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.One of the reasons Mycobacterium tuberculosis grows so slowly is the thick layer of mycolic acids. Mycobacterium tuberculosis has the longest chains of mycolic acids of any acid fast bacteria.
This prevents nutrients from easily entering the cell, thus slowing growth. Since the bacteria has such a thick cell wall, it’s very hard to treat tuberculosis. The antibiotic regimen you prescribe your patient will consist of multiple antibiotics that target different parts of cell division and will last at least two to four months, possibly continuing for another nine months. Armed with your knowledge of Mycobacterium tuberculosis you get ready to break the news to your patient.Okay, let’s now look at a different scenario. Imagine an island isolated from all human contact.
Only people with the most grotesque and feared diseases are sent here. This is a leprosy colony. Although none exist today, individuals suffering from infection of the acid fast bacteria Mycobacterium leprae were sent away from civilization to prevent infection hundreds of years ago. Transmission occurs through droplets released from the nose and mouth during close contact, thus the desire to separate lepers from the general population.
Mycobacterium leprae, a rod shaped bacteria, infects the skin, peripheral nerves, respiratory tract, and the eyes. It is also gram-positive, but the stain is weak so it is much better to use acid fast staining. It prefers colder temperatures, and thus sticks to infecting the extremities of a person and the upper respiratory tract.
Unlike other acid fast bacteria, Mycobacterium leprae can take years to culture in the laboratory and won’t grow on normal media plates at all. Scientists must use animals with low body temperatures, such as armadillos, to study the bacteria at all. Despite these challenges, scientists have improved leprosy treatment tremendously. Today, leprosy is treated with multi-drug therapy consisting of a combination of antibiotics. However, Mycobacterium leprae divides very slowly and can take five to twenty years to even manifest symptoms in patients. Thus, patients need to be on antibiotic courses for years, which can make it difficult to comply with treatment, especially in countries with poor healthcare. Although the disease proceeds slowly, the effects can be startling.
Patients suffer from a lack of sensation in their extremities, and eventually disfigurement in the infected areas.
Let’s review. Acid fast bacteria stain pink using the dye Carbol-fuchsin due to a high concentration of mycolic acids in their cell wall. The mycolic acids form a thick, waxy layer that makes them very resistant to environmental stressors, like antibiotics. However, it also causes a very slow rate of cell division, which makes antibiotic treatments long for infectious acid fast bacteria.
Two examples of acid fast bacteria are Mycobacterium tuberculosis and Mycobacterium leprae. Mycobacterium tuberculosis causes the respiratory disease tuberculosis and Mycobacterium leprae causes the disfiguring disease leprosy. Both are treatable with long-term multi-drug therapies.