238
cross-absorption of smooth strains, and passive haemagglutination and double
cross-absorption of rough colony types. It was determined that cross-reaction titres
for both antigen types were, in general, high, and cross-reactions between the Aer.
hydrophila isolate and three out of six thermostable Aer. salmonicida antisera were
very weak. In contrast, a strain of V. anguillarum and Ps. fl uorescens gave no reaction with the Aer. salmonicida antisera. The passive haemagglutination method was
more sensitive than whole-cell agglutination, as titres obtained for positive reactions were ten-fold higher in the former. When a double-diffusion method was
employed to study cell-free extracts, prepared from the bacteria used for the somatic
antigen study, strong cross-reactions among Aer. salmonicida , Aer. hydrophila and
V. anguillarum, and, to a lesser extent, with Ps. fl uorescens occurred . McCarthy and
Rawle ( 1975 ) concluded that no qualitative differences in serological composition
among Aer. salmonicida strains had been demonstrated, but noted that laboratory
maintenance of some Aer. salmonicida cultures resulted in progressive loss of serological reactivity, giving negative responses with their antisera. Therefore, they suggested that, when embarking on serological (or vaccination) studies, it is important
to include only fresh isolates. Hahnel et al. ( 1983 ) used micro-agglutination and
double-diffusion precipitin tests to study serological relatedness among virulent and
avirulent forms of eight isolates of Aer. salmonicida subsp. salmonicida. No serological differences were detected in the virulent isolates, but antigenic differences
were observed between the virulent and avirulent form of each culture. Thus, in
double-diffusion precipitin tests, the antigens of virulent sonicated cells formed an
additional precipitin line when compared with the homogeneous avirulent form.
Bacteriophage Typing
Bacteriophages have been used to study taxonomic relatedness between strains. The
fi rst isolation of bacteriophage specifi c for Aer. salmonicida was made by Todd
( 1933 ), although the usefulness for typing purposes was not demonstrated until the
work of Popoff ( 1971a , b ). It is considered that phage typing has value in epizootiological studies (Popoff 1984 ; Bast et al. 1988 ; Belland and Trust 1989 ). Essentially,
the bacteriophages may be divided into three morphological groups and ten serological types (Popoff 1984 ). Thus, using a set of eight phages, Popoff ( 1971b ) recognised 14 phage types. Also, Paterson et al. ( 1980 ) studied phage sensitivity as a
means of determining relationships between typical and atypical cultures. Pigmented
and achromogenic as well as aggregating and non-aggregating strains showed a
high sensitivity to two out of the three bacteriophages. In a further investigation by
Rodgers et al. ( 1981 ), 27 groups of Aer. salmonicida were defi ned on the basis of
sensitivity patterns to 18 bacteriophage isolates. Signifi cantly, the morphological
characteristics of the host bacterium, i.e. whether a rough, smooth or G-phase form,
infl uenced attachment of the bacteriophage. This was apparently attributed to the
varying quantities of LPS in the cell wall of the different morphological types.
5 Aeromonadaceae Representative (Aeromonas salmonicida)
cross-absorption of smooth strains, and passive haemagglutination and double
cross-absorption of rough colony types. It was determined that cross-reaction titres
for both antigen types were, in general, high, and cross-reactions between the Aer.
hydrophila isolate and three out of six thermostable Aer. salmonicida antisera were
very weak. In contrast, a strain of V. anguillarum and Ps. fl uorescens gave no reaction with the Aer. salmonicida antisera. The passive haemagglutination method was
more sensitive than whole-cell agglutination, as titres obtained for positive reactions were ten-fold higher in the former. When a double-diffusion method was
employed to study cell-free extracts, prepared from the bacteria used for the somatic
antigen study, strong cross-reactions among Aer. salmonicida , Aer. hydrophila and
V. anguillarum, and, to a lesser extent, with Ps. fl uorescens occurred . McCarthy and
Rawle ( 1975 ) concluded that no qualitative differences in serological composition
among Aer. salmonicida strains had been demonstrated, but noted that laboratory
maintenance of some Aer. salmonicida cultures resulted in progressive loss of serological reactivity, giving negative responses with their antisera. Therefore, they suggested that, when embarking on serological (or vaccination) studies, it is important
to include only fresh isolates. Hahnel et al. ( 1983 ) used micro-agglutination and
double-diffusion precipitin tests to study serological relatedness among virulent and
avirulent forms of eight isolates of Aer. salmonicida subsp. salmonicida. No serological differences were detected in the virulent isolates, but antigenic differences
were observed between the virulent and avirulent form of each culture. Thus, in
double-diffusion precipitin tests, the antigens of virulent sonicated cells formed an
additional precipitin line when compared with the homogeneous avirulent form.
Bacteriophage Typing
Bacteriophages have been used to study taxonomic relatedness between strains. The
fi rst isolation of bacteriophage specifi c for Aer. salmonicida was made by Todd
( 1933 ), although the usefulness for typing purposes was not demonstrated until the
work of Popoff ( 1971a , b ). It is considered that phage typing has value in epizootiological studies (Popoff 1984 ; Bast et al. 1988 ; Belland and Trust 1989 ). Essentially,
the bacteriophages may be divided into three morphological groups and ten serological types (Popoff 1984 ). Thus, using a set of eight phages, Popoff ( 1971b ) recognised 14 phage types. Also, Paterson et al. ( 1980 ) studied phage sensitivity as a
means of determining relationships between typical and atypical cultures. Pigmented
and achromogenic as well as aggregating and non-aggregating strains showed a
high sensitivity to two out of the three bacteriophages. In a further investigation by
Rodgers et al. ( 1981 ), 27 groups of Aer. salmonicida were defi ned on the basis of
sensitivity patterns to 18 bacteriophage isolates. Signifi cantly, the morphological
characteristics of the host bacterium, i.e. whether a rough, smooth or G-phase form,
infl uenced attachment of the bacteriophage. This was apparently attributed to the
varying quantities of LPS in the cell wall of the different morphological types.
5 Aeromonadaceae Representative (Aeromonas salmonicida)