248
these studies that the pathogen may be found in association with other aquatic animals, but it is apparent that many of the existing methods available for detection of
the pathogen are inadequate.
Data have pointed to the possibility that Aer. salmonicida may be disseminated
in aerosols (Wooster and Bowser 1996 ). In particular, experiments demonstrated
that the pathogen travelled 104.1 cm (the limit of the test chamber), via the airborne
route (Wooster and Bowser 1996 ). Thus, another possible means of spreading the
pathogen needs to be considered.
Diffi culties in Recovering Aeromonas salmonicida from the Aquatic
Environment It is relevant to digress, at this point, from the discussion of sources
of infection, in order to comment upon the diffi culties besetting the isolation of Aer.
salmonicida from environmental samples, other than fi sh. A dependable isolation
procedure for the pathogen is of critical importance to an understanding of the epizootiology of diseases caused by Aer. salmonicida . For example, if the pathogen is
capable of a free-living existence outside a fi sh host, the prevention and control of
diseases of Aer. salmonicida aetiology would be rendered much more diffi cult if not
impossible. However, currently Aer. salmonicida is defi ned as an obligate fi sh
pathogen not found in surface waters (Popoff 1984 ). This defi nition has no doubt
been formulated due to the paucity of conclusive evidence for a free-living existence of the pathogen. The organism, for instance, often cannot be isolated from
water on fi sh farms even during an epizootic of the disease (Cornick et al. 1969 ;
Kimura 1970 ; Allen 1982 ). Several reasons have been put forward to explain this
disconcerting phenomenon. One is that Aer. salmonicida is notoriously diffi cult to
isolate from mixed microbial populations as it is quickly outcompeted in growth by
most other commonly occurring aquatic bacteria. In addition, pigment production
on agar plates, heavily relied upon as a fi rst indication that Aer. salmonicida is present, is inhibited by the close proximity of colonies of other bacterial types. Therefore,
there is the perceived problem of recognising Aer. salmonicida in large mixed
microbial communities. Many of the problems with habitat and survival studies on
Aer. salmonicida are blamed on lack of adequate methodology. Both McCarthy
( 1980 ) and Michel and Dubois-Darnaudpeys ( 1980 ) stressed contamination diffi -
culties when employing non-selective media, e.g. TSA, for isolation of the pathogen. Cornick et al. ( 1969 ) reported that Aer. salmonicida was isolated most
frequently from environments containing few, if any, other bacterial species, particularly representatives of the genus Pseudomonas. Their preliminary experiments
suggested that some pseudomonad taxa obtained from water and fi sh inhibited the
growth of Aer. salmonicida in liquid and on solid culture media. Both cell-free fi ltrates and extracts of disrupted cells of the pseudomonads caused the inhibition,
believed to be due to antimicrobial activity. Dubois-Darnaudpeys ( 1977a ) also
examined the effects of the bacterial fl ora commonly occurring in surface water,
such as the Pseudomonas-Achromobacter and Flavobacterium groups, on the survival and growth of Aer. salmonicida. She found that the pathogen was inhibited at
all temperatures if the experiments were run in the presence of the other bacteria.
The ability of other micro-organisms, e.g. Acinetobacter, Aer. hydrophila,
5 Aeromonadaceae Representative (Aeromonas salmonicida)
these studies that the pathogen may be found in association with other aquatic animals, but it is apparent that many of the existing methods available for detection of
the pathogen are inadequate.
Data have pointed to the possibility that Aer. salmonicida may be disseminated
in aerosols (Wooster and Bowser 1996 ). In particular, experiments demonstrated
that the pathogen travelled 104.1 cm (the limit of the test chamber), via the airborne
route (Wooster and Bowser 1996 ). Thus, another possible means of spreading the
pathogen needs to be considered.
Diffi culties in Recovering Aeromonas salmonicida from the Aquatic
Environment It is relevant to digress, at this point, from the discussion of sources
of infection, in order to comment upon the diffi culties besetting the isolation of Aer.
salmonicida from environmental samples, other than fi sh. A dependable isolation
procedure for the pathogen is of critical importance to an understanding of the epizootiology of diseases caused by Aer. salmonicida . For example, if the pathogen is
capable of a free-living existence outside a fi sh host, the prevention and control of
diseases of Aer. salmonicida aetiology would be rendered much more diffi cult if not
impossible. However, currently Aer. salmonicida is defi ned as an obligate fi sh
pathogen not found in surface waters (Popoff 1984 ). This defi nition has no doubt
been formulated due to the paucity of conclusive evidence for a free-living existence of the pathogen. The organism, for instance, often cannot be isolated from
water on fi sh farms even during an epizootic of the disease (Cornick et al. 1969 ;
Kimura 1970 ; Allen 1982 ). Several reasons have been put forward to explain this
disconcerting phenomenon. One is that Aer. salmonicida is notoriously diffi cult to
isolate from mixed microbial populations as it is quickly outcompeted in growth by
most other commonly occurring aquatic bacteria. In addition, pigment production
on agar plates, heavily relied upon as a fi rst indication that Aer. salmonicida is present, is inhibited by the close proximity of colonies of other bacterial types. Therefore,
there is the perceived problem of recognising Aer. salmonicida in large mixed
microbial communities. Many of the problems with habitat and survival studies on
Aer. salmonicida are blamed on lack of adequate methodology. Both McCarthy
( 1980 ) and Michel and Dubois-Darnaudpeys ( 1980 ) stressed contamination diffi -
culties when employing non-selective media, e.g. TSA, for isolation of the pathogen. Cornick et al. ( 1969 ) reported that Aer. salmonicida was isolated most
frequently from environments containing few, if any, other bacterial species, particularly representatives of the genus Pseudomonas. Their preliminary experiments
suggested that some pseudomonad taxa obtained from water and fi sh inhibited the
growth of Aer. salmonicida in liquid and on solid culture media. Both cell-free fi ltrates and extracts of disrupted cells of the pseudomonads caused the inhibition,
believed to be due to antimicrobial activity. Dubois-Darnaudpeys ( 1977a ) also
examined the effects of the bacterial fl ora commonly occurring in surface water,
such as the Pseudomonas-Achromobacter and Flavobacterium groups, on the survival and growth of Aer. salmonicida. She found that the pathogen was inhibited at
all temperatures if the experiments were run in the presence of the other bacteria.
The ability of other micro-organisms, e.g. Acinetobacter, Aer. hydrophila,
5 Aeromonadaceae Representative (Aeromonas salmonicida)