Iron-regulated outer membrane proteins of Aeromonas salmonicida are important protective antigens in Atlantic salmon against furunculosis

Abstract

Four iron-regulated outer membrane proteins (IROMPs) of molecular weight 82, 77, 72 and 70 kilodaltons (kDa) were identified in 18 isolates of Aeromonas salmonicida grown in the presence of the synthetic iron chelators ethylene diamine di ( o-hydroxyphenylacetic acid) (EDDA) or 2,2′-dipyridyl. The IROMPs were shown to be immunogenic in Atlantic salmon by immunoblotting techniques. Reaction of immunoblots with a monospecific rabbit anti-IROMP antiserum indicated that the IROMPs were present and immunologically cross-reactive in each of 18 isolates of A. salmonicida most of which were obtained from Atlantic salmon with furunculosis. Outer membrane proteins (OMPs) of A. salmonicida grown in the presence of 2,2′-dipyridyl to induce IROMPs were evaluated for their ability to actively immunise Atlantic salmon. Compared with OMPs prepared from A. salmonicida grown in iron-replete media, IROMP preparations conferred significant protection against natural and experimental heterologous A. salmonicida challenge. Passive immunisation of Atlantic salmon with Atlantic salmon antisera containing antibodies to IROMPs and monospecific rabbit anti-IROMP antiserum or affinity purified immunoglobulin G (IgG), conferred significant protection against a heterologous A. salmonicida bath challenge. Both salmon antisera raised to IROMP preparations and the monospecific rabbit anti-IROMP antiserum were bactericidal for avirulent (A-layer negative) and virulent (A-layer positive) strains of A. salmonicida in vitro. The results indicate that IROMPs of A. salmonicida represent important protective antigens against furunculosis.