Characterization of attenuated Renibacterium salmoninarum strains and their use as live vaccines

Two nutritionally mutant strains of Renibacterium salmoninarum (Rs) were is olated that grew on tryticase soy agar (Rs TSA1) or brain heart infusion ag ar (Rs BHI1). These 2 strains could be continuously cultured on these media , whereas typical R. salmoninarum would only grow on KDM-2 agar. We determi ned no other phenotypic difference that could be used to distinguish them f rom wild-type R. salmoninarum. Both strains were found to be avirulent when 5 x 10(6) bacteria were intraperitoneally (IP) injected into Atlantic salm on. Rs TSA1, Rs BHI1, and Rs MT-239 (a R. salmoninarum strain previously sh own to be attenuated) were tested as live vaccines in 2 separate trials. Th e best protection was seen with Rs TSA1. Vaccinated Atlantic salmon had rel ative percent survival (RPS) of 50 at 74 d post-challenge in Trial 1 and 76 at 60 d post-challenge in Trial 2. In both trials, 100% of the control sal mon died from bacterial kidney disease (BKD) (within 40 d for Trial 1 and 5 0 d for Trial 2) after IP challenge with 5 x 10(6) live cells of the virule nt isolate Rs Margaree.