Genetic diversity and population structure of Mycobacterium marinum: new insights into host and environmental specificities

Mycobacterium marinum causes a systemic tuberculosis-like disease in fish, and skin infections in humans that might spread to deeper structures, resulting in tenosynovitis, arthritis and ostemomyelitis. However, little information is available concerning: (i) the intraspecific genetic diversity of M. marinum isolated from humans and animals; (ii) the M. marinum genotype circulation in the different ecosystems and (iii) the link between M. marinum genetic diversity and hosts (humans and fish). Here, we conducted a genetic study on 89 M. marinum isolates from humans (n = 68) and fish (n = 21) by using Mycobacterial Interspersed Repetitive Units-Variable Number of Tandem Repeats (MIRU-VNTR) typing. The results show that the M. marinum population is genetically structured not only according to the host, but also to the ecosystem as well as to tissue tropism in humans. This suggests the existence of different genetic pools in function of the biological and ecological compartments. Moreover, the presence of only certain M. marinum genotypes in humans suggests a different zoonotic potential of the M. marinum genotypes. Considering, the infection linked to the aquarium activity, a significant genetic difference was also detected when the human tissue tropism of M. marinum was taken into consideration, with a higher genetic polymorphism in strains isolated from patients with cutaneous forms than from individuals with deeper-structure infection. It appears that only few genotypes can produce deeper infections in humans, suggesting that the immune system might play a filtering role