A conserved Aeromonas salmonicida porin provides protective immunity to rainbow trout

A protein with an apparent M(r) of 28,000 was isolated from outer membrane preparations of Aeromonas salmonicida A440. The protein was tested for the ability to form pores, using a planar lipid bilayer model membrane system. The protein appeared to be a monomer with a single-channel conductance in 1.0 M KCl of 1.96 nS and a cation/anion permeability ratio of 2.91 +/- 0.68. These data show that the porin channel is comparable in size to OmpC and OmpF of Escherichia coli and is relatively nonselective, having some preference for cations over anions. The porin was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and a polyclonal antibody was raised. Immunoblot analysis showed that an immunologically cross-reactive protein was present in other Aeromonas strains but not in strains of Vibrio or Yersinia. The N-terminal amino acid sequence of the porin was determined and was found to show some homology to an Aeromonas hydrophila outer membrane protein. This is the second porin species of A. salmonicida to be described, and it differs from the other in subunit molecular weight, aggregation properties, peptidoglycan association, pore size, and antigenicity. Rainbow trout (Oncorhynchus mykiss) immunized intraperitoneally with the purified porin protein were significantly protected from experimental A. salmonicida challenge. This is the first report of successful vaccination against A. salmonicida with a purified outer membrane component.