226
organism, and were of the opinion that attempts to recognise and identify isolates
were being hampered by a lack of a complete description. This resulted in confusion
due to disagreement concerning the physiological and biochemical characterisation
of the organism. From the results of a study of ten isolates, it was concluded that
Bacterium salmonicida was extremely consistent in its general cultural and biochemical traits, and that problems in the past had arisen primarily from the use of
media that varied in composition among laboratories. Although Griffi n et al. ( 1953a )
ended their report by recommending the re-classifi cation of Bacterium salmonicida
to the newly created genus Aeromonas, as Aer. salmonicida, no defi nite reasons
were given for this move. However, with time, additional data have accumulated,
and the homogeneity and authenticity of the taxon has generally been supported.
Subsequent investigators have re-examined the homogeneity of the taxon, using
conventional and numerical phenotypic methods (Eddy 1960 ; Ewing et al. 1961 ;
Schubert 1961 ; Smith 1963 ; Eddy and Carpenter 1964 ; Popoff 1969 ). Thus, the
traditional description of Aer. salmonicida is of non-motile, fermentative, Gramnegative rods, which produce a brown water-soluble pigment on tryptone- containing
agar, which do not grow at 37 °C, and which produce catalase and oxidase
(Table 5.1 ). The circular chromosome is 4658 ± 30 kb (Umelo and Trust 1998 ).
Cells are found in, and are pathogenic to, salmonids and increasingly other fi sh species. Traditionally, the lack of motility has been accepted as one of the reliable
diagnostic traits used for the division of the aeromonads. However, this criterion has
been challenged by the report of McIntosh and Austin ( 1991a ) of motility (by polar
fl agella) in a strain of Aer. salmonicida subsp. salmonicida grown at elevated temperatures, i.e. 30–37 °C. The appearance of motility was also accompanied by variation in sugar fermentation patterns, the loss of ability to degrade complex molecules
and an increase in antibiotic resistance. Further evidence for a motile mode of existence of Aer. salmonicida was provided by the recovery of eight atypical isolates
from ulcers (but not from kidney tissue) on goldfi sh, carp and roach. The ulcerated
fi sh were obtained from aquaria, garden ponds and rivers in England (Austin 1993 ).
Interestingly, these isolates did not dissociate into different colony types, but grew
at 37 °C. Of course, there is always the concern that motile contaminants may have
been present in cultures, which were predominantly Aer. salmonicida. However, the
isolation of fl agella genes, fl aA and fl aB, which coded for unsheathed polar fl agella
at low frequency, has clinched the argument that Aer. salmonicida can be motile
under certain circumstances (Umelo and Trust 1997 ).
Certain traits, such as pigment production, captured the attention of fi sheries
scientists particularly because they were readily observable. In an examination of
pigment production, Griffi n et al. ( 1953b ) showed that its development was dependent upon medium composition, insofar as tyrosine or phenylalanine was deemed to
be essential. This was confi rmed by O’Leary et al. ( 1956 ). However, it was initially
assumed that this pigment was related to melanin, although subsequent investigation has refuted this possibility. Thus, Donlon et al. ( 1983 ) discovered that biosynthesis of the pigment from tyrosine differed substantially from melanogenesis and
not 3,4-dihydroxyphenylalanine as would have been expected of melanin synthesis.
Although production of the brown, water-soluble pigment constitutes a major diag5 Aeromonadaceae Representative (Aeromonas salmonicida)
organism, and were of the opinion that attempts to recognise and identify isolates
were being hampered by a lack of a complete description. This resulted in confusion
due to disagreement concerning the physiological and biochemical characterisation
of the organism. From the results of a study of ten isolates, it was concluded that
Bacterium salmonicida was extremely consistent in its general cultural and biochemical traits, and that problems in the past had arisen primarily from the use of
media that varied in composition among laboratories. Although Griffi n et al. ( 1953a )
ended their report by recommending the re-classifi cation of Bacterium salmonicida
to the newly created genus Aeromonas, as Aer. salmonicida, no defi nite reasons
were given for this move. However, with time, additional data have accumulated,
and the homogeneity and authenticity of the taxon has generally been supported.
Subsequent investigators have re-examined the homogeneity of the taxon, using
conventional and numerical phenotypic methods (Eddy 1960 ; Ewing et al. 1961 ;
Schubert 1961 ; Smith 1963 ; Eddy and Carpenter 1964 ; Popoff 1969 ). Thus, the
traditional description of Aer. salmonicida is of non-motile, fermentative, Gramnegative rods, which produce a brown water-soluble pigment on tryptone- containing
agar, which do not grow at 37 °C, and which produce catalase and oxidase
(Table 5.1 ). The circular chromosome is 4658 ± 30 kb (Umelo and Trust 1998 ).
Cells are found in, and are pathogenic to, salmonids and increasingly other fi sh species. Traditionally, the lack of motility has been accepted as one of the reliable
diagnostic traits used for the division of the aeromonads. However, this criterion has
been challenged by the report of McIntosh and Austin ( 1991a ) of motility (by polar
fl agella) in a strain of Aer. salmonicida subsp. salmonicida grown at elevated temperatures, i.e. 30–37 °C. The appearance of motility was also accompanied by variation in sugar fermentation patterns, the loss of ability to degrade complex molecules
and an increase in antibiotic resistance. Further evidence for a motile mode of existence of Aer. salmonicida was provided by the recovery of eight atypical isolates
from ulcers (but not from kidney tissue) on goldfi sh, carp and roach. The ulcerated
fi sh were obtained from aquaria, garden ponds and rivers in England (Austin 1993 ).
Interestingly, these isolates did not dissociate into different colony types, but grew
at 37 °C. Of course, there is always the concern that motile contaminants may have
been present in cultures, which were predominantly Aer. salmonicida. However, the
isolation of fl agella genes, fl aA and fl aB, which coded for unsheathed polar fl agella
at low frequency, has clinched the argument that Aer. salmonicida can be motile
under certain circumstances (Umelo and Trust 1997 ).
Certain traits, such as pigment production, captured the attention of fi sheries
scientists particularly because they were readily observable. In an examination of
pigment production, Griffi n et al. ( 1953b ) showed that its development was dependent upon medium composition, insofar as tyrosine or phenylalanine was deemed to
be essential. This was confi rmed by O’Leary et al. ( 1956 ). However, it was initially
assumed that this pigment was related to melanin, although subsequent investigation has refuted this possibility. Thus, Donlon et al. ( 1983 ) discovered that biosynthesis of the pigment from tyrosine differed substantially from melanogenesis and
not 3,4-dihydroxyphenylalanine as would have been expected of melanin synthesis.
Although production of the brown, water-soluble pigment constitutes a major diag5 Aeromonadaceae Representative (Aeromonas salmonicida)