IRON UPTAKE BY PASTEURELLA-PISCICIDA AND ITS ROLE IN PATHOGENICITY FOR FISH

We evaluated the iron uptake mechanisms in Pasteurella piscicida strai ns as well as the effect of iron overload on the virulence of these st rains for fish. With this aim, the capacity of the strains to obtain i ron from transferrin and heme compounds as well as their ability to ov ercome the inhibitory activity of fish serum was analyzed. All the P. piscicida strains grew in the presence of the iron chelator ethylene-d iamine-di (O-hydroxyphenyl acetic acid) or of human transferrin, which was used by a siderophore mediated mechanism. The chemical tests and cross-feeding assays showed that P. piscicida produced a siderophore w hich was neither a phenolate nor a hydroxamate. Cross-feeding assays a s well as preliminary chromatographic analysis suggest that this sider ophore may be chemically related to multocidin. All the P. piscicida i solates utilized hemin and hemoglobin as an iron source, since the vir ulence of the strains increased when the fish were preinoculated with these compounds. This effect was stronger in the avirulent strains (50 % lethal dose was reduced by 4 logs when fish were pretreated with hem in or hemoglobin). Only the pathogenic P. piscicida isolates were resi stant to the bactericidal action of the fresh fish serum. The nonpatho genic strains grew in fish serum only when it was heat-inactivated or when it was supplemented with ferric ammonium citrate, hemin, or hemog lobin. In all the strains, at least three iron-regulated outer membran e proteins (IROMPs) (105, 118, and 145 kDa) were increased when the st rains were cultured in iron-restricted medium. In addition, although a new IROMP (75 kDa) was expressed in some isolates, this protein was n ot associated with the virulence of the strains. All the IROMPs were i mmunologically related in all the P. piscicida isolates. It is notewor thy that iron also played a regulatory role in the synthesis of some p roteolytic enzymes, since these activities were expressed only when th e strains were cultured in iron depleted conditions.