172
In contrast to the usual characteristics of Aer. salmonicida, the majority of isolates of Aer. hydrophila are capable of growth at 37 °C and are, indeed, motile.
Some isolates have also been determined to produce diffusible brown pigments, as
does Aer. salmonicida, which could superfi cially confuse diagnosticians (Ross
1962 ; Paterson 1974 ; Allen et al. 1983a , b ; Austin et al. 1989 ). Santos et al. ( 1991 )
serotyped 62 motile Aeromonas spp. from rainbow trout. Of these, 55 isolates (89 %
of the total) were distributed between 17 serogroups, of which O3, O6, O11 and
O19 were dominant. Moreover, 40 (63 % of the total) of these isolates were pathogenic to fi sh. Nevertheless, antigenic cross-reactivity with Aer. salmonicida and Aer.
sobria has been noted (Leblanc et al. 1981 ). However, Shaw and Hodder ( 1978 )
reported that the core region of the LPS of Aer. hydrophila was distinct from that of
Aer. caviae and Aer. sobria.
Diagnosis
Phenotypic Methods
The API 20E profi le(s) for Aer. hydrophila are similar to those of Aer. allosaccharophila and Aer. sobria, and therefore use of this rapid identifi cation system could
give erroneous results . Toranzo et al. ( 1986 ) compared the API 20E rapid identifi cation system for Aer. hydrophila with Kaper’s medium (Kaper et al. 1979 ) and conventional biochemical tests. Kaper and co-workers formulated a single tube
medium, which was suitable for determining motility, inositol and mannitol fermentation, ornithine decarboxylase and deamination, and the production of H 2 S and
indole. Thus, bona fi de isolates of Aer. hydrophila gave an alkaline reaction on the
top of the medium, acid production in the butt, motility, and indole but not H 2 S
production (H 2 S production may occur on the top). Toranzo and colleagues pointed
to shortcomings of the API 20E system, insofar as many environmental isolates
were mis- identifi ed or not listed by the published profi le index. In contrast, Kaper’s
medium was effective for fast, presumptive identifi cation. Problems were encountered with the reliability of some conventional biochemical tests, notably the Voges
Proskauer reaction, fermentation and gas production from arabinose, gelatinase
production, and the lysine decarboxylase test. Ironically, these tests have also been
considered to be correlated with virulence in motile aeromonads.
Serology
Eurell et al. ( 1978 ) considered the effectiveness of slide agglutination especially for
fi eld use in the recognition of Aer. hydrophila infections, whereas tube- and macroagglutination were useful in laboratories. Kawahara and Kusuda ( 1987 ) reported
that FAT was superior to culturing for the diagnosis of Aer. hydrophila infections in
eels. Moreover, FAT was more successful than culturing for detecting Aer.
4 Aeromonadaceae Representatives (Motile Aeromonads)
In contrast to the usual characteristics of Aer. salmonicida, the majority of isolates of Aer. hydrophila are capable of growth at 37 °C and are, indeed, motile.
Some isolates have also been determined to produce diffusible brown pigments, as
does Aer. salmonicida, which could superfi cially confuse diagnosticians (Ross
1962 ; Paterson 1974 ; Allen et al. 1983a , b ; Austin et al. 1989 ). Santos et al. ( 1991 )
serotyped 62 motile Aeromonas spp. from rainbow trout. Of these, 55 isolates (89 %
of the total) were distributed between 17 serogroups, of which O3, O6, O11 and
O19 were dominant. Moreover, 40 (63 % of the total) of these isolates were pathogenic to fi sh. Nevertheless, antigenic cross-reactivity with Aer. salmonicida and Aer.
sobria has been noted (Leblanc et al. 1981 ). However, Shaw and Hodder ( 1978 )
reported that the core region of the LPS of Aer. hydrophila was distinct from that of
Aer. caviae and Aer. sobria.
Diagnosis
Phenotypic Methods
The API 20E profi le(s) for Aer. hydrophila are similar to those of Aer. allosaccharophila and Aer. sobria, and therefore use of this rapid identifi cation system could
give erroneous results . Toranzo et al. ( 1986 ) compared the API 20E rapid identifi cation system for Aer. hydrophila with Kaper’s medium (Kaper et al. 1979 ) and conventional biochemical tests. Kaper and co-workers formulated a single tube
medium, which was suitable for determining motility, inositol and mannitol fermentation, ornithine decarboxylase and deamination, and the production of H 2 S and
indole. Thus, bona fi de isolates of Aer. hydrophila gave an alkaline reaction on the
top of the medium, acid production in the butt, motility, and indole but not H 2 S
production (H 2 S production may occur on the top). Toranzo and colleagues pointed
to shortcomings of the API 20E system, insofar as many environmental isolates
were mis- identifi ed or not listed by the published profi le index. In contrast, Kaper’s
medium was effective for fast, presumptive identifi cation. Problems were encountered with the reliability of some conventional biochemical tests, notably the Voges
Proskauer reaction, fermentation and gas production from arabinose, gelatinase
production, and the lysine decarboxylase test. Ironically, these tests have also been
considered to be correlated with virulence in motile aeromonads.
Serology
Eurell et al. ( 1978 ) considered the effectiveness of slide agglutination especially for
fi eld use in the recognition of Aer. hydrophila infections, whereas tube- and macroagglutination were useful in laboratories. Kawahara and Kusuda ( 1987 ) reported
that FAT was superior to culturing for the diagnosis of Aer. hydrophila infections in
eels. Moreover, FAT was more successful than culturing for detecting Aer.
4 Aeromonadaceae Representatives (Motile Aeromonads)