Acid Fast Stain- Principle, Reagents, Procedure and Result Interpretation

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It is also know as Ziehl-Neelsen stain.

Objective of Acid Fast Stain

The main aim objective of this stain is to differentiate bacteria into acid fast group and non-acid fast groups.

Principle of Acid Fast Stain

Acid-fast mycobacteria contain mycolic acid in their outer membrane, making the cells waxy and resistant to staining with aqueous based stains such as the Gram stain. The primary stain, carbolfuchsin is applied to the cells, and heat and phenol are used to allow the stain to penetrate into the waxy surface of acid-fast microorganisms. The excess stain is removed with treatment by acid alcohol (ethanol and hydrochloric acid). A secondary stain, methylene blue, is then applied to the cells.

Ziehl-Neelsen Stain Reagents

Primary Stain: 0.3% Carbol-fuchsin. Dissolve 50 g phenol in 100 mL ethanol (95%) or methanol (95%). Dissolve 3 g Basic fuchsin in the mixture and add distilled water to bring the volume to 1 L.
Decolorization Solution: Add 30 mL hydrochloric acid to 1 L of 95% denatured alcohol. Cool and mix well before use. Alternate decolorizing reagent (without alcohol): Slowly add 250 mL sulfuric acid (at least 95%) to 750 mL distilled water. Cool and mix well before using.
Counterstain: 0.3% nethylene blue. Dissolve 3 g nethylene blue in 1 L distilled water.

Procedure of Acid Fast Stain

  1. Prepare and fix the specimen smear prior to staining.
  2. Place a small strip of blotting or filter paper over the top of the specimen, and place the slide over a boiling hot water bath on a mesh surface.
  3. Cover the filter paper with the primary stain, carbolfuchsin. Leave the slide on the water bath for 3 to 5 minutes. Continue to apply stain if the filter paper begins to dry.
  4. Remove the filter paper and rinse the slide with water until the solution runs clear.
  5. Run acid-alcohol decolorizer over the slide for approximately 10 to 15 seconds.
  6. Rinse the slide with water.
  7. Cover the smear with the secondary or counterstain, methylene blue, for 1 minute.
  8. Gently rinse the slide with water.
  9. Blot the slide dry with bibulous paper.

Result Interpretation of Acid Fast Stain

Acid fast: Bright red to intensive purple, Red, straight or slightly curved rods, occurring singly or in small groups, may appear beaded
Non-acid fast: Blue color; In addition, background material should stain blue.

Limitations

  1. The filter paper must remain moist and in contact with the specimen during heating to allow for proper penetration of the primary stain.
  2. Organisms cultivated on blood agar may experience nutrient deprivation, resulting in a lower lipid content in the outer membrane resulting in poor staining.

Examples

Acid-fast: Mycobacterium tuberculosis, Mycobacterium smegmatis.
Non-Mycobacterial bacteria: Nocardia
Coccidian Parasites: Cryptosporidium

About Author

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Sagar Aryal, PhD

Dr. Sagar Aryal is a microbiologist, researcher, and science communicator from Nepal. He is also co-founder and Research Head of the Department of Natural Products at the Kathmandu Research Institute for Biological Sciences (KRIBS), where he leads research on natural product discovery and microbial biodiversity. Dr. Aryal earned his Ph.D. in Microbiology from Tribhuvan University, Kathmandu, Nepal, in 2025, supported by a DAAD Research Grant and a University Grants Commission (UGC) Nepal Ph.D. Fellowship. His research interests include Actinomycetes, Myxobacteria, microbial taxonomy, and natural products, particularly the discovery of bioactive compounds with potential pharmaceutical applications. He isolated and identified Nocardia nepalensis, a novel bacterial species from Nepal, contributing to understanding microbial diversity in the region. Dr. Aryal has published more than 16 research articles in international journals and contributed three book chapters to established academic publications. He has presented scientific research findings and developed international research collaborations through visits to more than 18 countries across Asia, Europe, North America, and Oceania. From 2023 to 2025, he served as the American Society for Microbiology (ASM) Young Ambassador to Nepal. Since December 2025, he has served as an Editorial Board Member of Scientific Reports, a Nature Portfolio journal. In recognition of his scientific contributions, Dr. Aryal received the NAST Science and Technology Youth Award 2025 from the Nepal Academy of Science and Technology. He previously received the Frs. Locke and Stiller Research Award (LSRA) for the 2017-2018 cycle from St. Xavier's College, Kathmandu, and a Nepal Academy of Science and Technology (NAST) Research Grant for 2017-2018. Alongside his scientific career, Dr. Aryal has more than 15 years of experience in blogging, content writing, and search engine optimization (SEO).