328
septicaemia of catfi sh was determined to be most similar to Edw. tarda, i.e. 56–62 %
DNA homology, but suffi ciently distinct to warrant separate species recognition
(Hawke et al. 1981 ). Yet, comparison of 23S rRNA gene sequences confi rm a close
phylogenetic relationship between the two species (Zhang and Arias 2007 ). It
remains for further work to elucidate the relationship, if any, with Edwardsiella
phenon 12 of Johnson et al. ( 1975 ).
Cryptic plasmids, i.e. pCL1 and pCL2 of 5.7 kb and 4.9 kb respectively, have
been found in isolates of Edw. ictaluri (Lobb et al. 1993 ). However, an isolate from
green knife fi sh differed insofar as 4 plasmids, of 3.1, 4.1, 5.7 and 6.0 kb, were present. Of these, the 4.1 and 5.7 kb plasmids hybridised strongly (Lobb et al. 1993 ).
Diagnosis
Plasmid Profi ling A novel diagnostic approach concerns determination of plasmid
profi les for Edw. ictaluri (Lobb and Rhodes 1987 ; Speyerer and Boyle 1987 ).
Serology iFAT with monoclonal antibodies in an enzyme immunoassay has shown
promise for the recognition of Edw. ictaluri (Rogers 1981 ; Ainsworth et al. 1986 ).
A relevant development has been the use of a rapid iFAT to simultaneously detect
two pathogens, i.e. Edw. ictaluri and Fla. columnare using fl uorochromes with two
different spectra properties, Alexa Fluor 488 and 594 emitting green and red fl uorescence, respectively (Panangala et al. 2006 ). An indirect ELISA has been effective
at detecting the presence of antibodies to Edw. ictaluri in fi sh serum (Waterstrat
et al. 1989 ; Swain and Nayak 2003 ). A development of this approach involved the
use of tissue homogenisation (using 0.5 % v/v Triton X-100 in 0.05 M PBS [pH 7.2]),
fi ltration and then the ELISA (Earlix et al. 1996 ). This approach was used successfully to detect asymptomatic carriers, and permitted live bacteria to be fi ltered from
1 g quantities of tissue slurries, with a sensitivity of <10 colony forming units/g of
tissue. The fi lter-ELISA system detected Edw. ictaluri in 80 % of 98 channel catfi sh
compared to a detection of 24 % by culturing (Earlix et al. 1996 ).
Molecular Methods Real time PCR, which produces a result in 4–5 h, has been
developed for Edw. ictaluri, and detected the equivalent of 2.5 cells using DNA
samples from cultures and fi sh blood (Bilodeau et al. 2003). A later study confi rmed
a PCR for the detection of Edw. ictaluri and its differentiation from Edw. tarda
(Williams and Lawrence 2009 ).
Epizootiology
The natural reservoir for infection has not yet been fi rmly established. It is known
that the organism survives for limited periods, i.e. up to 8 days, in sterile pond water
(Hawke 1979 ). This suggests that Edw. ictaluri has only limited ability to survive in
6 Enterobacteriaceae Representatives
septicaemia of catfi sh was determined to be most similar to Edw. tarda, i.e. 56–62 %
DNA homology, but suffi ciently distinct to warrant separate species recognition
(Hawke et al. 1981 ). Yet, comparison of 23S rRNA gene sequences confi rm a close
phylogenetic relationship between the two species (Zhang and Arias 2007 ). It
remains for further work to elucidate the relationship, if any, with Edwardsiella
phenon 12 of Johnson et al. ( 1975 ).
Cryptic plasmids, i.e. pCL1 and pCL2 of 5.7 kb and 4.9 kb respectively, have
been found in isolates of Edw. ictaluri (Lobb et al. 1993 ). However, an isolate from
green knife fi sh differed insofar as 4 plasmids, of 3.1, 4.1, 5.7 and 6.0 kb, were present. Of these, the 4.1 and 5.7 kb plasmids hybridised strongly (Lobb et al. 1993 ).
Diagnosis
Plasmid Profi ling A novel diagnostic approach concerns determination of plasmid
profi les for Edw. ictaluri (Lobb and Rhodes 1987 ; Speyerer and Boyle 1987 ).
Serology iFAT with monoclonal antibodies in an enzyme immunoassay has shown
promise for the recognition of Edw. ictaluri (Rogers 1981 ; Ainsworth et al. 1986 ).
A relevant development has been the use of a rapid iFAT to simultaneously detect
two pathogens, i.e. Edw. ictaluri and Fla. columnare using fl uorochromes with two
different spectra properties, Alexa Fluor 488 and 594 emitting green and red fl uorescence, respectively (Panangala et al. 2006 ). An indirect ELISA has been effective
at detecting the presence of antibodies to Edw. ictaluri in fi sh serum (Waterstrat
et al. 1989 ; Swain and Nayak 2003 ). A development of this approach involved the
use of tissue homogenisation (using 0.5 % v/v Triton X-100 in 0.05 M PBS [pH 7.2]),
fi ltration and then the ELISA (Earlix et al. 1996 ). This approach was used successfully to detect asymptomatic carriers, and permitted live bacteria to be fi ltered from
1 g quantities of tissue slurries, with a sensitivity of <10 colony forming units/g of
tissue. The fi lter-ELISA system detected Edw. ictaluri in 80 % of 98 channel catfi sh
compared to a detection of 24 % by culturing (Earlix et al. 1996 ).
Molecular Methods Real time PCR, which produces a result in 4–5 h, has been
developed for Edw. ictaluri, and detected the equivalent of 2.5 cells using DNA
samples from cultures and fi sh blood (Bilodeau et al. 2003). A later study confi rmed
a PCR for the detection of Edw. ictaluri and its differentiation from Edw. tarda
(Williams and Lawrence 2009 ).
Epizootiology
The natural reservoir for infection has not yet been fi rmly established. It is known
that the organism survives for limited periods, i.e. up to 8 days, in sterile pond water
(Hawke 1979 ). This suggests that Edw. ictaluri has only limited ability to survive in
6 Enterobacteriaceae Representatives