340
There has been no diffi culty achieving experimental infections of fi sh with Edw.
tarda. Thus, using channel catfi sh, Meyer and Bullock ( 1973 ) established an infection by using the i.p. route of administration. At a water temperature of 27 °C, deaths
in 80 % of the population of 5–10 cm long fi ngerling fi sh followed within 10 days
of injecting an artifi cially high dose of 8.0 × 10
7 cells. Similar experiments, with
8 × 10
6 and 8 × 10
5 cells resulted in only 40 % cumulative mortalities within 10 days.
However, these workers pointed to the host specifi city of Edw. tarda , insofar as
brown trout, held at a water temperature of 13 °C, did not show any mortalities following injection with the pathogen. Subsequent investigations, however, showed
that Edw. tarda could indeed infect salmonids. Thus Amandi et al. ( 1982 ) demonstrated that the LD 50 for chinook salmon and steelhead trout, was 4.1 × 10
6 and
5.6 × 10
6 cells, respectively. It is worth emphasising that ictalurids were determined
to be more sensitive than salmonids, insofar as these workers determined the LD 50
for channel catfi sh to be only 4.0 × 10
5 cells. At lower water temperatures, e.g.
12 °C, the LD 50 was in the region of one order of magnitude higher. Ironically, a
water-borne challenge proved to be a failure, although this may refl ect the inability
of cells, grown in nutrient-rich media and therefore not typical of the natural physiological state, to survive in the aquatic environment. Nevertheless, Song et al.
( 1982 ) deduced that of 110 isolates, the lowest LD 50 (by water-borne challenge) was
3.1 × 10
7 cells/ml. Clearly, this is not conducive to the notion of a particularly virulent pathogen.
Using immersion, oral, and i.p. and i.m. routes, Mekuchi et al. ( 1995a ) succeeded
in infecting and killing Japanese fl ounder (by all routes). The LD 50 dose was calculated at 7.1 × 10
1 /fi sh (via i.m. injection), 1.2 × 10
2 /fi sh (via the i.p. route), 3.6 × 10
6 /
fi sh (by immersion) and 1.3 × 10
6 /fi sh (orally) (Mekuchi et al. 1995a )
The pathogenic mechanisms were investigated by Ullah and Arai ( 1983a , b ), who
reported that in contrast to Edw. ictaluri, pili were absent. Instead, cells were
observed to be surrounded by a slime layer. This may help with the adhesion to host
cells, and also protect the bacteria from host defences. Conversely, Sakai et al ( 2004 )
pointed to a role for fi mbriae on haemagglutination (this could also be induced or
increased by 3 % w/v NaCl; Yasunobu et al. 2006 ). A relation between motility and
virulence was indicated by Matsuyama et al. ( 2005 ), who observed that non-motile
(and not motile) cells were pathogenic by i.p. injection to red sea bream (there was
not any difference between motile and non-motile cultures recorded in Japanese
fl ounder or yellowtail). Following immersion challenge, Japanese fl ounder and red
sea bream died as exposure to motile and non-motile cultures, respectively
(Matsuyama et al. 2005 ). Opsonised virulent, but not avirulent, cells adhered, survived and even replicated in phagocytes whereas non opsonised avirulent cells
could also replicate intracellularly in phagocytes (Rao et al. 2001 ). Furthermore,
only avirulent cells enabled higher production of ROS intermediates by phagocytes,
which suggests that they are more likely to be susceptible to inactivation by this
means (Rao et al. 2001 ). A link to LPS, namely the O-antigen ligase gene waaL, has
been made insofar as deletions mutants were attenuated in terms of virulence, and
with markedly increased LD 50 dose (Xu et al. 2010 ). Furthermore, a comparison of
virulent and avirulent cultures pointed to the importance of LPS in pathogenicity,
6 Enterobacteriaceae Representatives
There has been no diffi culty achieving experimental infections of fi sh with Edw.
tarda. Thus, using channel catfi sh, Meyer and Bullock ( 1973 ) established an infection by using the i.p. route of administration. At a water temperature of 27 °C, deaths
in 80 % of the population of 5–10 cm long fi ngerling fi sh followed within 10 days
of injecting an artifi cially high dose of 8.0 × 10
7 cells. Similar experiments, with
8 × 10
6 and 8 × 10
5 cells resulted in only 40 % cumulative mortalities within 10 days.
However, these workers pointed to the host specifi city of Edw. tarda , insofar as
brown trout, held at a water temperature of 13 °C, did not show any mortalities following injection with the pathogen. Subsequent investigations, however, showed
that Edw. tarda could indeed infect salmonids. Thus Amandi et al. ( 1982 ) demonstrated that the LD 50 for chinook salmon and steelhead trout, was 4.1 × 10
6 and
5.6 × 10
6 cells, respectively. It is worth emphasising that ictalurids were determined
to be more sensitive than salmonids, insofar as these workers determined the LD 50
for channel catfi sh to be only 4.0 × 10
5 cells. At lower water temperatures, e.g.
12 °C, the LD 50 was in the region of one order of magnitude higher. Ironically, a
water-borne challenge proved to be a failure, although this may refl ect the inability
of cells, grown in nutrient-rich media and therefore not typical of the natural physiological state, to survive in the aquatic environment. Nevertheless, Song et al.
( 1982 ) deduced that of 110 isolates, the lowest LD 50 (by water-borne challenge) was
3.1 × 10
7 cells/ml. Clearly, this is not conducive to the notion of a particularly virulent pathogen.
Using immersion, oral, and i.p. and i.m. routes, Mekuchi et al. ( 1995a ) succeeded
in infecting and killing Japanese fl ounder (by all routes). The LD 50 dose was calculated at 7.1 × 10
1 /fi sh (via i.m. injection), 1.2 × 10
2 /fi sh (via the i.p. route), 3.6 × 10
6 /
fi sh (by immersion) and 1.3 × 10
6 /fi sh (orally) (Mekuchi et al. 1995a )
The pathogenic mechanisms were investigated by Ullah and Arai ( 1983a , b ), who
reported that in contrast to Edw. ictaluri, pili were absent. Instead, cells were
observed to be surrounded by a slime layer. This may help with the adhesion to host
cells, and also protect the bacteria from host defences. Conversely, Sakai et al ( 2004 )
pointed to a role for fi mbriae on haemagglutination (this could also be induced or
increased by 3 % w/v NaCl; Yasunobu et al. 2006 ). A relation between motility and
virulence was indicated by Matsuyama et al. ( 2005 ), who observed that non-motile
(and not motile) cells were pathogenic by i.p. injection to red sea bream (there was
not any difference between motile and non-motile cultures recorded in Japanese
fl ounder or yellowtail). Following immersion challenge, Japanese fl ounder and red
sea bream died as exposure to motile and non-motile cultures, respectively
(Matsuyama et al. 2005 ). Opsonised virulent, but not avirulent, cells adhered, survived and even replicated in phagocytes whereas non opsonised avirulent cells
could also replicate intracellularly in phagocytes (Rao et al. 2001 ). Furthermore,
only avirulent cells enabled higher production of ROS intermediates by phagocytes,
which suggests that they are more likely to be susceptible to inactivation by this
means (Rao et al. 2001 ). A link to LPS, namely the O-antigen ligase gene waaL, has
been made insofar as deletions mutants were attenuated in terms of virulence, and
with markedly increased LD 50 dose (Xu et al. 2010 ). Furthermore, a comparison of
virulent and avirulent cultures pointed to the importance of LPS in pathogenicity,
6 Enterobacteriaceae Representatives