425
RFLP led to the recognition of genetic heterogeneity (Izumi et al. 2003 ), with four
RFLP pattern types and 9 sequence types recognised among isolates from Ontario,
Canada (Hesami et al. 2009). MLST of 560 Fla. psychrophilum isolates from various Nordic countries collected between 1983 and 2012 revealed 81 different
sequence types belonging to 12 clonal complexes and 30 single member sequence
types. The largest clonal complex, CC-ST10, included mostly isolates from rainbow
trout and comprised the dominant genotype, ST2. In Norway, ST2 was the only
isolated CC-ST10 genotype, which suggested a recent introduction of an epidemic
clone. Most of the single member sequence types were isolated from the environment or non-rainbow trout species. It was considered likely that dissemination of
the putatively virulent clonal complex, CC-ST10, was associated with the movement of fi sh or their products (Nilsen et al. 2014 ). In the case of Switzerland, MLST
was used to study the relationships among 112 isolates from rainbow trout and
brown trout, with the outcome that 27 sequence types were defi ned. Most isolates
were recovered in two clonal complexes, CC-ST2 and CC-ST90, which were represented by several sequence types. Eight single member sequence types were unique,
and could not be accommodated in any clonal complex (Strepparava et al. 2013 ).
Using MLST with Japanese isolates, 35 mostly unique sequence types were delineated, suggesting a comparatively recent introduction of Fla. psychrophilum into
Japan (Fujiwara-Nagata et al. 2013 ). Moreover, MLST was used to examine 66 isolates from rainbow trout in France with the recognition of 15 sequence types, of
which 14 were regarded as being unique. These sequence types were grouped in one
clonal complex of 8 genetically-related sequence types plus 7 single members
(Siekoula-Nguedia et al. 2012 ).
Evidence has been presented that isolates may well contain prophage. Thus, PCR
examination of 49 isolates from Chile, Denmark and the USA revealed the presence
of four bacteriophage genes, i.e. integrase, tail tape protein and two hypothetical
proteins, in the majority (80 %). Therefore, it would appear that temporate bacteriophage has lysogenized in a large proportion of Fla. psychrophilum isolates (Castillo
et al. 2014 ).
On the basis of the phenotypic characteristics, the conclusion was reached that
the causal agent of coldwater disease could be equated with the genus Cytophaga,
as defi ned by Stanier ( 1942 ), but was suffi ciently distinct from existing species to
warrant description as a new species. Hence, Cyt. psychrophila was named (Borg
1960 ). However, Lewin and Lounsberry (cited in Leadbetter 1974 ) disagreed with
this notion, and proposed that the organisms would be better classifi ed in the genus
Flexibacter, as Fle. aurantiacus. Bernardet and Grimont ( 1989 b) agreed with the
opinion that the organisms belong in the genus Flexibacter, but proposed Fle. psychrophilum. Then with general improvements in the understanding of the yellowpigmented bacteria resulting from phylogenetic data, the taxon was transferred to
Flavobacterium, as Fla. psychrophilum (Bernardet et al. 1996 ). It is agreed that the
taxon comprises a homogeneous group of bacteria (Lorenzen et al. 1997 ).
Flavobacterium psychrophilum
RFLP led to the recognition of genetic heterogeneity (Izumi et al. 2003 ), with four
RFLP pattern types and 9 sequence types recognised among isolates from Ontario,
Canada (Hesami et al. 2009). MLST of 560 Fla. psychrophilum isolates from various Nordic countries collected between 1983 and 2012 revealed 81 different
sequence types belonging to 12 clonal complexes and 30 single member sequence
types. The largest clonal complex, CC-ST10, included mostly isolates from rainbow
trout and comprised the dominant genotype, ST2. In Norway, ST2 was the only
isolated CC-ST10 genotype, which suggested a recent introduction of an epidemic
clone. Most of the single member sequence types were isolated from the environment or non-rainbow trout species. It was considered likely that dissemination of
the putatively virulent clonal complex, CC-ST10, was associated with the movement of fi sh or their products (Nilsen et al. 2014 ). In the case of Switzerland, MLST
was used to study the relationships among 112 isolates from rainbow trout and
brown trout, with the outcome that 27 sequence types were defi ned. Most isolates
were recovered in two clonal complexes, CC-ST2 and CC-ST90, which were represented by several sequence types. Eight single member sequence types were unique,
and could not be accommodated in any clonal complex (Strepparava et al. 2013 ).
Using MLST with Japanese isolates, 35 mostly unique sequence types were delineated, suggesting a comparatively recent introduction of Fla. psychrophilum into
Japan (Fujiwara-Nagata et al. 2013 ). Moreover, MLST was used to examine 66 isolates from rainbow trout in France with the recognition of 15 sequence types, of
which 14 were regarded as being unique. These sequence types were grouped in one
clonal complex of 8 genetically-related sequence types plus 7 single members
(Siekoula-Nguedia et al. 2012 ).
Evidence has been presented that isolates may well contain prophage. Thus, PCR
examination of 49 isolates from Chile, Denmark and the USA revealed the presence
of four bacteriophage genes, i.e. integrase, tail tape protein and two hypothetical
proteins, in the majority (80 %). Therefore, it would appear that temporate bacteriophage has lysogenized in a large proportion of Fla. psychrophilum isolates (Castillo
et al. 2014 ).
On the basis of the phenotypic characteristics, the conclusion was reached that
the causal agent of coldwater disease could be equated with the genus Cytophaga,
as defi ned by Stanier ( 1942 ), but was suffi ciently distinct from existing species to
warrant description as a new species. Hence, Cyt. psychrophila was named (Borg
1960 ). However, Lewin and Lounsberry (cited in Leadbetter 1974 ) disagreed with
this notion, and proposed that the organisms would be better classifi ed in the genus
Flexibacter, as Fle. aurantiacus. Bernardet and Grimont ( 1989 b) agreed with the
opinion that the organisms belong in the genus Flexibacter, but proposed Fle. psychrophilum. Then with general improvements in the understanding of the yellowpigmented bacteria resulting from phylogenetic data, the taxon was transferred to
Flavobacterium, as Fla. psychrophilum (Bernardet et al. 1996 ). It is agreed that the
taxon comprises a homogeneous group of bacteria (Lorenzen et al. 1997 ).
Flavobacterium psychrophilum