2011/702 Pathogenesis, prevalence and persistence of pathogenic bacteria associated with farmed Tasmanian Atlantic Salmon

By Eva Hatje

Farming Atlantic Salmon and other aquaculture species is an important industry in Australia. A major limitation is the outbreak of bacterial disease amongst the farms, which can result in huge economic and fish stock losses. This thesis examined the presence and persistence of potentially pathogenic bacterial populations in farmed Tasmanian Atlantic Salmon (Salmo salar L.) and assessed their dynamics as a factor of seasonal water temperature and diet. Special attention was paid to the virulence factors of these bacteria in fish and other marine animals such as farmed Banana Prawns. The ability of lactic acid bacteria (LAB) to inhibit pathogens associated with fish disease as well as a comparison of the virulence properties of these bacteria isolated from different sources was also examined.

This thesis demonstrated that Vibrio and Pseudomonas spp. are present and able to persist in the gastrointestinal tract of farmed Atlantic Salmon. A factor that influences the presence of these bacteria in the salmon gut is the surrounding water temperature. This population is not very diverse, with only a few species, namely V. ichthyoenteri and P. fragi dominating. These bacteria also carried virulence properties involved in adhesion to fish cells, production of biofilm, genes coding for toxins and were resistant to some antibiotics. Moreover, virulence factors usually found in human pathogens are shared amongst closely related bacteria regardless of their source of isolation, as shown with Aeromonas spp. These bacteria were also able to interact with a model of the human gut epithelium and translocate, even though their adhesion was reduced in the presence of LAB.