Evaluation of strain specific variation in outer membrane vesicles isolated from P. salmonis and their ability to protect against SRS in adult zebrafish

Salmoid rickettisal septicemia (SRS) is caused by the intracellular pathogen Piscirickettsia salmonis, which over the last decade has caused a significant loss in the Chilean aquaculture, due to high mortality in salmon as a result of SRS. Due to the bacterium’s intracellular nature vaccination has been problematic, and thus, there is a need for further studies of the bacterium in order to develop new vaccine strategies against SRS. The use of outer membrane vesicles has been shown to provide a good protection when used as a vaccine candidate in both fish and humans, and might therefor be an alternative vaccine against SRS. We have previously shown that OMVs isolated from P. salmonis type strain (LF-89), provided protection against SRS in adult zebrafish (FOTS application 8507). There is, however, little know if OMVs from other strains of P. salmonis will provide a similar effect, and what the effect of different OMVs could potentially be in adult zebrafish. Thus, the main aim of the experiment is to compare the ability of OMVs isolated from different strains of P. salmonis to protect against SRS, as well as comparing the virulence of the different strains of bacteria. To evaluate OMVs the experiment is divided into two setups with a total of 830 zebrafish, where part one will evaluate the effect of increasing vaccine doses and bacteria, and part two evaluate the vaccine's ability to provide protection. The efficacy of the vaccine has previously been tested in cell cultures and a zebrafish embryo model to identify effects, but further testing of the vaccine requires a fully developed humoral and cellular immune response, it is, thus, desirable to use a more complex challenge model as adult zebrafish