Mycobacterium marinum – Culture; Molecular diagnosis (PCR)


Mycobacterium marinum is a gram-positive and acid-alcohol-resistant bacterium of the genus Mycobacterium, family Mycobacteriaceae. The infection by this microorganism has been identified in different species of fish, both from cold, warm, fresh and salty waters, although it can also affect people, in which it causes a disease called "pool granuloma" or "aquarium granuloma".

Mycobacterium marinum survives in macrophages and has the ability to sporulate. Unlike other Mycobacterium species, most strains of Mycobacterium marinum cannot grow at the usual incubation temperature of 37°C, but grow between 30 and 33°C. It belongs to the group 1 of the Runyon classification, it is a slow growing mycobacteria and photochromogenic: its colonies are cream-colored and turn yellow when exposed to light.

This microorganism is found in the soil and in the fresh and salty water lakes, pools and aquariums. Due to their presence in these media, infection by Mycobacterium marinum can be a risk for people with occupations or hobbies with exposure to sea water, fish or shellfish. The transmission of Mycobacterium marinum occurs through the external inoculation of skin abrasions, wounds or trauma, in contact with contaminated water from swimming pools and aquariums. Although the infection can be caused by a direct injury with fish fins or bites, most are acquired during the handling and cleaning of aquariums. In addition, indirect infections can occur when using utensils that have previously contacted fish tanks.

In fish, Mycobacterium marinum infection causes a chronic progressive disease, similar to active tuberculosis, manifested through weight loss, open ulcers that do not heal, distended abdomen, loss of appetite, fin erosion, unusual coloration, bulging eyes, deformities in the spine and changes in behavior. The predominant lesion in the final stages of the infection is the presence of necrotizing granulomas.

In people, Mycobacterium marinum has an incubation period of 2 to 8 weeks after inoculation, after which, a papulonodular lesion appears at the level of the port of entry, which generally evolves towards a painful and purplish nodule. The two most common manifestations of this infection are a single large granuloma, or an ascending lymphangitic granuloma (a series of small nodules usually beginning in the hand and progressing in an ascending line to the arm). Although granulomas are usually unique, they often evolve to multiple, describing superficial papules on the skin and erythematous plaques that may present suppuration and ulceration. The lesions can be painful, without pain or with intermittent pain. The injuries usually occur on the elbows, knees and feet of those infected in swimming pools, and on the hands and fingers of the owners of the aquarium. The inhibition of the growth of Mycobacterium marinum at 37°C is related to its ability to infect the colder parts of the body, especially the extremities. The infection is usually limited to the skin, though, less frequently, cases of involvement of the underlying tissues have been described with arthritis, tenosynovitis, osteomyelitis or bursitis. Rarely, a type of disease has been described where the infection is spread, observed in patients with severe immunosuppression, that can be fatal.

Recommended tests for diagnosis:

The diagnosis is based on the microscopic identification, culture methods or molecular diagnostic methods (PCR).

Tests carried out in IVAMI:

  • Microscopic exam.
  • Detection by isolation by cultivation in selective media.
  • Molecular diagnosis (PCR), to detect Mycobacterium marinum DNA.

Recommended sample:

  • Skin lesion biopsy.

Conservation and shipment of the sample:

  • Refrigerated (preferred) for less than 2 days.
  • Frozen: more than 2 days.

Delivery of results:

  • Culture and identification: 15 to 30 days.
  • Molecular diagnosis (PCR): 24 to 48 hours.

Cost of the test:

  • Microscopic exam, conventional culture and culture by MGIT system: Consult to