Characterization and Vaccine Potential of Outer Membrane Vesicles from subsp. .

subsp. () is a Gram-negative fish pathogen with worldwide distribution and broad host specificity that causes heavy economic losses in aquaculture. Although was first identified more than 50 years ago, its pathogenicity mechanisms are not completely understood. In this work, we report that secretes large amounts of outer membrane vesicles (OMVs) when cultured in vitro and during in vivo infection. These OMVs were morphologically characterized and the most abundant vesicle-associated proteins were identified. We also demonstrate that OMVs protect cells from the bactericidal activity of fish antimicrobial peptides, suggesting that secretion of OMVs is part of the strategy used by to evade host defense mechanisms. Importantly, the vaccination of sea bass () with adjuvant-free crude OMVs induced the production of anti- antibodies and resulted in partial protection against infection. These findings reveal new aspects of biology and may provide a basis for developing new vaccines against this pathogen.