Yellowtail kingfish (YTK), Seriola lalandi , is a marine, pelagic, piscivorus species with a circumglobal distribution. The high market value and its adaptability to sea cage culture make YTK a viable candidate for commercial production in Australia. However, one of the major impediments to the growth of Australian YTK industry is infectious diseases. Photobacterium damselae subsp. damselae (Pdd) is a gram-negative, halophilic bacterium which has been isolated from a wide variety of fish species around the globe. In recent years, the bacterium has been reported for the first time in wild and cultured fish species, including YTK, throughout Australia . The major virulence factors of Pdd are plasmid (pPHDD1)- encoded phospholipase-D damselysin and pore- forming toxin phobalysin P. However, strains of Pdd without plasmid have also demonstrated pathogenicity due to the presence of chromosome I-encoded virulence factors. The present study was conducted to test the pathogenicity of plasmid-positive and plasmid- negative Australian isolates of Pdd towards YTK.
An infection trial was conducted in a flow- through system with healthy, unvaccinated cultured YTK, weighing an average 152.0±17.7 g . T hree isolates of Pdd (one plasmid-positive, AS-16-0963#3; and two plasmid-negative , AS-15-3942#7 and AS-16-0963#1 ) were selected for experimental infection. Throughout the experiment, w ater temperature was maintained at 20-22 °C and fish were fed twice per day to satiation with commercial feed. Dissolved oxygen was monitored daily and kept with in 80-100%. Fish were randomly allocated to 20 (300 L) tanks , with 10 fish per tank. There were six experimental treatments, with intraperitoneal injection of one of the three test isolates of Pdd at two different concentrations, 104 CFU/fish and 107 CFU/fish, and three replicate tanks per treatment. Control fish in two replicate tanks were sham-injected . Fish mortality was recorded daily for 10 days post-challenge. Blood samples from infected fish after 4 days of infection were collected to measure hematology indices.
M ortality rates for the plasmid-positive isolate, AS-16-0963#3, were 47% and 100% at 104 CFU/fish and 107 CFU/fish respectively, compared to13- 20% and 3- 13% respectively for the two plasmid-negative isolates. No mortality was observed in the control fish. There was little difference in hematology parameters among isolates . These results suggest that, while presence of the pPHDD1 plasmid is not essential for pathogenicity, the plasmid does increase the virulence of Photobacterium damselae subsp. damselae to Seriola lalandi in Australia .