233
isolates were very closely related, with minimal divergence. It is a pity that the exact
homology values were not presented.
In a study of 26 typical and atypical isolates by DNA:DNA re-association methods, Belland and Trust ( 1988 ) found that typical isolates were recovered in a homogeneous group, whereas the atypical representatives were more diverse. Of these,
one biotype consisted of isolates obtained from goldfi sh (obtained from a wide geographical range), whereas the second group accommodated isolates derived from
carp in Europe. From the results of a numerical taxonomic and DNA:DNA hybridisation study, Austin et al. ( 1989 ) made similar conclusions regarding the homogeneity of typical isolates of Aer. salmonicida. However using 16S rRNA sequencing
techniques, Martínez-Murcia et al. ( 1992 ) reported that subspecies achromogenes
and masoucida were indistinguishable, and only differed from subspecies salmonicida by two bases.
There is overwhelming evidence that the so-called ‘atypical’ isolates are distinct
from typical Aer. salmonicida (e.g. Hänninen and Hirvelä-Koski 1997 ; Austin et al.
1998 ; Wiklund and Dalsgaard 1998 ; Umelo and Trust 1998 ). Yet, it has so far
proved to be impossible to include the atypical isolates into a meaningful classifi cation. Moreover, there has been incongruence reported between the results of molecular (PCR, RAPD and ribotyping and phenotypic methods, in terms of group
membership (Austin et al. 1998 ). Høi et al. ( 1999 ) recognised 4 PCR groups among
205 atypical isolates. The problems of inter-laboratory differences and lack of standardisation in test methods has been highlighted by Dalsgaard et al. ( 1998 ). Some
studies have indicated homogeneity among atypical isolates; a sentiment which is
not endorsed by others. For example, Kwon et al. 1997 ) carried out a RAPD analyses of 29 atypical isolates from 8 species of fi sh in Japan, and concluded that the
profi ling was identical, thereby indicating genetic homogeneity. However, heterogeneity was apparent between these atypical isolates and reference cultures from the
validly described subspecies (Kwon et al. 1997 ); a notion which has been confi rmed
by others (e.g. Austin et al. 1998 ). After studying 51 isolates from Finland by ribotyping, plasmid profi ling and phenotyping, it was concluded that pigment- producing
strains could be separated from achromogenic cultures. Also, oxidase-negative isolates were distinct from oxidase-positive atypical isolates in terms of ribotypes and
phenotypes (Hänninen and Hirvelä-Koski 1997 ).
There have been several reports of acid production in sucrose fermentation tests
among typical isolates (see Fryer et al. 1988 ; Wiklund et al. 1992 ). This is relevant
because hitherto this was one of the tests used to differentiate typical from atypical
isolates of the pathogen (Martin-Carnahan and Joseph 2005 ).
Plasmid Profi les of Aeromonas salmonicida
Plasmids carried by typical (14 strains) and atypical (11 strains) forms of Aer. salmonicida have also provided additional genetic evidence for the classifi cation of
typical and atypical isolates into separate taxa (Belland and Trust 1989 ). These
Aeromonas salmonicida
isolates were very closely related, with minimal divergence. It is a pity that the exact
homology values were not presented.
In a study of 26 typical and atypical isolates by DNA:DNA re-association methods, Belland and Trust ( 1988 ) found that typical isolates were recovered in a homogeneous group, whereas the atypical representatives were more diverse. Of these,
one biotype consisted of isolates obtained from goldfi sh (obtained from a wide geographical range), whereas the second group accommodated isolates derived from
carp in Europe. From the results of a numerical taxonomic and DNA:DNA hybridisation study, Austin et al. ( 1989 ) made similar conclusions regarding the homogeneity of typical isolates of Aer. salmonicida. However using 16S rRNA sequencing
techniques, Martínez-Murcia et al. ( 1992 ) reported that subspecies achromogenes
and masoucida were indistinguishable, and only differed from subspecies salmonicida by two bases.
There is overwhelming evidence that the so-called ‘atypical’ isolates are distinct
from typical Aer. salmonicida (e.g. Hänninen and Hirvelä-Koski 1997 ; Austin et al.
1998 ; Wiklund and Dalsgaard 1998 ; Umelo and Trust 1998 ). Yet, it has so far
proved to be impossible to include the atypical isolates into a meaningful classifi cation. Moreover, there has been incongruence reported between the results of molecular (PCR, RAPD and ribotyping and phenotypic methods, in terms of group
membership (Austin et al. 1998 ). Høi et al. ( 1999 ) recognised 4 PCR groups among
205 atypical isolates. The problems of inter-laboratory differences and lack of standardisation in test methods has been highlighted by Dalsgaard et al. ( 1998 ). Some
studies have indicated homogeneity among atypical isolates; a sentiment which is
not endorsed by others. For example, Kwon et al. 1997 ) carried out a RAPD analyses of 29 atypical isolates from 8 species of fi sh in Japan, and concluded that the
profi ling was identical, thereby indicating genetic homogeneity. However, heterogeneity was apparent between these atypical isolates and reference cultures from the
validly described subspecies (Kwon et al. 1997 ); a notion which has been confi rmed
by others (e.g. Austin et al. 1998 ). After studying 51 isolates from Finland by ribotyping, plasmid profi ling and phenotyping, it was concluded that pigment- producing
strains could be separated from achromogenic cultures. Also, oxidase-negative isolates were distinct from oxidase-positive atypical isolates in terms of ribotypes and
phenotypes (Hänninen and Hirvelä-Koski 1997 ).
There have been several reports of acid production in sucrose fermentation tests
among typical isolates (see Fryer et al. 1988 ; Wiklund et al. 1992 ). This is relevant
because hitherto this was one of the tests used to differentiate typical from atypical
isolates of the pathogen (Martin-Carnahan and Joseph 2005 ).
Plasmid Profi les of Aeromonas salmonicida
Plasmids carried by typical (14 strains) and atypical (11 strains) forms of Aer. salmonicida have also provided additional genetic evidence for the classifi cation of
typical and atypical isolates into separate taxa (Belland and Trust 1989 ). These
Aeromonas salmonicida