Growth, metabolism, ultrastructure and chemotherapy of Spironucleus vortens

Spironucleus species are anaerobic, flagellated protozoa, which can be either parasitic or commensal and are related to the ubiquitous human parasite, Giardia intestinalis. These parasites are responsible for devastating outbreaks of systemic infections in a wide range of food and ornamental fish and thus represent a significant problem in aquaculture. Despite the detrimental impact of Spironucleus species on an industry of growing importance, they have been little investigated, and information on their metabolism, host-parasite interactions, geographical range, pathogenesis and chemotherapy is scarce. A better understanding of the biology of these parasites is therefore essential for effective disease management. This thesis aimed at advancing the knowledge of Spironucleus parasites, by providing novel information on their growth, metabolism, ultrastructure and treatment. Spironucleus vortens, a parasite of cichlids, was used as a model organism in these investigations. Following optimization of in vitro growth conditions, S. vortens proved to be a non-fastidious organism, and grew to high densities (2.6 x 106 cells, ml"1) with a very short doubling time (1.79 h), but demonstrated an unusual biphasic pattern of growth. Despite being categorized as anaerobic, the organism exhibited both an unusually high tolerance and affinity for O2. Garlic and allium-derived thiosulfinates, which are know for their powerful, broad spectrum antimicrobial properties, had a dose-dependant inhibitory effect on S. vortens, albeit at doses higher than that required for the inhibition of most microorganisms investigated so far. Metabolic investigations demonstrated that the organism could use glucose, but that the compound was not its preferred substrate. The organism was also found to contain large pools of endogenous substrates, exhibited high proteolytic activity and was capable of rapid phagocytosis of non-soluble particles. Although Spironucleus species were described as lacking hydrogenosomes, S. vortens produced H2 at a very high rate, and ultrastructural and enzymatic studies revealed that despite previous reports, the organism possessed hydrogenosome-like, redox-active organelles. Besides hydrogenosomes, mitosome-like organelles were also detected. Such combination is currently unique in the eukaryotic kingdom and has important implications for current theories of organelles and eukaryotic evolution