| Summary: | Aeromonas salmonicida subsp. achromogenes is a Gram negative fish pathogen causing atypical furunculosis in Arctic charr, Salvelinus alpinus L., and several other farmed fish species. The disease can cause significant reconomic losses in the aquaculture industry. The aim of this study was to increase knowledge of the molecular virulence mechanisms of A. salmonicida subsp. achromogenes and host-pathogen interactions with focus on the toxic caseinolytic metalloendopeptidase, AsaP1, which is a major extracellular virulence factor of the bacterium and highly immunogenic. Furthermore, the aim was to develop a new vaccination strategy against atypical furunculosis based on an AsaP1-toxoid. A. salmonicida possesses the LuxI/R type quorum sensing system, termed AsaI/R. The role of quorum sensing regulation of A. salmonicida subsp. achromogenes virulence and brown pigment production was investigated. An A. salmonicida subsp. achromogenes asaI knock-out strain was constructed by allelic insertion of a kanamycin-cassette. The mutation leads to a total knock-out of the quorum sensing system. AsaP1, cytotoxic activity, and a brown pigment secreted by the bacterium, were found to be quorum sensing regulated, but the cell associated virulence factors A-layer protein and LPS were not regulated by quorum sensing. Furthermore, the virulence of the quorum sensing knock-out strain was significantly impaired compared to the wild type (wt) strain. A reverse transcription quantitative real-time PCR (RT-qPCR) based analysis of the innate and adaptive immune response in head-kidney, liver, and spleen of Arctic charr during a 7 d period of infection with A. salmonicida subsp. achromogenes was performed, using the wt strain and an isogenic AsaP1-deficient mutant. The results revealed an initial regulation of innate immune response parameters indicated by up-regulation of pro-inflammatory cytokine and chemokine expression. Furthermore, the immune response triggered by the two strains was significantly different, e.g., significant ...
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