Microbiology · Ziehl-Neelsen stained smear, ×1000

Acid-Fast Bacilli (Ziehl-Neelsen Stain)

Slender pink-red rods on a blue background — acid-fast bacilli of Mycobacterium tuberculosis on a Ziehl-Neelsen stain.

Micrograph with thin red rod-shaped bacteria scattered on a pale blue background
Mycobacterium tuberculosis, Ziehl-Neelsen stain, ×1000 (CDC PHIL #5789).Image: CDC / Dr. George P. Kubica, via Wikimedia Commons — Public domain.

Written & reviewed by: Kinase Medical Team · Last reviewed: 30 September 2026 · Editorial policy

Key features

  • Mycobacteria retain carbol fuchsin after acid-alcohol decolorisation → fine red rods against the blue methylene-blue counterstain, singly, in pairs or in groups.
  • Acid-fastness is due to the high mycolic acid content of the cell wall.
  • M. tuberculosis is a slender, slightly curved rod; it is often neutral or only weakly positive on Gram stain.
  • Few bacilli may be present, so reading needs a trained eye; auramine-rhodamine fluorescence is an alternative stain.
  • Culture on solid media takes about 3–6 weeks; PCR-based tests allow rapid diagnosis.

Differential diagnosis

Differential diagnosis of Acid-Fast Bacilli (Ziehl-Neelsen Stain)
ConditionHow to tell it apart
NocardiaOnly weakly (partially) acid-fast — shown with a modified acid-fast stain that separates it from Actinomyces.
Cryptosporidium / Isospora (Cystoisospora)Acid-fast protozoa (seen in stool), not bacilli.

How it appears in NEET PG

Stain images are a common microbiology format; ZN questions ask for the organism, the cell-wall component responsible or the decolouriser.

Frequently asked questions

Why are mycobacteria acid-fast?

Their cell wall is rich in mycolic acids and other lipids, which hold the carbol fuchsin dye even after decolorisation with acid-alcohol.

What colour are acid-fast bacilli on a ZN stain?

Red (pink-red) fine rods, sometimes slightly curved or granular, against the blue methylene-blue counterstain.

Which other organisms are acid-fast?

Nocardia (weakly), Rhodococcus, Legionella micdadei, and the protozoa Cryptosporidium and Isospora.

Sources

  1. CDC / WHO / IUATLD — Acid-Fast Direct Smear Microscopy: A Laboratory Training Program
  2. Harrison's Principles of Internal Medicine (textbook)
  3. Nelson Essentials of Pediatrics (textbook)

Keep practising

For exam preparation and medical education only — not a substitute for clinical judgement, diagnosis or treatment of any patient.