28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 Importance Of Pili For Virulence Of Aeromonas salmonicida In Salmo salar Jessica Boyd, Andrew Dacanay, Sandra Sperker, Michael Reith and Stewart Johnson National Research Council, Institute for Marine Biosciences, 1411 Oxford St. Halifax, Nova Scotia, Canada B3H 3Z1 Aeromonas salmonicida is the causative agent of the fish disease furunculosis. This bacterium can infect a wide variety of fish species and is particularly important as a pathogen of Atlantic salmon (Salmo salar).At the National Research Council, Institute for Marine Biosciences, we are sequencing the genome of A. salmonicida. Although no pili are seen in electron micrographs, our genome sequence program has identified four pili. There were no differences in morbidity or mortality following an intraperitoneal (ip.) challenge with either the mutants or the parental strain. However following a dip challenge there was significantly reduced mortality with the mutants compared with the parental strain. Results suggested that these pili are important for colonization of some external surface(s) of the fish, but are not necessary once the epithelial barriers are breached. The prevalence of covert infections in the dip challenge survivors was determined by a stress test that used both corticosteroid and temperature stressors. Both mutants had significantly higher prevalence of covert infections compared to the parental strain. These results suggested that the host can clear the infection more easily if the pili are present. If the pili are absent, the bacterium can escape the immune response and initiate a covert infection more easily. Thus the pili are important for crossing the epithelial barriers and establishing infection, but the pili may become a liability once the bacterium is inside a host. Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page