial smear

  • Ziehl's carbol fuchsin

  • Acid-alcohol

  • Methylene blue

  • Blotting paper

  • Water bath/heating setup

  • Compound microscope

  • Immersion oil

Procedure



  1. Prepare a bacterial smear on a clean glass slide and allow it to dry. Fix the smear appropriately.

  2. Place a strip of blotting paper over the smear.

  3. Saturate the blotting paper with Ziehl's carbol fuchsin.

  4. Place the slide over a water bath and gently heat it for approximately 3–5 minutes. Do not allow the stain to boil or dry.

  5. If the stain begins to dry, add more carbol fuchsin.

  6. Remove the blotting paper and gently rinse the slide with water.

  7. Flood the slide with acid-alcohol for approximately 10–15 seconds.

  8. Rinse the slide with water.

  9. Flood the slide with methylene blue for approximately 1 minute.

  10. Rinse with water and gently blot the slide dry.

Examine the stained smear under a compound microscope using the 100× oil-immersion objective.

Observation

  • Acid-fast bacteria appear as red or pink rods.

  • Non-acid-fast organisms and the background appear blue.

Result

Acid-fast bacteria are identified by their red/pink appearance against a blue background after Ziehl–Neelsen staining.

Precautions

  • Do not allow the carbol fuchsin to dry during heating.

  • Do not boil the stain.

  • Avoid excessive decolorization.

  • Use a clean glass slide and properly prepared smear.

  • Handle stains and biological specimens carefully.

  • Use immersion oil only with the 100× oil-immersion objective.

Conclusion

Ziehl–Neelsen staining is a differential staining technique used to identify acid-fast bacteria. The acid-fast organisms retain carbol fuchsin because of the high mycolic-acid content of their cell wall and therefore appear red/pink, while non-acid-fast organisms appear blue after counterstaining with methylene blue.