231
duction, Voges Proskauer reaction, H 2 S and lysine decarboxylase production, and
fermentation of sucrose (Table 5.1 ). In the eighth edition of Bergey’s Manual of
Determinative Bacteriology, Schubert ( 1974 ) regarded these non-pigmented isolates as Aer. salmonicida subsp. achromogenes and Aer. salmonicida subsp. masoucida, respectively. Typical strains were classifi ed by him as Aer. salmonicida subsp.
salmonicida. This classifi cation into subspecies, has been retained in the recent literature (Martin-Carnahan and Joseph 2005 ). This is interesting, because in an earlier publication (Popoff 1970 ), it was contended that subspecies achromogenes and
masoucida were more closely related to Aer. hydrophila than to Aer. salmonicida. In
fact, Paterson et al. ( 1980 ) suggested that ‘masoucida’ bridged the gap between
typical Aer. salmonicida cultures and Aer. hydrophila, insofar as the subspecies possessed similar physiological and growth characteristics to the latter. However, Aer.
salmonicida subsp. masoucida is non-motile, sensitive to Aer. salmonicida bacteriophages, possesses an antigenic component specifi c to Aer. salmonicida, and has a
DNA homology of 103 % with Aer. salmonicida (MacInnes et al. 1979 ). The relationship between the subspecies is certainly not sacrosanct, insofar they could be
combined or kept separate according to which methods happen to be in vogue
(Austin et al. 1998 ). For example by PCR, there would be good reason to consider
combining subspecies achromogenes and masoucida, a view that is not substantiated by ribotyping and RAPD analyses (Austin et al. 1998 ). Phenetic data suggested
that there would be a case for combining subspecies masoucida with salmonicida,
and subspecies achromogenes with Haemophilus piscium. Indeed, examination of
the small subunit rRNA gene sequences revealed a profound (99.9 %) homology of
an authentic strain of Haemophilus piscium with Aer. salmonicida subsp. salmonicida (Thornton et al. 1999 ). Yet, methods point to the comparative uniqueness of
subspecies smithia (Austin et al. 1998 ).
Certainly, a species subdivided into four subspecies should not be considered as
unworkable; however, the classifi cation is complicated by other factors. Thus, the
existence of aberrant strains from a wide range of fi sh hosts and geographical locations is well established, and new reports are continually being made. According to
McCarthy ( 1980 ), the existence of such strains is no doubt more common than even
their documentation in the published literature suggests. As Mawdesley-Thomas
( 1969 ) pointed out, there is no sound reason why a pathogen that affects one family
of freshwater fi shes should not infect others. He contended that emphasis had been
placed on food and game fi sh, and that only the absence of detailed investigations of
fi sh diseases generally had given the false impression that each fi sh has its own
specifi c set of diseases. McCarthy ( 1980 ) and McCarthy and Roberts ( 1980 ) made
the valid point that the original description of atypical strains, as reported by
Schubert ( 1974 ), was based upon data for only a few isolates. These authors submitted that revision is now both possible and necessary. According to a comparative
phenetic and genotypic analysis of 29 atypical isolates, in addition to 144 other
Aeromonas spp., McCarthy ( 1977a ) delineated four phenetic groups. Of these, one
cluster comprised typical isolates of Aer. salmonicida, a second group was composed of atypical isolates of Aer. salmonicida derived from salmonids including
representatives of subspecies achromogenes and masoucida, a third group contained
Aeromonas salmonicida
duction, Voges Proskauer reaction, H 2 S and lysine decarboxylase production, and
fermentation of sucrose (Table 5.1 ). In the eighth edition of Bergey’s Manual of
Determinative Bacteriology, Schubert ( 1974 ) regarded these non-pigmented isolates as Aer. salmonicida subsp. achromogenes and Aer. salmonicida subsp. masoucida, respectively. Typical strains were classifi ed by him as Aer. salmonicida subsp.
salmonicida. This classifi cation into subspecies, has been retained in the recent literature (Martin-Carnahan and Joseph 2005 ). This is interesting, because in an earlier publication (Popoff 1970 ), it was contended that subspecies achromogenes and
masoucida were more closely related to Aer. hydrophila than to Aer. salmonicida. In
fact, Paterson et al. ( 1980 ) suggested that ‘masoucida’ bridged the gap between
typical Aer. salmonicida cultures and Aer. hydrophila, insofar as the subspecies possessed similar physiological and growth characteristics to the latter. However, Aer.
salmonicida subsp. masoucida is non-motile, sensitive to Aer. salmonicida bacteriophages, possesses an antigenic component specifi c to Aer. salmonicida, and has a
DNA homology of 103 % with Aer. salmonicida (MacInnes et al. 1979 ). The relationship between the subspecies is certainly not sacrosanct, insofar they could be
combined or kept separate according to which methods happen to be in vogue
(Austin et al. 1998 ). For example by PCR, there would be good reason to consider
combining subspecies achromogenes and masoucida, a view that is not substantiated by ribotyping and RAPD analyses (Austin et al. 1998 ). Phenetic data suggested
that there would be a case for combining subspecies masoucida with salmonicida,
and subspecies achromogenes with Haemophilus piscium. Indeed, examination of
the small subunit rRNA gene sequences revealed a profound (99.9 %) homology of
an authentic strain of Haemophilus piscium with Aer. salmonicida subsp. salmonicida (Thornton et al. 1999 ). Yet, methods point to the comparative uniqueness of
subspecies smithia (Austin et al. 1998 ).
Certainly, a species subdivided into four subspecies should not be considered as
unworkable; however, the classifi cation is complicated by other factors. Thus, the
existence of aberrant strains from a wide range of fi sh hosts and geographical locations is well established, and new reports are continually being made. According to
McCarthy ( 1980 ), the existence of such strains is no doubt more common than even
their documentation in the published literature suggests. As Mawdesley-Thomas
( 1969 ) pointed out, there is no sound reason why a pathogen that affects one family
of freshwater fi shes should not infect others. He contended that emphasis had been
placed on food and game fi sh, and that only the absence of detailed investigations of
fi sh diseases generally had given the false impression that each fi sh has its own
specifi c set of diseases. McCarthy ( 1980 ) and McCarthy and Roberts ( 1980 ) made
the valid point that the original description of atypical strains, as reported by
Schubert ( 1974 ), was based upon data for only a few isolates. These authors submitted that revision is now both possible and necessary. According to a comparative
phenetic and genotypic analysis of 29 atypical isolates, in addition to 144 other
Aeromonas spp., McCarthy ( 1977a ) delineated four phenetic groups. Of these, one
cluster comprised typical isolates of Aer. salmonicida, a second group was composed of atypical isolates of Aer. salmonicida derived from salmonids including
representatives of subspecies achromogenes and masoucida, a third group contained
Aeromonas salmonicida