This dissertation. reports the first detailed study of the possible pathogenic mechanisms and virulence determinants of the fish pathogen V. anguillarum carried out in association with histo-pathological studies using electron microscopy and electrocardiography In addition, possible infection route (s) were investigated and the effect of environmental variables on pathogenesis examined. Pathogenicity was found to be species specific with strains which were avirulent in eels being highly virulent in grey mullet. Following injection of virulent bacteria into eels all tissues were rapidly colonised, death being characterised by severe haemorrhagic septicaemia. In contrast, avirulent strains were rapidly eliminated from the eel host to levels below detection. The heart was particularly affected during pathogenesis. Vibriosis was found to be temperature related, disease being retarded by low temperature. Pathogenic properties investigated included production of enzymes and haemolysins and examined in vitro phagocytosis and growth rates. Membrane proteins were extracted by various techniques and separated using SDS-polyacrylamide electrophoresis thereby elucidating interstrain variation in protein profile, most strains containing a major outer membrane protein thought to be a porin. Ultrastructural studies revealed some bacteria to have up to three polar flagella per cell, with multiflagellate forms only being observed in virulent strains. Analysis of plasmid DNA revealed a partial correlation between possession of a 47 megadalton plasmid and colistin resistance. Experimental vibriosis was characterised by deposition of haemosiderin in liver tissues, thought to be a poorly studied host defence mechanism, and large numbers of tissue bacteria surrounded by an electron lucent zone which was non-capsular in nature. Features of the disease included desquamation of the intestinal mucosa with excessive loss of ions into the gut lumen. The most likly route of infection was thought to be via the gut, as osmoregulatory processes provided a direct means for waterborne V. anguillarum to enter the gut, to which this bacterium was found to be particularly well adapted. Gut traversal was thought to be the precursor to a possible latent infection in the kidney.

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