Photobacterium damselae subsp. damselae, an emerging pathogen in Danish rainbow trout, Oncorhynchus mykiss (Walbaum), mariculture

Abstract

A selection of 16 field isolates of Photobacterium damselae from marine rainbow trout farms in Denmark was subjected to phenotypic and genotypic characterization and pathogenicity to fish. All isolates belonged to the subspecies damselae, being positive for haemolysis, motility and urease. There were considerable differences in haemolytic properties, some isolates presenting a broad zone of haemolysis and others only a narrow zone. Pulsed-field gel electrophoresis revealed a high diversity indicating that P. damselae subsp. damselae is an opportunistic, not clonal pathogen in Danish marine rainbow trout. Virulence of the strains to rainbow trout was highly variable with LD(50) values ranging from 3.9 x 10(3) to 1.5 x 10(8) cfu at 20 degrees C. The virulence was significantly higher at 20 degrees C than at 13 degrees C. The strains with the strongest haemolytic properties were the most virulent suggesting a strong involvement of haemolysin in the pathogenesis. The pathological changes were consistent with a bacterial septicaemia and the haemorrhages were more pronounced than for most other bacterial infections.

Toxicity of Photobacterium damselae subsp. damselae strains isolated from new cultured marine fish

“…damselae ECPs tested were very low (Table 1), without production of caseinase or gelatinase (Table 2), but they exhibited other activities such as phospholipase, lipase, amylase, phosphatases, or glycosidases (Tables 2 & 3). Nevertheless, none of these activities could be related with the high degree of toxicity both in vivo and in vitro presented by the ECPs, in contrast to results reported for other fish pathogens (Esteve et al 1995).On the other hand, several authors have demonstrated that the pathogenicity of bacterial fish pathogens was related to the capability to hemolyse the host erythrocytes (Borrego et al 1991, Fouz et al 1993, Pedersen et al 2009). The differences found in the hemolytic activity of the Photobacterium damselae subsp.…”

“…On the other hand, several authors have demonstrated that the pathogenicity of bacterial fish pathogens was related to the capability to hemolyse the host erythrocytes (Borrego et al 1991, Fouz et al 1993, Pedersen et al 2009). The differences found in the hemolytic activity of the Photobacterium damselae subsp.…”

Chromosome-Encoded Hemolysin, Phospholipase, and Collagenase in Plasmidless Isolates of Photobacterium damselae subsp. damselae Contribute to Virulence for Fish

“…In this regard, only a few studies detected gelatinolytic activity in a small fraction of P. damselae subsp. damselae isolates (16,17), while other studies reported negative results for this trait (14). No correlation could be established between gelatin degradation and the isolation source or geographical origin.…”

“…1). Although previous studies reported that plasmidless strains show virulence for a variety of fish species (7,17), our study constitutes evidence that hlyA ch (the gene that encodes PhlyC) is a major contributor to virulence of plasmidless strains.…”

“…Previous studies detected phospholipase activity (but not protease production) in pPHDD1-harboring and plasmidless P. damselae subsp. damselae strains (4,14,15), and other studies reported that some strains exhibited collagenase activity (16,17), but the genetic basis of these activities remains uncharacterized.…”