Mycobacterium marinum characteristics, taxonomy, morphology

Sherman Hoover

Mycobacterium marinum it is a bacterium that belongs to the broad group of mycobacteria. It is an almost exclusive pathogen of fish and some amphibians. However, sometimes and under certain conditions it is capable of causing pathology in humans..

It was isolated for the first time in 1926 from fish tissues, but it was not determined until 1951 that it is an opportunistic pathogen in humans. The first described case of pool granuloma dates from this year, the name given to the infection caused by Mycobacterium marinum.


Over time and through various studies it was established that those who are more likely to suffer an infection caused by this bacterium are those who practice water sports, those who have fish tanks at home or those who have a work occupation in which they are in contact with aquatic environments.

Mycobacterium marinum it is a widely studied bacterium. However, there are still many aspects of its physiology to be elucidated. The studies carried out on it have been important in determining the treatment guidelines to follow in case of infection..

Article index

  • 1 Taxonomy
  • 2 Morphology
  • 3 Features
    • 3.1 It is slow developing
    • 3.2 It is of free life
    • 3.3 It is mesophilic
    • 3.4 Habitat
    • 3.5 It is aerobic
    • 3.6 They are alcohol - acid resistant
    • 3.7 It is photochromic
    • 3.8 Are Ziehl-Nielsen positive and Gram positive
    • 3.9 It is catalase positive
    • 3.10 It is urease positive
    • 3.11 It is pathogenic
  • 4 Diseases it generates
  • 5 Pathogenesis
  • 6 Symptoms
  • 7 Diagnosis
  • 8 Treatment
  • 9 References


The taxonomic classification of Mycobacterium marinum it is:

Domain: Bacterium

Edge: Actinobacteria

Class: Actinobacteria

Order: Actinomycetales

Suborder: Corynebacterineae

Family: Mycobacteriaceae

Gender: Mycobacterium.

Species: Mycobacterium marinum.


The Mycobacterium marinum is a bacterium whose cells are shaped like a slightly curved rod. They have an average size of 0.2-0.4 microns wide by 2-10 microns long. They look like individual cells under the microscope.

In the cultures, cream-colored, circular-sized colonies are observed, which may turn yellow when exposed to light..

The bacterial cell does not present any type of extensions such as flagella or cilia. It is surrounded by a cell wall that has a fairly complex structure.

It has a thick cell wall, characteristic of bacteria of the genus Mycobacterium. This contains a large amount of lipids, which makes it hydrophobic. It also contains mycolic acids and a peptidoglycan known as lipoarabinomannan..


The Mycobacterium marinum it is an atypical species within the group of mycobacteria. Its features include:

It is slow developing

This bacterium is characterized by slow growth. In crops it has been observed that it takes an average of 2 to 8 weeks to grow.

It's free life

The Mycobacterium marinum It is a bacterium that does not need to be inside a host in order to carry out its life cycle. Bacteria can develop freely in their habitat.

It is mesophilic

Through experimental studies it has been possible to determine that the development temperature of this bacterium ranges between 30 ° C and 37 ° C. The optimum temperature is 32 ° C.


This is a ubiquitous bacterium in aquatic environments. This means that it can be found in freshwater habitats (rivers, lakes, ponds) and saltwater habitats (oceans and seas)..

It's aerobic

It is aerobic, because the Mycobacterium marinum it necessarily requires oxygen to carry out its metabolic processes. Taking this into account, the bacteria need to be in an environment with high availability of this chemical element..

They are alcohol - acid resistant

This is a physical property that prevents bacterial cells from being able to resist the discoloration of a pigment known as basic fuchsin. This pigment penetrates the cell and is retained by the cell membrane. This is due to the presence of mycolic acid.

The most common bleaching procedures involve the use of an acid-alcohol combination. In the case of Mycobacterium marinum, this bleaching is not successful.

It is photochromic

In the presence of light, the Mycobacterium marinum is capable of synthesizing very yellow carotenoid pigments.

They are Ziehl - Nielsen positive and Gram positive

Despite the Mycobacterium marinum they do not follow the patterns of gram positive bacteria, that is, they do not retain the dye and therefore do not adopt the typical violet coloration, they are known as acid-resistant gram positive bacteria.

Likewise, the type of stain used to study these bacteria is known as the Ziehl-Nielsen stain. In this stain, broadly speaking, a dye is added that stains the bacteria red to later add methylene blue as a contrast..

Reddish bacteria can be seen under a microscope with a blue background.

It is catalase positive

These bacteria synthesize the enzyme catalase, capable of breaking down the hydrogen peroxide molecule in water and oxygen..

It is urease positive

Urease is an enzyme that has urea as a substrate and hydrolyzes it into ammonia and carbon dioxide, according to the following reaction:

(NHtwo) 2CO + HtwoOR __________________ COtwo + 2NH3

The Mycobacterium marinum synthesize this enzyme. This is a characteristic that is used to differentiate this bacterium from others..

It is pathogenic

This bacterium is a fish pathogen, causing fish tuberculosis. Likewise, it is a known opportunistic pathogen in humans..

The infection develops when the bacteria enter the body through an injury or erosion in the skin. This occurs when the skin in these conditions is in contact with contaminated water.

Diseases it generates

As a pathogen it mainly attacks fish. Occasionally it can generate in humans a pathology known as "Granuloma de las Piscinas".

People become infected when they come into contact with contaminated water. It generally occurs in people who have aquariums in their homes or have jobs related to this environment..


The incubation period for this bacteria is normally 2 to 4 weeks, although occasionally it can be 2 months..

Once the bacteria enter the body through a wound or skin lesion, the immune system is activated and the bacterial cells are phagocytosed by macrophages.

Within macrophages, thanks to various virulence factors, the formation of lysosomes is interrupted, which are those that contain the enzymes that can cause the lysis of the bacteria.

As the lysosome-phagosome binomial does not exist, the bacterium is capable of dodging the defenses of the immune system, begin to reproduce and generate injuries in the body.


The first symptom to appear is a lump or a sore that does not heal in some part of the body that has been exposed to contaminated water.

It begins as a papulonodular lesion that later becomes a painful, purplish-colored nodule that can occasionally exude some fluid and ulcerate..

Lesion caused by Mycobacterium marinum. Source: By UnknownUnknown author @ CDC NIOSH [Public domain], via Wikimedia Commons

Another way in which it can present is with several nodular and ulcerative lesions that extend linearly towards the site of inoculation..

In 95% of cases the lesions appear on the upper limbs, preferably on the hands and forearms. It is rare for regional lymph nodes to become swollen.


To make an accurate diagnosis, one of the tools that the specialist has is the medical history. This must be detailed in order to determine if there is any history of contact with fish or possibly contaminated water.

However, the definitive diagnosis is given by a biopsy of the lesion and the subsequent culture in which the bacterial forms of Mycobacterium marinum can be evidenced..


Like any infection whose causative agent is bacteria, antibiotics are the treatment option.

According to various studies and health experience, Mycobacterium marinum is sensitive to rifampicin, cotrimazole, ethambutol, sulfonamides, and clarithromycin. The bacteria have been shown to be resistant to isoniazid and pyrazinamide.

The doses and duration of treatment depend on the criteria of the doctor. The most important thing is to follow the instructions given by him to the letter..


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