430
post- hatch (Eckman et al. 2003 ). Moreover, bathing rainbow trout fry for 24 in a
logarithmic- phase culture, i.e. 24 h, led to higher mortalities than use of other phases
of the culture growth cycle (Aoki et al. 2005 ). Bathing (also with stress caused by
treatment with formalin) and i.p. injection of 10
4 CFU/rainbow trout (average
weight = 1 g) [10
7 cells were needed for larger fi sh] led to reproducible infections,
with results refl ecting the nature of the bacterial strain, the stocking density, and the
source and weight of the fi sh (Madsen and Dalsgaard 1999 ). Bathing yellowtail in a
suspension of the pathogen led quickly to adherence of the organism to external
surfaces, i.e. lower jaw, pectoral fi n, gills and skin. Then, there was an increase in
pathogen numbers in the gills followed by increase internally, i.e. in the blood, kidney and spleen (Nagano et al. 2011a ). These workers concluded that gills are the
primary target for invasion by the pathogen (Nagano et al. 2011a , b). Abrasion of
skin and its mucus enhanced invasion of Fla. psychrophilum via immersion and
cohabitation, with the shedding rate from infected fi sh refl ecting water temperature,
i.e. 15 rather than 4 °C, and the presence of dead fi sh (Madetoja et al. 2000 ). Highly
virulent cultures are better capable of adhering to intestinal explants and gill tissue
in gill perfusion models from rainbow trout than less virulent isolates (Nematollahi
et al. 2003 , 2005b ). The presence of 2 g/l of organic material or 5 mg/l of nitrite
increased adhesion to gills (Nematollahi et al. 2003 ). Adherence and temperaturemediated (greater at 5 than 15 °C) agglutination of yeast cells and erythrocytes as a
function of serology, i.e. serotype, has been documented, with haemagglutination
inhibited by sialic acid, heating to 65 °C or treatment with proteinase K (Møller
et al. 2003 ). Electron microscopy of haemagglutinating and non-haemagglutinating
cultures revealed the presence of a thin capsule (in both types of cultures), an
absence of pili, but the presence of long, tubular blebs particularly in iron-restricted
media that released membrane vesicles into the supernatant. Only the membrane
vesicles of haemagglutinating cultures had haemagglutinating activity. The conclusion reached by the workers (Møller et al. 2005 ) was that via surface blebbing the
pathogen releases membrane vesicles with some proteolytic activity that may somehow impede the immune response of the host.
Following infection, the bacteria may be seen embedded in the mucus along the
fi ns which is where necrosis starts (Martínez et al. 2004 ). Although the precise
pathogenicity mechanism of the organism eluded scientists for many years, it is now
appreciated that Fla. psychrophilum produces extracellular components, including
the ability to degrade gelatin and type II but not type I and IV collagen and has zinc
metalloprotease like activity (Ostland et al. 2000 ) which in other organisms would
be associated with virulence. Moreover, a psychrophilic metalloprotease, termed
Fpp2 and with a molecular weight of ~62 kDa, has been described (Secades et al.
2001 , 2003 ). This protease is different from Fpp1, which is a 55 kDa metalloprotease and cleaved actin and myosin, i.e. components of muscle (Secades et al. 2003 ).
An OMP of 18 kDa, termed P18, was described, and linked with the S-layer
(Massius et al. 2004). Once inside fi sh, Fla. psychrophilum appears to associate
with kidney macrophages for which there is a role for sialic acid in the binding
process (Wiklund and Dalsgaard 2003 ). Moreover, the pathogen has been found to
become internalized in spleen phagocytes of rainbow trout, with bacterial numbers
7 Flavobacteria and Cytophagas
post- hatch (Eckman et al. 2003 ). Moreover, bathing rainbow trout fry for 24 in a
logarithmic- phase culture, i.e. 24 h, led to higher mortalities than use of other phases
of the culture growth cycle (Aoki et al. 2005 ). Bathing (also with stress caused by
treatment with formalin) and i.p. injection of 10
4 CFU/rainbow trout (average
weight = 1 g) [10
7 cells were needed for larger fi sh] led to reproducible infections,
with results refl ecting the nature of the bacterial strain, the stocking density, and the
source and weight of the fi sh (Madsen and Dalsgaard 1999 ). Bathing yellowtail in a
suspension of the pathogen led quickly to adherence of the organism to external
surfaces, i.e. lower jaw, pectoral fi n, gills and skin. Then, there was an increase in
pathogen numbers in the gills followed by increase internally, i.e. in the blood, kidney and spleen (Nagano et al. 2011a ). These workers concluded that gills are the
primary target for invasion by the pathogen (Nagano et al. 2011a , b). Abrasion of
skin and its mucus enhanced invasion of Fla. psychrophilum via immersion and
cohabitation, with the shedding rate from infected fi sh refl ecting water temperature,
i.e. 15 rather than 4 °C, and the presence of dead fi sh (Madetoja et al. 2000 ). Highly
virulent cultures are better capable of adhering to intestinal explants and gill tissue
in gill perfusion models from rainbow trout than less virulent isolates (Nematollahi
et al. 2003 , 2005b ). The presence of 2 g/l of organic material or 5 mg/l of nitrite
increased adhesion to gills (Nematollahi et al. 2003 ). Adherence and temperaturemediated (greater at 5 than 15 °C) agglutination of yeast cells and erythrocytes as a
function of serology, i.e. serotype, has been documented, with haemagglutination
inhibited by sialic acid, heating to 65 °C or treatment with proteinase K (Møller
et al. 2003 ). Electron microscopy of haemagglutinating and non-haemagglutinating
cultures revealed the presence of a thin capsule (in both types of cultures), an
absence of pili, but the presence of long, tubular blebs particularly in iron-restricted
media that released membrane vesicles into the supernatant. Only the membrane
vesicles of haemagglutinating cultures had haemagglutinating activity. The conclusion reached by the workers (Møller et al. 2005 ) was that via surface blebbing the
pathogen releases membrane vesicles with some proteolytic activity that may somehow impede the immune response of the host.
Following infection, the bacteria may be seen embedded in the mucus along the
fi ns which is where necrosis starts (Martínez et al. 2004 ). Although the precise
pathogenicity mechanism of the organism eluded scientists for many years, it is now
appreciated that Fla. psychrophilum produces extracellular components, including
the ability to degrade gelatin and type II but not type I and IV collagen and has zinc
metalloprotease like activity (Ostland et al. 2000 ) which in other organisms would
be associated with virulence. Moreover, a psychrophilic metalloprotease, termed
Fpp2 and with a molecular weight of ~62 kDa, has been described (Secades et al.
2001 , 2003 ). This protease is different from Fpp1, which is a 55 kDa metalloprotease and cleaved actin and myosin, i.e. components of muscle (Secades et al. 2003 ).
An OMP of 18 kDa, termed P18, was described, and linked with the S-layer
(Massius et al. 2004). Once inside fi sh, Fla. psychrophilum appears to associate
with kidney macrophages for which there is a role for sialic acid in the binding
process (Wiklund and Dalsgaard 2003 ). Moreover, the pathogen has been found to
become internalized in spleen phagocytes of rainbow trout, with bacterial numbers
7 Flavobacteria and Cytophagas