The Use of Extracellular Membrane Vesicles for Immunization against Francisellosis in Nile Tilapia () and Atlantic Cod ( L.).

Francisellosis in fish is caused by the facultative intracellular Gram-negative bacterial pathogens ssp. and . The disease is affecting both farmed and wild fish worldwide and no commercial vaccines are currently available. In this study, we tested isolated membrane vesicles (MVs) as possible vaccine candidates based on previous trials in zebrafish () indicating promising vaccine efficacy. Here, the MV vaccine-candidates were tested in their natural hosts, Atlantic cod ( L.) and Nile tilapia (). Injection of MVs did not display any toxicity or other negative influence on the fish and gene expression analysis indicated an influence on the host immune response. However, unlike in other tested fish species, a protective immunity following vaccine application and immunization period could not be detected in the Atlantic cod or tilapia. Further in vivo studies are required to achieve a better understanding of the development of immunological memory in different fish species.