Virulence factors of bacteria pathogenic for coldwater fish

The importance of cell surface associated properties and extracellular products (ECP) as virulence factors varies among the fish pathogens considered in this review. Among members of the genus Vibrio, V. salmonicida, and V. ordalii do not secrete proteases, hemolysins, or cytotoxins, while V. anguillarum, V. vulnificus, and V. damsela are strong exotoxin producers. In V. salmonicida, the presence of a hydrophobic surface antigen, VS-P1 (40 Kd), has been described as a possible virulence determinant protecting the bacterium against the action of host serum. Although in V. anguillarum surface properties related to adherence and invasiveness, as well as different exoenzymes (i.e. hemolysins, cytotoxins, and dermatotoxins) can contribute to the development of infections, metallo-proteases, and undetermined low MW substances are the main toxins responsible for the lethality of their ECP. Whereas V. vulnificus seems to have a different mechanism of virulence for eels and mice, with the presence of a capsule being associated only with its pathogenicity for mammals, all V. damsela strains possess similar virulence determinants for poikilotherm and homoiothermic hosts secreting a potent lethal phospholipase toxin with hemolytic and cytotoxic activities. Although cell surface characteristics linked to the A layer can play a role in disease produced by Aeromonas salmonicida, the pathogenesis of furuncle formation in vivo is due to a combined effect of two enzymes, a 70 Kd serine protease and a 25 Kd phospholipase. Pseudomonas anguilliseptica, Flavobacterium branchiophilum, and the gliding bacteria (Flexibacter-Cytophage spp) are producers of proteolytic enzymes, but the involvement of ECP in the first attack of external fish tissues has only been demonstrated in Flavobacterium. In P. anguilliseptica, however, a relationship between the presence of K antigens and virulence has been clearly confirmed. Yersinia ruckeri lacks most of the cell-surface virulence properties present in other pathogenic Yersinia spp., with proteases and/or hemolysins being the main toxins responsible of the lethal effects of the ECP. In contrast, ECP from Renibacterium salmoninarum are devoid of proteolytic, hemolytic, and cytotoxic activities, and are not lethal for fish. The virulence of R. salmoninarum is strongly correlated with the nature and properties of the cell envelope such as hydrophobicity, autoaggregation, hemagglutination, and the presence of a 57 Kd antigenic protein.

Although in some of these bacteria, the iron-acquisition systems can play a role in their pathogenicity, only in V. anguillarum has it been conclusively demonstrated that a direct relationship exists between virulence for fish and the possesion of two siderophore-mediated iron transport mechanisms coded, respectively, by plasmidic and chromosomal genes.