The Mycobacterium genus comprises several pathogenic species that can infect a broad range of hosts, animal or human. The severity of the disease can range from skin lesions to pulmonary or brain infections. Mycobacterium marinum is a non-tuberculous mycobacteria inducing a disease similar to the human tuberculosis in freshwater and marine vertebrates. At the cellular level, the stages of M. marinum infection are highly similar to the ones of Mycobacterium tuberculosis. Once inside host cells, both bacteria manipulate the phagosome maturation process to reside in a non-degradative vacuole called Mycobacterium-containing vacuole (MCV). During this transition, M. marinum and M. tuberculosis induce membrane damage that is a major event in the infection cycle. Indeed, it leads to a battle with the host, which aims at constraining the bacteria inside the MCV by involving major repair processes such as the ESCRT machinery and the autophagy pathway. The MCV damaging ability of M. marinum and M. tuberculosis depends on one of their type VII secretion systems, ESX-1, and its secreted peptide, EsxA. Scattered in vitro experiments document that EsxA’s membranolytic activity depend on the host vacuole membrane composition. In addition, their infection cycle leads to the characteristic induction of lipid accumulation in their host cells. These lipids are required for their own metabolism and virulence.
This thesis aims at 1) deciphering the contribution of host membrane composition to M. marinum infection and at 2) investigating the role of sterol transporters, Niemann-pick C1type C1 (NPC1) in both the social amoeba Dictyostelium discoideum and murine microglial BV-2 cells.
1) We demonstrated that microdomain components, organiser proteins and sterols, gradually accumulate at the MCV at all stages and that microdomains are required for successful M. marinum infection. In addition, we show in vitro that microdomains are necessary for EsxA partitioning into membrane and in vivo that microdomain disruption leads to a decrease of M. marinum induced MCV damage and cytosolic access. Importantly, we demonstrate that this phenomenon is conserved in D. discoideum and BV-2 cells.
2) We showed that NPC1 localises and accumulates at the MCV and npc expression is modulated during M. marinum infection. In D. discoideum knockout of the two NPC1 homologs strongly affects intracellular growth of M. marinum. The underlying mechanisms are still to be elucidated.
This thesis highlights an evolutionary conserved role of host sterols during M. marinum infection with a major impact on bacterial virulence.
