Difference Between Acid Fast and Non Acid Fast Bacteria

Acid Fast vs Non Acid Fast Bacteria

The difference between acid fast and non acid fast bacteria is in their cell wall, basically. Bacteria are generally identified and observed with differential staining procedures. Acid fast staining is one of such methods to distinguish a certain type of bacteria from others. This method was first discovered by Franz Ziehl and Friedrich Neelsen. At that time, Mycobacterium that cause tuberculosis, was unable to be stained and observed using other staining procedures such as gram stain. Neelsen and Ziehl stained this bacteria by adding phenol (carbolic acid) and basic fuchsin(e) along with acid alcohol, so the dye is known as Carbol Fuchsin(e) solution or Ziehl – Neelsen stain.

Acid Fast Staining Procedure

In order to complete the understanding of the acid fast and non acid fast bacteria, first we will go through the staining procedure. Acid fastness is a property of a bacterium that is resistant to decolorization by acids or acid alcohols during the staining procedure. This is initially described by Paul Ehrlich. Following three steps are carried out during the staining procedure.

1. Application of primary dye – Carbolfuchsin is the primary die that is flooded over a smear of bacteria that is heat fixed on a clean slide. Heat is applied to ensure the penetration of dye up to the cytoplasm.

2. Decolorization – acid-alcohol treatment to remove primary dye.

3. Counterstaining – Methylene blue is applied to view the colorless bacteria.

What are Acid Fast Bacteria?

Bacteria that have acid fastness are known as acid fast bacteria. In other words, bacteria that are still stained with red color after the decolorization step during the acid fast staining procedure are known as acid fast bacteria. What makes these bacteria to have acid fastness? Well, if we consider a cross section of the cell wall of an acid fast bacterium, it can be easily understand.

Acid fast stain (or carbolfuchsin) binds only to bacteria that have a waxy cell wall. This cell wall contains a hydrophobic waxy lipid known as mycolic acid, which occupies the 60% of the cell wall. Due to the hydrophobic property, water soluble materials are prevented from entering the cytoplasm. That is why this bacteria is unable to be stained by water soluble dyes such as methylene blue. Carbolfuchsin is composed of phenol and fuchsin so that it can be penetrated up to the cytoplasm.

During the acid alcohol decolorization step, the acid alcohol is prevented entering the cytoplasm due to the presence of hydrophobic mycolic acid, thus it is unable to remove carbolfuchsin from the bacteria cell. So the primary dye will remain in the cytoplasm even after the decolorization step.

Acid fast bacteria includes several genera such as Mycobacterium and Nocardia, which are pathogenic to human, causing tuberculosis and nocardiosis, respectively.

Difference Between Acid Fast and Non Acid Fast Bacteria

Acid Fast Bacteria is in red

What are Non Acid Fast Bacteria?

If a bacterium lacks the acid fastness it is called nonacid fast bacteria. After following the acid fast staining procedure, these bacteria will stain with blue. This is because nonacid fast bacteria have a thin cell wall and lack mycolic acid in the cell wall. This allows the penetration of carbolfuchsin into the cytoplasm. However, it is removed with acid alcohol treatment, making nonacid fast bacterial cells colorless. In order to clearly observe and differentiate from acid fast bacteria, methylene blue will be useful here.

Nonacid fast bacteria can be stained using gram stain or any other simple staining procedure. Examples of nonacid fast bacteria are Escherichia coli, Pseudomonas sp.

Acid Fast vs Non Acid Fast Bacteria

Non acid fast bacteria is in blue

What is the difference between Acid Fast and Non Acid Fast Bacteria?

• Acid Fastness:

• Acid fast bacteria show acid fastness.

• Non acid fast bacteria lack acid fastness.

• Cell Wall:

• Acid fast bacteria contain a thick cell wall with mycolic acid layer.

• Non acid fast bacteria lack this layer.

• Gram Stain:

• Acid fast bacteria are harder to be stained using gram stain.

• Non acid fast bacteria can be stained using gram stain.

• Pathogenic or Nonpathogenic:

• Most of acid fast bacteria are pathogenic.

• Non acid fast bacteria can be pathogenic or nonpathogenic.

• Bacilli or Cocci:

• Acid fast bacteria are mostly bacilli.

• Non acid fast bacteria can be bacilli or cocci.


Images Courtesy:

  1. Mycobacterium tuberculosis (stained red) in tissue (blue) via Wikicommons (Public Domain)
  2. Gastric signet ring cell carcinoma histopathology, PAS stain by KGH(CC BY-SA 3)