Flavobacterium psychrophilum, cause of ... - cop.eXtension.org

cytotoxic for the rainbow trout kidney phagocytes, which suggested that although the
adhesin may facilitate attachment, a phagocytic toxin was not necessary for virulence.
Borg (1960) originally concluded that F. psychrophilum did not produce extracellular
toxins, because cell-free extracts that he injected into fish did not produce mortality.
The proteolytic nature of F. psychrophilum, however, suggested that extracellular
proteases may contribute to the pathogenicity of this organism (Pacha 1968). The ability
to degrade elastin, for example, has been associated with virulence in some isolates
(Madsen and Dalsgaard 1999a) and such a heat stabile metalloprotease was shown to
induce severe muscle necrosis and necrotic myositis (Ostland et al. 2000).
Gross and microscopic lesions, which are similar to the lesions produced by injection of
viable bacteria, have been observed among steelhead trout injected with extracellular
products from this bacterium (Otis 1984). A 55-kDa psychrophilic metalloprotease
(Fpp1), which depended upon calcium ion concentration for activity and thermal stability,
was active between 25 to 40 o C but activity was greatly diminished at 45 o C (Secades et al.
2001). Cultures of F. psychrophilum produced lesser amounts of Fpp1 when incubated in
the presence of calcium at 18 o C versus 12 o C; a relationship that directly mimics the
correlation of temperature and the expression of this disease in nature. Furthermore,
calcium concentrations necessary for optimal in vitro induction of Fpp1 were similar to
those naturally found in the blood of fish. The protease cleaved gelatin, laminin,
fibrnectin, fibrinogen, collagen type IV, and to a leaser extent collagen. It also degraded
actin and myosin, which are basic elements of the musculature. Thus, Fpp1 may