VIRULENCE AND ANTIGENIC CHARACTERISTICS OF A CULTURED RICKETTSIALES-LIKE ORGANISM ISOLATED FROM FARMED ATLANTIC SALMON SALMO-SALAR IN EASTERN CANADA

The present study describes culture, virulence and antigenic character istics of a Rickettsiales-like organism (RLO) associated with mortalit y in farmed Atlantic salmon in eastern Canada. Clinical disease was re produced in naive Atlantic salmon parr by intraperitoneal (IP) inocula tion with kidney homogenate from naturally infected fish. Pure culture s of RLO were isolated into chinook salmon embryo (CHSE) cells from ki dney of experimentally infected fish. The RLO caused cytopathic effect in cultured CHSE-214 typified by coalescing areas of swollen cells th at eventually detached from the substrate. Bacteria in infected cultur e supernatants reacted with Piscirickettsia salmonis-specific polyclon al sera or monoclonal antibody (MAb) in an indirect fluorescent antibo dy test. TP inoculation with cultured RLO resulted in mortalities of 1 00, 62, 22.5 and 0% in Atlantic salmon, coho salmon, rainbow trout and common carp, respectively. Cultured RLO were sensitive to chloramphen icol, flumequine, oxytetracycline and oxolinic acid and insensitive to gentamicin and amphotericin B. RLO antigens were compared with those of 3 strains of P. salmonis from Chilean salmon by SDS-PAGE and immuno blotting. A silver-staining band of about 12 kDa was detected in prote inase K (PK) digests of all RLO strains, and a diffuse band of about 1 5 kDa was observed in 2 Chilean strains only. No other silver-stained bands were visible in PK digests of any strain examined. The polyclona l serum recognized 9 protein bands and multiple non-protein bands exte nding from less than 20 kDa to greater than 95 kDa in all isolates. Th e MAb reacted with an epitope in PK digests that occurred in all 4 str ains on structures of widely ranging molecular masses, resulting in a ladder pattern similar to that obtained with polyclonal serum. Treatme nt of PK digests with periodic acid abolished reactivity with MAb and polyclonal serum. Co-elution of 2-keto-3-deoxyoctonate and MAb reactiv ity following size exclusion chromatography of solubilized P. salmonis suggested that the MAb recognized a lipopolysaccharide-associated epi tope in all 4 RLO iso lates. Cultural, virulence and antigenic similar ities among the strains examined in the present study indicate that th e eastern Canadian salmonid RLO should be considered a strain of P. sa lmonis.