Virulence properties of Moritella viscosa extracellular products

Moritella viscosa is the causative agent of winter ulcer disease of marine fish. Knowledge of its pathogenicity is limited and there are no reports comparing the virulence properties of a collection of bacterial isolates. The in vivo and in vitro virulence of the extracellular products (ECP) of 22 M. viscosa isolates was screened. Two non‐virulent Canadian isolates and a Norwegian isolate with reduced virulence produced non‐lethal ECP. Correlation was obtained between cytotoxin and haemolysin production of M. viscosa. Isolates from salmon produced ECP with lower cytotoxic and haemolytic activities than ECP of isolates originating from other hosts. Correlation was not found between lethality of ECPs in salmon and cytotoxic or haemolytic activities. All isolates secreted esterases and a metallopeptidase (MvP1), degraded starch and produced siderophores. Variable levels of ECP protein concentration, different enzymatic activities and siderophore production could not explain differences in virulence. The results show that virulent M. viscosa isolates secrete a lethal toxic factor of unknown nature and that cytotoxin production may reflect host adaptation. Cell‐culture models may not be optimal for determining the virulence of M. viscosa, as no association between cytotoxicity and bacterial virulence was obtained. Non‐virulent strains may be useful in future research on M. viscosa virulence, as construction of mutants has not been successful.