236
examination of the specifi cities of aeromonad extracellular antigens, Liu ( 1961 )
observed that cross-reactions occurred between Aer. salmonicida and the motile
aeromonads. Liu suggested that the absence of serological cross-reactions with
other Gram-negative bacteria indicated that organisms belonging to the genus
Aeromonas comprise a distinct group. Studies, using bacteriophage, demonstrated
the sensitivity of some Aer. hydrophila cultures to Aer. salmonicida bacteriophages,
whereas organisms not belonging to Aeromonas showed complete resistance to the
virus (Popoff 1971a , b ). In particular, studies employing molecular genetic techniques support the retention of Aer. salmonicida in the genus Aeromonas. MacInnes
et al. ( 1979 ) determined that percentage DNA homologies of motile aeromonads
with Aer. salmonicida subsp. salmonicida ranged between 31 and 80 %, whereas
those hybridised against Aer. hydrophila were between 31 and 100 %. The Aer. salmonicida strains exhibited a relatively high degree of homology when hybridised
against Aer. hydrophila. Thus, the values for 10 out of 11 isolates were in the range
of 51 to 69 %. From these results, MacInnes et al. ( 1979 ) concluded that non-motile
aeromonads demonstrated a legitimate genetic relationship to the motile species of
Aeromonas. In fact, some motile strains shared a higher level of sequence homology
with Aer. salmonicida than with the reference motile aeromonads which according
to MacInnes could be attributed to the smaller genome size of Aer. salmonicida. In
another investigation, 56–65 % binding between Aer. salmonicida and Aer. hydrophila DNA strands was recorded (McCarthy 1978 ). These homology values indicate
strong genetic relationships between the principal Aeromonas species (Paterson
et al. 1980 ; Belland and Trust 1988 ).
It is well established that Aer. salmonicida, as a species, is phenotypically distinct from its motile counterparts. The increasing evidence from genetic and other
molecular biology studies pertaining to intrageneric relationships between Aer. salmonicida and other aeromonad species, however, appear to support the retention of
the pathogen within the genus. Certainly, the situation is not aided by ongoing
manoeuvres in the classifi cation of the motile aeromonads. Therefore, since the
arrangement of the genus is in transition, and yet to be defi nitively resolved, we
believe that nothing would be gained by the re-classifi cation of Aer. salmonicida.
To reiterate, it appears to be the consensus of opinion that the area in need of
further work is the intraspecifi c relationships between typical and atypical isolates.
It is anticipated that such work would improve the classifi cation of the genus
Aeromonas.
Serology
Additional approaches to ascertaining the intra- and interspecifi c relationships of
Aer. salmonicida have been adopted. These include serological techniques and bacteriophage typing. Certainly, the antigenicity of Aer. salmonicida has been the focus
of much attention, primarily with a view to its signifi cance in vaccine development.
Unfortunately, the early investigations of Williamson ( 1929 ) were halted by the
5 Aeromonadaceae Representative (Aeromonas salmonicida)
examination of the specifi cities of aeromonad extracellular antigens, Liu ( 1961 )
observed that cross-reactions occurred between Aer. salmonicida and the motile
aeromonads. Liu suggested that the absence of serological cross-reactions with
other Gram-negative bacteria indicated that organisms belonging to the genus
Aeromonas comprise a distinct group. Studies, using bacteriophage, demonstrated
the sensitivity of some Aer. hydrophila cultures to Aer. salmonicida bacteriophages,
whereas organisms not belonging to Aeromonas showed complete resistance to the
virus (Popoff 1971a , b ). In particular, studies employing molecular genetic techniques support the retention of Aer. salmonicida in the genus Aeromonas. MacInnes
et al. ( 1979 ) determined that percentage DNA homologies of motile aeromonads
with Aer. salmonicida subsp. salmonicida ranged between 31 and 80 %, whereas
those hybridised against Aer. hydrophila were between 31 and 100 %. The Aer. salmonicida strains exhibited a relatively high degree of homology when hybridised
against Aer. hydrophila. Thus, the values for 10 out of 11 isolates were in the range
of 51 to 69 %. From these results, MacInnes et al. ( 1979 ) concluded that non-motile
aeromonads demonstrated a legitimate genetic relationship to the motile species of
Aeromonas. In fact, some motile strains shared a higher level of sequence homology
with Aer. salmonicida than with the reference motile aeromonads which according
to MacInnes could be attributed to the smaller genome size of Aer. salmonicida. In
another investigation, 56–65 % binding between Aer. salmonicida and Aer. hydrophila DNA strands was recorded (McCarthy 1978 ). These homology values indicate
strong genetic relationships between the principal Aeromonas species (Paterson
et al. 1980 ; Belland and Trust 1988 ).
It is well established that Aer. salmonicida, as a species, is phenotypically distinct from its motile counterparts. The increasing evidence from genetic and other
molecular biology studies pertaining to intrageneric relationships between Aer. salmonicida and other aeromonad species, however, appear to support the retention of
the pathogen within the genus. Certainly, the situation is not aided by ongoing
manoeuvres in the classifi cation of the motile aeromonads. Therefore, since the
arrangement of the genus is in transition, and yet to be defi nitively resolved, we
believe that nothing would be gained by the re-classifi cation of Aer. salmonicida.
To reiterate, it appears to be the consensus of opinion that the area in need of
further work is the intraspecifi c relationships between typical and atypical isolates.
It is anticipated that such work would improve the classifi cation of the genus
Aeromonas.
Serology
Additional approaches to ascertaining the intra- and interspecifi c relationships of
Aer. salmonicida have been adopted. These include serological techniques and bacteriophage typing. Certainly, the antigenicity of Aer. salmonicida has been the focus
of much attention, primarily with a view to its signifi cance in vaccine development.
Unfortunately, the early investigations of Williamson ( 1929 ) were halted by the
5 Aeromonadaceae Representative (Aeromonas salmonicida)