MYCOBACTERIUM AVIUM COMPLEX: CHARACTERISTICS AND MORPHOLOGY - BIOLOGY - 2025

Mycobacterium avium complex is a group of bacteria made up of three species: Mycobacterium intracellulare, Mycobacterium avium and Mycobacterium chimaera. The three species have similar properties and characteristics, so when talking about them, it is common to refer to Mycobacterium avium complex.

This type of bacteria is part of the group of atypical mycobacteria. They are widely distributed throughout the globe. They are also found in a multitude of environments, so it is possible that every human being is in contact with them.

Mycobacterium avium complex. Source: Wikipedia

The Mycobacterium avium complex bacteria became the most common non-tuberculous mycobacteria since the onset of AIDS. This is because they frequently affect various tissues such as gastrointestinal, lung or lymphatic, as well as disseminated infections in people with depressed immune systems.

That is why when there are cases of patients affected by this type of bacteria, one of the steps to follow is to do a serology for HIV. Likewise, when the patient is known to be HIV positive and experiences the typical symptoms of these bacteria, it is important to make a differential diagnosis in order to apply the appropriate treatment.

In any case, the knowledge of these bacteria is important because it allows to identify them and thus determine the conduct to follow in case of proven infection.

Taxonomy

The taxonomic classification of the Mycobacterium avium complex is as follows:

Domain: Bacteria

Phylum: Actinobacteria

Class: Actinobacteria

Subclass: Actinobacteridae

Order: Actinomycetales

Suborder: Corynebacterineae

Family: Mycobacteriaceae

Genus: Mycobacterium

Species: Mycobacterium avium, Mycobacterium chimaera and Mycobacterium intracellulare.

Morphology

Bacteria belonging to the Mycobacterium avium complex group are rod-shaped with rounded ends. They do not present any type of extensions on their cell surface such as cilia and flagella. They are generally found as individual bacteria. They do not form cords or chains.

They have a cell wall that is made up of three sheets, a lipophilic outer layer and a base layer of peptidoglycan. This is linked through covalent bonds to mycolic acid. This is important, because it is what contributes to the cell's surface being hydrophobic and waxy.

In the cultures, it is observed that the colonies can appear in two forms: smooth transparent or opaque dome-shaped.

Regarding its genetic material, it is made up of a single circular chromosome in which all the genes of the bacterium are contained. The DNA of the bacterium is 5,575,491 nucleotides in length, predominantly nucleotides made up of the nitrogenous bases guanine and cytosine.

Likewise, approximately 90% of DNA are genes that code for protein expression. In total, these bacteria synthesize a total of 5,120 proteins with the most diverse uses.

General characteristics

As already mentioned, Mycobacterium avium complex is a group of bacteria that have many of their characteristics in common.

They are aerobic organisms

These bacteria necessarily need to be in an environment with a wide availability of oxygen. They require this chemical element to carry out their metabolic processes.

They are mesophiles

These bacteria have an optimum growth temperature of 37 ° C.

They are slow growing

Mycobacterium avium complex bacteria are very slow growing. In solid media cultures, colonies take 10-21 days to develop. This is because they synthesize long chain fatty acids (60-80 carbon atoms) that make up their outer membrane.

Habitat

These bacteria are characterized by their ubiquity. This implies that they can be found anywhere in the world. These bacteria have been isolated in a multitude of environments such as dust, water, and in various animals.

They are pathogens

This type of bacteria is capable of generating pathologies in both animals and man. In the case of humans, it attacks both immunocompetent and immunosuppressed people.

It is especially an opportunistic organism in people who are infected with the acquired immunodeficiency virus (AIDS).

They are catalase positive

The bacteria of the Mycobacterium avium complex synthesize the enzyme catalase, which allows them to break down the hydrogen peroxide (H ₂ O ₂) molecule into water and oxygen. This is an important characteristic that allows the identification and differentiation of bacteria on an experimental level.

They are urease negative

These bacteria do not synthesize the enzyme urease, so they are not capable of hydrolyzing urea to obtain ammonia and carbon dioxide.

They are not photochromic

Photochromic bacteria are those whose cultures produce deep yellow carotenoid pigments when exposed to light.

They are acid - alcohol resistant

This property has to do with the ability of bacterial cells to resist discoloration when subjected to staining processes.

They are Ziehl - Nielsen positive

Due to their condition of acid-alcohol resistant bacteria, the most used stain to observe them is that of Ziehl-Nielsen. In this, the bacterial sample is subjected to a red dye and later methylene blue is added to make a contrast.

When observed under the microscope, the blue background can be seen and on it the red bacilli, corresponding to the Mycobacterium avium complex.

Antibiotic resistance

Mycobacterium avium complex bacteria are resistant to the following antibiotics: isoniazid, ethambutol, rifampin, and streptomycin.

Diseases

In humans, the Mycobacterium avium complex bacteria are pathogens that usually affect people with compromised immune systems (for example, those infected with HIV). In people who do not have any immune conditions, infection with this bacteria is extremely rare.

Among the pathologies related to these bacteria are:

• Lymphadenitis
• Lung disease
• Disseminated disease

Symptoms

Lymphadenitis

It is characterized by a painless increase in the size of the lymph nodes, specifically the cervical and submandibular. No other symptoms or signs are present, including fever.

Lung disease

It is a pathology similar to tuberculosis, but unlike it, it is not contagious from person to person. Its symptoms include:

• Persistent productive cough
• General discomfort
• Fatigue
• Fever

Disseminated disease

This pathology is the consequence of the spread of the bacteria throughout the body, through the bloodstream. In people who have a healthy immune system, it does not usually occur.

It is very common in those with a depressed immune system, especially those who are HIV positive. Among its most characteristic symptoms are:

• High fever
• Slimming
• Night sweats
• Fatigue
• Diarrhea
• Abdominal pain

Treatment

The infections generated by the bacteria Mycobacterium avium complex are treated with the use of antibiotics.

Generally, those most commonly used are: clarithromycin, azithromycin, rifampin, ethambutol, and rifabutin. Due to how serious this infection can become, a combination treatment is chosen. Multiple antibiotics are often given for a reasonable period of time.

Of course, the one indicated to prescribe the ideal treatment is the doctor, who proposes the treatment once the susceptibility and resistance of the bacterial strain found in the patient have been studied.

As in the treatment of any bacterial infection, the important thing is to follow the guidelines and instructions of the doctor 100%, since otherwise, it could be counterproductive for the patient.

References

1. Cespedes, M. and Aberg, J. Mycobacterum avium complex. Obtained from: antimicrobe.org.
2. Horsburgh, C. (1991). Mycobacterium avium complex infection in the Acquired Immunodeficiency Syndrome. The New England Journal of Medicine. 324. 1332-1338.
3. Mycobacterium avium complex. Retrieved from: Microbewiki.kenyon.edu
4. Saldías, F., Tirapegui, F. and Díaz, O. (2013). Lung infection by Mycobacterium avium complex in the immunocompetent host. Chilean Journal of Respiratory Diseases. 29. 162-167
5. Santos, M. and Gobernado, M. Mycobacterium complex: microbiological aspects. Retrieved from: seimc.org
6. Seville, I. (2007). Molecular characterization, detection and resistance of Mycobacterium avium subspecies paratuberculosis. Obtained from: Euskadi.es
7. Surco, V. and Gavincha, C. (2014). Mycobacteria. Journal of clinical update and research. 49.