369
The precise taxonomic position of the causal agent of ERM has intrigued bacteriologists since the initial isolation of the organism. Ross et al. ( 1966 ) realised that
heated O-antigens prepared from 14 cultures of the ERM organism agglutinated
strongly (titre = 1:320 or 1:640) with the corresponding antigens of Sal. enterica
subsp. arizonae O group 26, and weakly (titre = 1:20) with O group 29. Conversely,
there was no reaction with O-antigens prepared from Salmonella, Ent. liquefaciens,
Citrobacter or Serratia. In addition, this team pointed to the biochemical similarities with Ent. liquefaciens, Ser. marcescens subsp . kiliensis and Sal. enterica subsp.
arizonae. Furthermore, Stevenson and Daly ( 1982 ) indicated serological crossreactions with Haf. alvei . However after an examination of the phenotypic and
molecular traits of 33 isolates, Ewing et al. ( 1978 ) elevated the pathogen to species
status, as Y. ruckeri. It is interesting to note that the pathogen was included in the
genus Yersinia because of only a 30–31 % DNA homology to Y. enterocolitica and
Y. pseudotuberculosis. This compares to DNA homologies of 24–28 % and 31 %
with Ser. marcescens and Ser. liquefaciens, respectively (Steigerwalt et al. 1976 ).
Therefore from the DNA hybridisation experiments, it is diffi cult to determine the
reasons for including the ERM organism with Yersinia rather than Serratia.
However, it would be relevant to extend the existing data bases by comparing the
homology with Sal. enterica subsp. arizonae. This is especially relevant because
Green and Austin ( 1982 ) have shown a greater phenotypic relationship of Y. ruckeri
with Sal. enterica subsp. arizonae than with Y. enterocolitica or Y. pseudotuberculosis. In fact, the causal agent of ERM may belong in a new genus of the
Enterobacteriaceae, an idea which has been mooted by Bercovier and Mollaret
( 1984 ). The distinctiveness was confi rmed by an examination of phylogeny based
on 16S rRNA sequences, when Y. ruckeri formed a distinct node, albeit associated
with other yersinias (Ibrahim et al. 1993 ).
Whereas the taxon is phenotypically homogeneous, it is serologically diverse
insofar as fi ve serotypes have been recognised (O’Leary 1977 ; Bullock et al. 1978 ;
Stevenson and Airdrie 1984a ). Three serotypes have been referred to colloquially as
the “Hagerman strain” (the most common and the most virulent), the ‘Big Creek
strain’ (relatively avirulent) and the ‘Australian strain’ (appears to be avirulent)
(Busch 1981 ). These serotypes have been designated as Type 1 (Hagerman), Type 2
(O’Leary) and Type 3 (Australian) (Bullock and Anderson 1984 ) and the newly
described serovars IV and V (Stevenson and Airdrie 1984a ). Serotype 2 may be
distinguished from serotype 1 by its ability to ferment sorbitol (O’Leary 1977 ).
However, caution needs to be advocated since this may well be a plasmid-mediated
trait. Nevertheless, serotypes/serovars 2, 3, 4 and 5 appear to be highly related in
terms of DNA homology (De Grandis et al. 1988 ). Also, two LPS profi les have been
recognised among 23 Portuguese isolates, corresponding to serotypes O1 and O3.
Greater heterogeneity was recorded for OMP, with 7 profi les recognised, and there
was 10 ribotypes, 6 of which accommodated serotype O1 isolates (Sousa et al.
2001 ).
During the early 1990’s in England, a new form of the pathogen was recovered
among rainbow trout that had been previously vaccinated by immersion with commercial enteric redmouth vaccines. This form, which was initially considered to
Yersinia ruckeri
The precise taxonomic position of the causal agent of ERM has intrigued bacteriologists since the initial isolation of the organism. Ross et al. ( 1966 ) realised that
heated O-antigens prepared from 14 cultures of the ERM organism agglutinated
strongly (titre = 1:320 or 1:640) with the corresponding antigens of Sal. enterica
subsp. arizonae O group 26, and weakly (titre = 1:20) with O group 29. Conversely,
there was no reaction with O-antigens prepared from Salmonella, Ent. liquefaciens,
Citrobacter or Serratia. In addition, this team pointed to the biochemical similarities with Ent. liquefaciens, Ser. marcescens subsp . kiliensis and Sal. enterica subsp.
arizonae. Furthermore, Stevenson and Daly ( 1982 ) indicated serological crossreactions with Haf. alvei . However after an examination of the phenotypic and
molecular traits of 33 isolates, Ewing et al. ( 1978 ) elevated the pathogen to species
status, as Y. ruckeri. It is interesting to note that the pathogen was included in the
genus Yersinia because of only a 30–31 % DNA homology to Y. enterocolitica and
Y. pseudotuberculosis. This compares to DNA homologies of 24–28 % and 31 %
with Ser. marcescens and Ser. liquefaciens, respectively (Steigerwalt et al. 1976 ).
Therefore from the DNA hybridisation experiments, it is diffi cult to determine the
reasons for including the ERM organism with Yersinia rather than Serratia.
However, it would be relevant to extend the existing data bases by comparing the
homology with Sal. enterica subsp. arizonae. This is especially relevant because
Green and Austin ( 1982 ) have shown a greater phenotypic relationship of Y. ruckeri
with Sal. enterica subsp. arizonae than with Y. enterocolitica or Y. pseudotuberculosis. In fact, the causal agent of ERM may belong in a new genus of the
Enterobacteriaceae, an idea which has been mooted by Bercovier and Mollaret
( 1984 ). The distinctiveness was confi rmed by an examination of phylogeny based
on 16S rRNA sequences, when Y. ruckeri formed a distinct node, albeit associated
with other yersinias (Ibrahim et al. 1993 ).
Whereas the taxon is phenotypically homogeneous, it is serologically diverse
insofar as fi ve serotypes have been recognised (O’Leary 1977 ; Bullock et al. 1978 ;
Stevenson and Airdrie 1984a ). Three serotypes have been referred to colloquially as
the “Hagerman strain” (the most common and the most virulent), the ‘Big Creek
strain’ (relatively avirulent) and the ‘Australian strain’ (appears to be avirulent)
(Busch 1981 ). These serotypes have been designated as Type 1 (Hagerman), Type 2
(O’Leary) and Type 3 (Australian) (Bullock and Anderson 1984 ) and the newly
described serovars IV and V (Stevenson and Airdrie 1984a ). Serotype 2 may be
distinguished from serotype 1 by its ability to ferment sorbitol (O’Leary 1977 ).
However, caution needs to be advocated since this may well be a plasmid-mediated
trait. Nevertheless, serotypes/serovars 2, 3, 4 and 5 appear to be highly related in
terms of DNA homology (De Grandis et al. 1988 ). Also, two LPS profi les have been
recognised among 23 Portuguese isolates, corresponding to serotypes O1 and O3.
Greater heterogeneity was recorded for OMP, with 7 profi les recognised, and there
was 10 ribotypes, 6 of which accommodated serotype O1 isolates (Sousa et al.
2001 ).
During the early 1990’s in England, a new form of the pathogen was recovered
among rainbow trout that had been previously vaccinated by immersion with commercial enteric redmouth vaccines. This form, which was initially considered to
Yersinia ruckeri