Mycobacterium characteristics, morphology and pathogenesis

1983
Jonah Lester

Mycobacterium is a genus of bacteria characterized, among other things, by being stick-shaped and not staining adequately using the Gram stain method. It constitutes a large group of bacteria, quite heterogeneous that in many cases are pathogenic agents for humans.

Mycobacteria have certain characteristics that make them an object of interest to all specialists in the field of microbiology. However, there are species of which many aspects are still unknown, including the specific conditions that are required to generate any pathology. Due to this, there are many studies that have been raised to try to elucidate it..

Mycobacterium tuberculosis bacterial cells. Source: By NIAID (Mycobacterium tuberculosis Bacteria) [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Among the diseases that are caused by bacteria of the genus Mycobacterium there are two that for years have been thousands of deaths: tuberculosis and leprosy. The first is caused by Mycobacterium tuberculosis and the second for the Mycobacterium leprae. Due to their pathogenic potential, they are perhaps the best known and most studied mycobacteria.

On the contrary, there are others totally unknown. However, in general, there is a vision of the genre Mycobacterium as a group of bacteria with well-marked common and particular characteristics. This makes them one of the most interesting organisms in the Bacteria domain..

Article index

  • 1 Taxonomy
  • 2 Morphology
  • 3 General characteristics
  • 4 Pathogenesis
    • 4.1 Virulence factors
  • 5 References

Taxonomy

The taxonomic classification of the genus Mycobacterium is the next:

Domain: Bacterium

Edge: Actinobacteria

Order: Actinomycetales

Suborder: Corynebacterineae

Family: Mycobacteriaceae

Gender: Mycobacterium.

Morphology

Bacteria belonging to the genus Mycobacterium They are shaped like an elongated bar. Its measurements are: 0.2 - .04 microns wide by 2 - 10 microns long. Some species have rounded edges, as well as others have straight edges.

They all have a fairly complex cell wall. This complexity distinguishes it from the rest of the prokaryotic organisms. Among its most outstanding characteristics is the abundance of lipids known as mycolic acids.

Similarly, in the cell wall they contain a peptidoglycan called lipoarabinomannan, which is linked through phosphodiester type bonds to a polysaccharide called arabinogalactan..

The complexity of the cell wall of bacteria belonging to the genus Mycobacterium It lies in the bonds that are established between the molecules of lipoarabinomannan, arabinogalactan and mycolic acids.

Bacterial cells of this genus, in general, do not have cilia or flagella.

The genome of mycobacteria is confined to a single circular chromosome that is made up of a sequence of nucleotides, with cytosine and guanine representing 65% of the total..

The number of genes will depend on the species you are talking about. For example, the Mycobacterium tuberculosis has one of the longest genomes known to date.

General characteristics

It is slow growing

Most of the species that make up this genus are slow growing. This means that they take more than 7 days to generate observable colonies in the cultures..

The only exceptions are the Mycobacterium smeagmatis and the Mycobacterium fortuitum, which have shown a pattern of rapid growth.

They are aerobic

The bacteria that make up this genus are characterized by being aerobic. This means that they necessarily need an environment with a wide availability of oxygen to be able to develop properly and carry out their different metabolic processes..

They are acid - alcohol resistant

The gender Mycobacterium It has a peculiarity, and that is that the species that make it up are resistant to discoloration by acids or alcohols.

In the various staining procedures, one of the essential steps is bleaching using acidic substances or alcohols. In the case of mycobacteria, they cannot be discolored through this method. This is mainly due to the mycolic acids present in the cell wall, which provide low absorption..

They are catalase positive

All members of the genus Mycobacterium they synthesize the enzyme catalase. This enzyme acts on hydrogen peroxide (HtwoORtwo) and unfold it into oxygen and water, with the consequent release of bubbles.

This property is very important, since together with other tests it allows the identification of bacteria at an experimental level..

There are some species, like Mycobacterium tuberculosis that produce a catalase called thermostable that continues to exert its function after passing 68 ° C for about 20 minutes.

They are capable of producing pigments

Mycobacteria have the ability to produce pigments when in the presence or absence of light.

Those in which light induces the production of pigments are known as Photochromic. Clear examples of this type of bacteria are Mycobacterium kansasii, Mycobacterium simiae Y Mycobacterium marinum.

On the contrary, those that produce pigments in the absence of light are called scotochromogens. Among these we can mention: Mycobacterium scrofulaceum, Mycobacterium szulgai Y Mycobacterium flavescens.

Has the ability to reduce nitrates

Some of the bacteria members of the genus Mycobacterium synthesize an enzyme known as nitratoreductase, which catalyzes the chemical reaction in which nitrates are reduced to nitrites:

NOT3- + 2 ë - NOtwo + HtwoOR

The mycobacteria that synthesize this enzyme are Mycobacterium tuberculosis, Mycobacterium kansasii, Mycobacterium fortuitum, Mycobacterium chelonae, among other.

Stained by the Ziehl Neelsen technique

Due to the constitution of their cell wall, mycobacteria cannot be colored through the Gram staining process..

However, they can be processed through other procedures such as that of Ziehl Neelsen. In this, the coloration is subjected to a previous heating so that it can pass through the cell wall that is composed of mycolic acids (fatty acids).

Later it is cooled with water, causing the fatty acids to solidify, retaining the colorant. Finally, methylene blue is added to create a contrast between acid-fast and non-acid-fast bacterial cells..

Synthesize the enzyme urease

Various species of bacteria belonging to the genus Mycobacterium synthesize an enzyme known as urease, which catalyzes the reaction in which urea is hydrolyzed to form ammonia and carbon dioxide.

Among these bacteria we can mention Mycobacterium bovis, Mycobacterium africanum Y Mycobacterium malmoense.

Growth temperature

Depending on the species of mycobacterium, the growth temperature varies. However, it can be stated that most of them grow optimally at a temperature of 37 ° C.

Likewise, there are exceptions, for example, Mycobacterium marinum Y Mycobacterium haemophilum require a temperature of 30 ° C to develop, while Mycobacterium thermoresistibile does it at 52 ° C.

Pathogeny

Not all mycobacterial species pose a threat to living beings, especially man.

Among the mycobacteria that are strictly pathogenic can be mentioned Mycobacterium tuberculosis, Mycobacterium leprae, Mycobacterium bovis Y Mycobacterium africans.

On the contrary, there are some that are occasional pathogens. This means that to generate a pathology they require certain conditions, such as immunosuppression of the host. Among these we can mention Mycobacterium xenopi, Mycobacterium abscessus Y Mycobacterium chelonae.

The pathogenic process of these bacteria, in general, is as follows: the bacterium enters the bloodstream and immediately the defense mechanisms of the immune system are activated, specifically macrophages. These are cells that specialize in phagocytosis of foreign agents.

Once inside the macrophage, through various biochemical survival strategies, the bacterium avoids the lethal activity of lysosomes (a kind of sacs that contain lytic enzymes) and begins to reproduce and spread to generate lesions in the various tissues..

Virulence factors

Virulence factors are elements to take into account in the development of a bacterial infection, since they are the ones that determine the ability of a bacterium to enter a host and generate some pathology.

In the case of mycobacteria, virulence factors serve several functions:

  • Promote the entry and reproduction of the bacteria in host cells.
  • Interfere with the host's natural defense mechanisms so that the bacteria are not harmed.

Among the most well-known and studied virulence factors of mycobacteria, we can mention the Cordon Factor, sulphatides and lipo-arabic mannan..

References

  1. Alderwick, L., Harrison, J., Lloyd, G. and Birch, H. (2015, August). The Mycobacterial cell Wall - Peptidoglycan and Arabinogalactan. Cold Spring Harbor Perspectives in Medicine. 5 (8).
  2. Biology of mycobacteria. Obtained from: fcq.uach.mx
  3. Imperiale, B., Morcillo, N. and Bernardelli, A. (2007). Phenotypic identification of mycobacteria. Biochemistry and clinical pathology. 71 (2). 47-51
  4. Mycobacterial infections. Retrieved from: medlineplus.gov
  5. Retrieved from: microbewiki.com
  6. Special Issue: "Mechanism of Mycobacterium tuberculosis Pathogenesis". Retrieved from: mdpi.com
  7. Obtained from: hygiene. edu.uy

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