Acid fast Bacteria – Its Composition and Structure:

History of acid fast bacteria

        Paul Ehrlich was the first to explain the bacterium’s ability to withstand acid quickly. By including phenol (carbolic acid) and basic fuschin, Friedrich Nesslsen and Franz Ziehl improved the stain in the 1890s. Because they withstand acid washing, they are known as “acid-fast bacteria.”

Acid fast bacteria: What are they?

      The bacteria that retain the red color of carbon fuchsin after the decolorizing step are called acid fast.  A group of bacteria with the property of acid fastness is known as acid fast bacteria, often known as acid fast bacilli or simply AFB. The technique of staining is called acid fast staining and the stain used in it are acid fast stains.

      A bacterium’s capacity to withstand acid decolorization during the staining process is known as acid fastness. There are various types of staining techniques that utilize different dyes or stains.

    As a result, acid fast bacteria are those that keep the red color of the dye after being stained with carbol fuchsin and do not immediately become decolorized by acid-alcohol. Additionally, the cell walls of these bacteria contain significant amounts of lipids that have a potent affinity for the dye carbol fuchsin.

What are examples of acid fast bacteria?

   Some examples of acid-fast bacteria include;

  • Mycobacterium tuberculosis; This bacterium is the causal agent of tuberculosis in humans, with the lungs as the infection site.
  • Mycobacterium leprae; This bacterium is the causative agent of leprosy.
  • Mycobacterium Bovis; This bacterium is the causal agent of tuberculosis in cattle.
  • Mycobacterium avium; This bacterium is the causal agent in birds.
  • Nocardia species; They are a causal agent of nocardiosis and some cutaneous infections.

What are the structure and composition of the acid fast bacterial cell wall?

Gram positive bacteria that are acid-fast also contain large amounts of glycolipids, particularly mycolic acids, which make up about 60% of the acid-fast cell wall in the genus Mycobacterium. These glycolipids are found in the outer membrane or envelope of the acid-fast cell wall in addition to peptidoglycan.

Layer 1:

         A thin, inner layer of peptidoglycan exists in the Mycobacterium acid-fast cell wall.

Layer 2:

        Arabinogalactan is connected to the peptidoglycan layer through this layer (D-arabinose and D-galactose).

Layer 3:

       The arabinogalactan is subsequently joined to an outer membrane made of mycolic acids with a high molecular weight. A layer of polypeptides and mycolic acids made up of free lipids, glycolipids, and peptidoglycolipids is deposited on top of the arabinogalactan/mycolic acid layer. A few other glycolipids are phosphatidylinositol mannosidase and lipoarabinomannan (PIM). Porins are necessary to move small hydrophilic molecules across the outer membrane of the acid-fast cell wall, much like they are in the gram-negative cell wall’s outer membrane.

Layer 4:

          Surface proteins that vary depending on the strain and species of the bacterium are embedded in the acid-fast cell wall’s outer surface.

Layer 5:

         Between the peptidoglycan and the cytoplasmic membrane is the gelatinous substance known as the periplasm.

What are the functions of Acid fast bacterial cell walls?

 Layer 1:

          In the first layer, the peptidoglycan stops osmotic lysis.

Layer 2:

          The arabinogalactan layer, which is connected to the peptidoglycan and the mycolic acid outer membrane, is thought to give the cell wall additional tensile strength.

Layer 3:

            In addition to preventing the entry of chemicals, mycolic acids and other glycolipids also give the organisms a slower growth rate and greater resistance to chemicals and lysosomal phagocyte components than other bacteria. The pores are substantially longer and there are a lot fewer porins in the acid-fast cell wall than there are in the gram-negative cell wall. This is expected to play a substantial role in acid-fast bacteria having less permeability. 

Layer 4:

          Depending on the strain and species, the surface proteins in the acid-fast cell wall perform several functions, such as acting as enzymes and adhesins, which allow the bacteria to colonize and resist flushing by sticking closely to host cells and other surfaces.

Layer 5:

          Enzymes for breaking down nutrients can be found in the periplasm.

What is the summary of the acid fast bacterial cell wall?

  • Bacteria that are acid-fast stain red following acid-fast staining because of the structure of their cell walls.
  • Just a few bacteria have an acid-fast cell wall, including those in the genera Mycobacterium and Nocardia.
  • An exterior membrane made of mycolic acids and covered with a variety of polypeptides and glycolipids is connected to the acid-fast cell wall by a thin, inner layer of peptidoglycan that is linked to a layer of arabinogalactan.
  • The outer membrane of the acid-fast cell wall must be crossed by porins to carry tiny hydrophilic molecules.
  • Innate and adaptive immune defenses are both triggered by the acid-fast cell wall in the body.
  • Pathogen-associated molecular patterns, or PAMPs, which are specific to microbes and are not linked to human cells, are recognized by the body to activate innate immunity. PAMPs cause the production of inflammatory cytokines by binding to Pattern-recognition receptors (PRRs) on defense cells.
  • However, excessive inflammation can be dangerous and even fatal to the body. Inflammation is the body’s method of delivering defense cells and defense chemicals to an infection site.
  • PAMPs linked to the acid-fast cell wall include arabinogalactan, mycolic acids, and peptidoglycan monomers.
  • The development of antibody molecules against bacterial cell wall antigens is one example of how cell wall molecules might activate adaptive immunity.
  • A few antimicrobial chemotherapeutic substances prevent the production of acid-fast cell walls.
  • An adaptive immune response is started when an antigen interacts with antigen receptors on cells.


Rimsha Bashir
Rimsha Bashir

Rimsha Saith is a highly knowledgeable microbiologist with a keen interest in the field. Her expertise and passion are in her writing for Microbiology. As a writer, Rimsha has authored numerous articles that have been well-received by both health and medical students and industries.

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