234
workers found that typical isolates possessed a very homologous plasmid content
comprised of a single large (70–145 kb) plasmid and three low molecular weight
plasmids. Livesley et al. ( 1997 ) reported that 5 plasmids were most common among
the 18 isolates examined; Giles et al. ( 1995 ) found 4 or 6 plasmids with 4 smaller
plasmids of 4.3–8.1 kb being often observed in isolates from the Atlantic coast of
Canada, but 6 plasmids of 4.2–8.9 kb among cultures from the Pacifi c coast of
Canada. A total of 23 plasmids and 40 different plasmid profi les were recognised
among 124 isolates from Denmark, Norway, Scotland and North America (Nielsen
et al. 1993 ). An earlier theme was repeated insofar as all isolates had one large plasmid of 60–150 kb, and two low molecular weight plasmids of 5.2 and 5.4 kb. In
addition, two plasmids of 5.6 and 6.4 kb were frequently present (Nielsen et al.
1993 ). A larger investigation of 383 isolates over a 6 year period concluded that 1–4
plasmids of 52–105 mDa were inevitably present, casting doubt on the relevance of
plasmid typing for epizootiology (Sørum et al. 1993a , b ). Again, oxytetracycline
and streptomycin resistant isolates from the Atlantic and Pacifi c coasts contained 4
or 6 plasmids, with 4 smaller plasmids of 4.3 to 8.1 kb being often observed. Some
slight variation in plasmid content was noted between sources of the isolates (Giles
et al. 1995 ). Atypical isolates possessed two to four different plasmid types (Belland
and Trust 1989 ). Moreover, there was a correlation between plasmid composition
and source of the atypical isolates. This observation may prove useful in epizootiological studies, where plasmid content of atypical isolates could serve as useful
markers (Belland and Trust 1989 ). In a subsequent investigation of 113 cultures of
atypical isolates from a wide range of geographical locations, 7 groupings were
defi ned; 18 cultures did not have any common plasmid profi le. Of interest, the two
type strains NCIMB 1110 and ATCC 27013 were recovered in different groups. For
some groups, i.e. I, III, V and VII (these isolates were catalase-negative), there was
an association with the origin of the cultures, i.e. the location of the farm. Also,
some differences in phenotype were apparent between members of some groups.
Again, the value for epizootiological investigations was stressed (Sørum et al. 2000 ).
The Taxonomic Dilemma
Using molecular techniques, a consistent view about the genetic relatedness of the
long established subspecies of Aer. salmonicida emerges. The outstanding dilemma
concerns the poor correlation between phenetic and genotypic data (Austin et al.
1989 ). This problem needs to be addressed before the defi nitive classifi cation of
Aeromonas results. Nevertheless, it may be concluded that unlike the atypical isolates, Aer. salmonicida subsp. salmonicida is extremely homogeneous; a conclusion
which is supported by phenotypic and molecular data (Austin et al. 1989 ; Dalsgaard
et al. 1994 ; Hänninen et al. 1995 ; Miyata et al. 1996 ; Umelo and Trust 1998 ).
Despite the existence of typical and atypical representatives of Aer. salmonicida,
the current theme in operation for speciation has proved tenable. Nevertheless, new
‘atypical’ isolates, which do not fi t into existing classifi cations of Aer. salmonicida,
5 Aeromonadaceae Representative (Aeromonas salmonicida)
workers found that typical isolates possessed a very homologous plasmid content
comprised of a single large (70–145 kb) plasmid and three low molecular weight
plasmids. Livesley et al. ( 1997 ) reported that 5 plasmids were most common among
the 18 isolates examined; Giles et al. ( 1995 ) found 4 or 6 plasmids with 4 smaller
plasmids of 4.3–8.1 kb being often observed in isolates from the Atlantic coast of
Canada, but 6 plasmids of 4.2–8.9 kb among cultures from the Pacifi c coast of
Canada. A total of 23 plasmids and 40 different plasmid profi les were recognised
among 124 isolates from Denmark, Norway, Scotland and North America (Nielsen
et al. 1993 ). An earlier theme was repeated insofar as all isolates had one large plasmid of 60–150 kb, and two low molecular weight plasmids of 5.2 and 5.4 kb. In
addition, two plasmids of 5.6 and 6.4 kb were frequently present (Nielsen et al.
1993 ). A larger investigation of 383 isolates over a 6 year period concluded that 1–4
plasmids of 52–105 mDa were inevitably present, casting doubt on the relevance of
plasmid typing for epizootiology (Sørum et al. 1993a , b ). Again, oxytetracycline
and streptomycin resistant isolates from the Atlantic and Pacifi c coasts contained 4
or 6 plasmids, with 4 smaller plasmids of 4.3 to 8.1 kb being often observed. Some
slight variation in plasmid content was noted between sources of the isolates (Giles
et al. 1995 ). Atypical isolates possessed two to four different plasmid types (Belland
and Trust 1989 ). Moreover, there was a correlation between plasmid composition
and source of the atypical isolates. This observation may prove useful in epizootiological studies, where plasmid content of atypical isolates could serve as useful
markers (Belland and Trust 1989 ). In a subsequent investigation of 113 cultures of
atypical isolates from a wide range of geographical locations, 7 groupings were
defi ned; 18 cultures did not have any common plasmid profi le. Of interest, the two
type strains NCIMB 1110 and ATCC 27013 were recovered in different groups. For
some groups, i.e. I, III, V and VII (these isolates were catalase-negative), there was
an association with the origin of the cultures, i.e. the location of the farm. Also,
some differences in phenotype were apparent between members of some groups.
Again, the value for epizootiological investigations was stressed (Sørum et al. 2000 ).
The Taxonomic Dilemma
Using molecular techniques, a consistent view about the genetic relatedness of the
long established subspecies of Aer. salmonicida emerges. The outstanding dilemma
concerns the poor correlation between phenetic and genotypic data (Austin et al.
1989 ). This problem needs to be addressed before the defi nitive classifi cation of
Aeromonas results. Nevertheless, it may be concluded that unlike the atypical isolates, Aer. salmonicida subsp. salmonicida is extremely homogeneous; a conclusion
which is supported by phenotypic and molecular data (Austin et al. 1989 ; Dalsgaard
et al. 1994 ; Hänninen et al. 1995 ; Miyata et al. 1996 ; Umelo and Trust 1998 ).
Despite the existence of typical and atypical representatives of Aer. salmonicida,
the current theme in operation for speciation has proved tenable. Nevertheless, new
‘atypical’ isolates, which do not fi t into existing classifi cations of Aer. salmonicida,
5 Aeromonadaceae Representative (Aeromonas salmonicida)