Acid-Fast Bacteria: Definition & Examples

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  • 0:00 What Are Acid-Fast Bacteria?
  • 0:48 Cell Wall Structure
  • 1:42 Acid-Fast Bacteria Stain
  • 2:14 Examples
  • 5:56 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, you'll learn the basics of acid fast bacteria. Learn how these bacteria are identified and about the unique structure of their cell walls that classify them as such. By the end, you'll be able to explain some examples important to medicine.

What Are Acid Fast Bacteria?

If you look around you, the surfaces in your life are probably pretty clean. Even your skin looks clear of any dirt. However, looking under a microscope tells a different story. An unseen world exists that is composed of bacteria, or single-celled organisms that lack any membrane-bound compartments called organelles.

Although you might think bacteria are all the same, think again! There are tens of thousands of bacteria species, and some scientists even estimate millions. Today, we're going to look at one type of bacteria in detail, acid fast bacteria, which are bacteria with a cell wall rich in special acids, called mycolic acids.

Cell Wall Structure

To understand what acid fast bacteria are, we have to review a little about the cell wall, or the rigid outer coating made of proteins, lipids, and carbohydrates that protects the bacteria.

In acid fast bacteria, like all bacteria cell membranes, a thin layer of peptidoglycan covers the cell membrane. Peptidoglycan is a mesh-like layer composed of sugars and proteins. The peptidoglycan is attached to other sugars, including galactan and arabinan, which connect the peptidoglycan to the identifying characteristic of all acid fast bacteria: mycolic acids. Mycolic acids are long chains of fatty acids that create a strong barrier for the bacteria against the outside world.

The mycloic acid layer is enclosed by a capsule, or a slippery structure that is composed of sugars and lipids, which also helps make the cell wall impenetrable to outside threats.

Acid Fast Bacteria Stain

Due to the unique cell wall structure, scientists can identify these bacteria using a stain called an acid fast stain. During the stain, bacteria are applied to a microscope slide. The bacteria are covered with a solution called Carbol-fuchsin. The Carbol-fuchsin dye attaches to the mycolic acids, staining them pink. The excess solution is rinsed out using alcohol and if the bacteria contain mycolic acids, they will appear pink under the microscope.


There are fewer acid fast bacteria compared to other types, such as gram-positive or gram-negative bacteria. However, one of the most important examples is Mycobacterium, which causes chronic diseases like tuberculosis and leprosy.

Imagine being a doctor in the infectious disease clinic. A patient comes to you complaining of a chronic cough, which produces bloody sputum and a high fever, characteristic of Mycobacterium tuberculosis. The patient says she has been sick for weeks. A test for strep throat comes up negative, and after further inquiry, you find your patient has recently traveled to Asia, a region with high rates of tuberculosis infection. Blood tests reveal your patient has antibodies to tuberculosis, indicating that she was infected during her life.

To confirm she has an active tuberculosis infection, you order a culture of her sputum to test for the bacteria. However, Mycobacterium tuberculosis grows very slowly and the cultures can take eight weeks to get results. You know you'll be looking for a rod shaped, acid fast bacteria under the microscope when the results come in.

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