230
tively whereas the former has two large plasmids of 166 and 155 kb (Reith et al.
2008 ; Charette et al. 2012 ).
Atypical Isolates of Aeromonas salmonicida
When compared to the so-called motile aeromonads, the description of Aer. salmonicida suggests a very homogeneous group of organisms. Alas, as so often happens in biology, a multitude of exceptions have disturbed the apparent idyllic
situation. ‘Atypical’ strains deviate from the classical description of the taxon over
a number of biochemical, physiological and genetic properties, e.g. AFLP fi ngerprints, making typing diffi cult (Hirvelä-Koski et al. 1994 ; Austin et al. 1998 ;
Wiklund and Dalsgaard 1998 ; Dalsgaard et al. 1998 ; Høi et al. 1999 ; Lund et al.
2002 ). For example, Japanese isolates of so-called atypical Aer. salmonicida were
recovered in four groups, which were defi ned after 16S rDNA sequencing. There
was not any host specifi city with these groups (Yamada et al. 2000 ). The most common reasons for describing isolates as ‘atypical’ are:
• lack of, weak or slow pigment production (Nakatsugawa 1994 ; Koppang et al.
2000 )
• catalase-negativity (Kaku et al. 1999 )
• oxidase-negativity (Wiklund and Bylund 1993 ; Wiklund et al. 1994 ; Wiklund
and Dalsgaard 1995 ; Pedersen et al. 1994 , 1996 ; Kaku et al. 1999 )
• nutritional fastidiousness, i.e. for blood or blood products (Austin 1993 )
• slow growth, i.e. ≥5 days to obtain visible colonies (Austin 1993 ; Kaku et al.
1999 )
• different hosts from salmonids, i.e. cyprinids (e.g. Austin 1993 ; Kaku et al. 1999 )
and marine fi sh, including shotted halibut ( Eopsetta grigorjewi; Nakatsugawa
1994 ), dab, plaice and fl ounder ( Platichthys fl esus ) (Wiklund et al. 1994 ; Wiklund
and Dalsgaard 1995 ), common wolffi sh ( Anarhichas lupus ) (Hellberg et al.
1996 ), turbot ( Scophthalmus maximus ) (Pedersen et al. 1994 ), greenling
( Hexagrammos otakii ) , Japanese fl ounder ( Paralichthys olivaceus ) and Schlegel’s
black rockfi sh ( Sebastes schlegeli ) (Iida et al. 1997 ), where the disease is often
ulceration.
With the last mentioned example, the justifi cation for describing the isolates as
atypical was based on the host rather than the characteristics of the cultures.
One of the earliest indications that aberrant strains occurred was provided by
Smith ( 1963 ), who examined six isolates of non-pigmented cultures, which were
clustered as Group I from a numerical taxonomy study. These organisms were
related at the 75.6 % similarity level to typical pigment-producing isolates. Smith
( 1963 ) proposed a separate new species for Group I, i.e. with the specifi c epithet
achromogenes, although the recommendation was not adopted. A second nonpigmented group was recognised by Kimura ( 1969a ) as Aer. salmonicida subsp.
masoucida. This subspecies differed from typical strains on account of indole pro5 Aeromonadaceae Representative (Aeromonas salmonicida)
tively whereas the former has two large plasmids of 166 and 155 kb (Reith et al.
2008 ; Charette et al. 2012 ).
Atypical Isolates of Aeromonas salmonicida
When compared to the so-called motile aeromonads, the description of Aer. salmonicida suggests a very homogeneous group of organisms. Alas, as so often happens in biology, a multitude of exceptions have disturbed the apparent idyllic
situation. ‘Atypical’ strains deviate from the classical description of the taxon over
a number of biochemical, physiological and genetic properties, e.g. AFLP fi ngerprints, making typing diffi cult (Hirvelä-Koski et al. 1994 ; Austin et al. 1998 ;
Wiklund and Dalsgaard 1998 ; Dalsgaard et al. 1998 ; Høi et al. 1999 ; Lund et al.
2002 ). For example, Japanese isolates of so-called atypical Aer. salmonicida were
recovered in four groups, which were defi ned after 16S rDNA sequencing. There
was not any host specifi city with these groups (Yamada et al. 2000 ). The most common reasons for describing isolates as ‘atypical’ are:
• lack of, weak or slow pigment production (Nakatsugawa 1994 ; Koppang et al.
2000 )
• catalase-negativity (Kaku et al. 1999 )
• oxidase-negativity (Wiklund and Bylund 1993 ; Wiklund et al. 1994 ; Wiklund
and Dalsgaard 1995 ; Pedersen et al. 1994 , 1996 ; Kaku et al. 1999 )
• nutritional fastidiousness, i.e. for blood or blood products (Austin 1993 )
• slow growth, i.e. ≥5 days to obtain visible colonies (Austin 1993 ; Kaku et al.
1999 )
• different hosts from salmonids, i.e. cyprinids (e.g. Austin 1993 ; Kaku et al. 1999 )
and marine fi sh, including shotted halibut ( Eopsetta grigorjewi; Nakatsugawa
1994 ), dab, plaice and fl ounder ( Platichthys fl esus ) (Wiklund et al. 1994 ; Wiklund
and Dalsgaard 1995 ), common wolffi sh ( Anarhichas lupus ) (Hellberg et al.
1996 ), turbot ( Scophthalmus maximus ) (Pedersen et al. 1994 ), greenling
( Hexagrammos otakii ) , Japanese fl ounder ( Paralichthys olivaceus ) and Schlegel’s
black rockfi sh ( Sebastes schlegeli ) (Iida et al. 1997 ), where the disease is often
ulceration.
With the last mentioned example, the justifi cation for describing the isolates as
atypical was based on the host rather than the characteristics of the cultures.
One of the earliest indications that aberrant strains occurred was provided by
Smith ( 1963 ), who examined six isolates of non-pigmented cultures, which were
clustered as Group I from a numerical taxonomy study. These organisms were
related at the 75.6 % similarity level to typical pigment-producing isolates. Smith
( 1963 ) proposed a separate new species for Group I, i.e. with the specifi c epithet
achromogenes, although the recommendation was not adopted. A second nonpigmented group was recognised by Kimura ( 1969a ) as Aer. salmonicida subsp.
masoucida. This subspecies differed from typical strains on account of indole pro5 Aeromonadaceae Representative (Aeromonas salmonicida)