Pathogenesis and Cell Biology of the Salmon Parasite Spironucleus salmonicida
Abstract: Spironucleus species are classified as diplomonad organisms, diverse eukaryotic flagellates found in oxygen-deprived environments. Members of Spironucleus are parasitic and can infect a variety of hosts, such as mice and birds, while the majority are found to infect fish. Massive outbreaks of severe systemic infection caused by a Spironucleus member, Spironucleus salmonicida (salmonicida = salmon killer), have been reported in farmed salmonids resulting in large economic impacts for aquaculture.In this thesis, the S. salmonicida genome was sequenced and compared to the genome of its diplomonad relative, the mammalian pathogen G. intestinalis (Paper I). Our analyses revealed large genomic differences between the two parasites that collectively suggests that S. salmonicida is more capable of adapting to different environments. As S. salmonicida can infiltrate different host tissues, we provide molecular evidence for how the parasite can tolerate oxygenated environments and suggest oxygen as a potential regulator of virulence factors (Paper III). To further investigate the molecular responses of the parasite and in addition, its host, during infection we set up an interaction system of S. salmonicida and ASK (Atlantic salmon kidney) cells (Paper VI).To study the cell biology in S. salmonicida we optimized an enzymatic proximity labeling method using ascorbate peroxidase (APEX) as a reporter for transmission electron microscopy (TEM) (Paper IV). As the system is robust and versatile, we showed the localization and performed ultrastructural characterization of numerous proteins in S. salmonicida and G. intestinalis. We furthermore utilized the APEX system to study the annexin protein family in S. salmonicida (Paper II). Super resolution microscopy and TEM were applied to show that the annexins are mostly associated with cytoskeletal and membranous structures. In addition, we performed phylogenetic analyses concluding that the annexin gene family is expanded in diplomonads.We performed experimental infection in Atlantic salmon and derived a potential model for the route of infection (Paper V). The results suggested multiple routes of transmission between hosts for the parasite.To conclude, the comprehensive work in this thesis has provided valuable insights into the pathogenesis and cell biology of the highly adaptable diplomonad parasite S. salmonicida.
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