533
heightened when the bacterial cells were starved for 2 and 8 days (Larsen et al.
2004 ) . Evidence points to a 40.1 kDa fl agellin A protein (encoded by the fl aA gene)
being essential for virulence (Milton et al. 1996 ). Thus, loss of fl agella by transposon mutagenesis led to a 500-fold reduction in virulence following an immersion
challenge (O’Toole et al. 1996 ). Flagellum production and virulence by the
water- borne but not i.p. route was correlated with RpoN (O’Toole et al. 1997 ). As a
cautionary note, it is possible that other changes to the bacterial cells may have
occurred with the loss of fl agella. Ransom ( 1978 ) postulated that infection probably
begins with colonisation of the posterior gastro-intestinal tract and rectum. This
conclusion resulted from the observation that V. anguillarum was seen initially in
these sites. Using GFP-labelled V. anguillarum cells and immersion challenge, the
gastrointestinal tract of zebra fi sh was the fi rst site where the pathogen was observed,
with chemotactic motility being regarded as essential for the association with the
host surface (O’Toole et al. 2004 ). Horne and Baxendale ( 1983 ) reported adhesion
of V. anguillarum to intestinal sections derived from rainbow trout. All regions of
the intestine were colonised (approximately 10
3 cells/cm
2
), with maximum attachment occurring within 100 min. Using gnotobiotic sea bass larvae and 10
8 CFU/ ml
of GFP-labelled V. anguillarum HI-610 serovar O2a , which was originally isolated
from cod, Rekecki et al. ( 2012 ) observed colonisation of gut enterocytes within 2 h
or oral uptake; it took 48 h for the bacterial cells to be recognised in the swim bladder. V. anguillarum made contact with the host in the oesophageal mucosa, with
likely attachment to the microvilli of the mid- and hindgut enterocytes (Rekecki
et al. 2012 ). V. anguillarum cells were detected in the gut of experimentally infected
gnotobiotic sea bass larvae by TEM and immunogold labelling. In particular, bacteria were observed to be in close contact with the apical brush border in the gut
lumen. Some luminal cells were invaginated, and demonstrated likely engulfment
of the pathogen by pseudopod-like formations. Immunogold positive thread-like
structures secreted by the pathogen were suggestive of the presence of outer membrane vesicles, which may transport virulence factors to gut cells (Rekecki et al.
2013 ). It is interesting to note that serogroup O1 but not O2 isolated demonstrated
the ability to adhere to mucus from Atlantic salmon epithelial surfaces, i.e. foregut,
gills, hindgut, pyloric caeca and skin (Knudsen et al. 1999 ). Orally administered V.
anguillarum survived in the stomach of juvenile turbot for several hours, persisted
in the intestine, and proliferated in faeces (Olsson et al. 1998 ). This view has been
reinforced by a study, which concluded that >50 % of the spleens of turbot contained cells of V. anguillarum after infection via the oral and rectal routes (Olsson
et al. 1996 ). The skin appears to become colonised within 12 h of immersion in a
virulent culture (Kanno et al. 1990 ). Then, invasion of the liver, spleen, muscle, gills
and intestine follows (Muroga and De La Cruz 1987 ). Resistance to the potential
debilitating effect of fi sh serum (Trust et al. 1981 ) may hasten the invasion processes. Some degree of host specifi city has been indicated, insofar as strains from
rainbow trout were poorly pathogenic to saithe, and vice versa (Egidius and
Andersen 1978 ). This raises the question concerning the size of inoculum necessary
to achieve clinical disease. Levine et al. ( 1972 ) reported lesions at the site of infection in winter fl ounder after exposure to only 640 cells. These were administered by
Vibrio anguillarum
heightened when the bacterial cells were starved for 2 and 8 days (Larsen et al.
2004 ) . Evidence points to a 40.1 kDa fl agellin A protein (encoded by the fl aA gene)
being essential for virulence (Milton et al. 1996 ). Thus, loss of fl agella by transposon mutagenesis led to a 500-fold reduction in virulence following an immersion
challenge (O’Toole et al. 1996 ). Flagellum production and virulence by the
water- borne but not i.p. route was correlated with RpoN (O’Toole et al. 1997 ). As a
cautionary note, it is possible that other changes to the bacterial cells may have
occurred with the loss of fl agella. Ransom ( 1978 ) postulated that infection probably
begins with colonisation of the posterior gastro-intestinal tract and rectum. This
conclusion resulted from the observation that V. anguillarum was seen initially in
these sites. Using GFP-labelled V. anguillarum cells and immersion challenge, the
gastrointestinal tract of zebra fi sh was the fi rst site where the pathogen was observed,
with chemotactic motility being regarded as essential for the association with the
host surface (O’Toole et al. 2004 ). Horne and Baxendale ( 1983 ) reported adhesion
of V. anguillarum to intestinal sections derived from rainbow trout. All regions of
the intestine were colonised (approximately 10
3 cells/cm
2
), with maximum attachment occurring within 100 min. Using gnotobiotic sea bass larvae and 10
8 CFU/ ml
of GFP-labelled V. anguillarum HI-610 serovar O2a , which was originally isolated
from cod, Rekecki et al. ( 2012 ) observed colonisation of gut enterocytes within 2 h
or oral uptake; it took 48 h for the bacterial cells to be recognised in the swim bladder. V. anguillarum made contact with the host in the oesophageal mucosa, with
likely attachment to the microvilli of the mid- and hindgut enterocytes (Rekecki
et al. 2012 ). V. anguillarum cells were detected in the gut of experimentally infected
gnotobiotic sea bass larvae by TEM and immunogold labelling. In particular, bacteria were observed to be in close contact with the apical brush border in the gut
lumen. Some luminal cells were invaginated, and demonstrated likely engulfment
of the pathogen by pseudopod-like formations. Immunogold positive thread-like
structures secreted by the pathogen were suggestive of the presence of outer membrane vesicles, which may transport virulence factors to gut cells (Rekecki et al.
2013 ). It is interesting to note that serogroup O1 but not O2 isolated demonstrated
the ability to adhere to mucus from Atlantic salmon epithelial surfaces, i.e. foregut,
gills, hindgut, pyloric caeca and skin (Knudsen et al. 1999 ). Orally administered V.
anguillarum survived in the stomach of juvenile turbot for several hours, persisted
in the intestine, and proliferated in faeces (Olsson et al. 1998 ). This view has been
reinforced by a study, which concluded that >50 % of the spleens of turbot contained cells of V. anguillarum after infection via the oral and rectal routes (Olsson
et al. 1996 ). The skin appears to become colonised within 12 h of immersion in a
virulent culture (Kanno et al. 1990 ). Then, invasion of the liver, spleen, muscle, gills
and intestine follows (Muroga and De La Cruz 1987 ). Resistance to the potential
debilitating effect of fi sh serum (Trust et al. 1981 ) may hasten the invasion processes. Some degree of host specifi city has been indicated, insofar as strains from
rainbow trout were poorly pathogenic to saithe, and vice versa (Egidius and
Andersen 1978 ). This raises the question concerning the size of inoculum necessary
to achieve clinical disease. Levine et al. ( 1972 ) reported lesions at the site of infection in winter fl ounder after exposure to only 640 cells. These were administered by
Vibrio anguillarum