249
Chromobacterium, Esch. coli, Flavobacterium and Pseudomonas, and their metabolites, to inhibit the survival of Aer. salmonicida in non-sterile seawater was also
reported by Effendi and Austin ( 1991 ). Thus, because the isolation and study of the
viability of the pathogen is complicated by competition and inhibition by other
organisms, it is not surprising that investigations into survival of the pathogen,
which could help to establish whether or not it is capable of a free-living existence
outside of fi sh, have invariably retreated to laboratory-based experiments using sterilized water. A fi ltration method tested by Maheshkumar et al. ( 1990 ) attempted,
with some degree of success, to overcome the diffi culties of isolation of Aer. salmonicida posed by the presence of other bacteria, i.e. the possible overgrowth by
small numbers of cells of the pathogen. In their studies, up to 5 l of hatchery water
was seeded with Aer. salmonicida and passed through 1
-MDS electropositive fi lters.
This technique was used in combination with removal of the fi lters after backwashing, soaking them in a small volume of 3 % beef extract solution, and followed by
the scraping of the fi lters to remove trapped bacteria. When all the eluates from the
fi lters were combined, recovery of Aer. salmonicida was determined to be 35 %.
Thus, Maheshkumar et al. ( 1990 ) concluded that the fi ltration technique demonstrated greater sensitivity than the direct examination of water. Also, the enumeration of Aer. salmonicida was not overly effected by the presence of other bacteria
because in the water samples the pathogen retained the ability to produce a brown
pigment. However, these authors noted that biochemical/serological tests would be
necessary for the detection of atypical non-pigmenting isolates. Nonetheless the
classic dilemma of many ecological studies persists, i.e. how does the response of
an organism in laboratory-based experiments relate to its performance in the natural
environment where it needs to interact in a diverse and heterogeneous community?
It is a problem still in search of a reliable solution.
It has often been stated that ecological investigations of Aer. salmonicida are
hampered by the lack of an effective selective isolation medium specifi cally
formulated for the pathogen. It was, for example, the opinion of Cornick et al.
( 1969 ) that the development of a selective medium for Aer. salmonicida could quite
possibly change the present views on the habitat and viability of the pathogen.
McCarthy ( 1980 ) also believed a selective medium would greatly assist ecological
work. No doubt such a medium would be extremely useful; however, it is unlikely
that its existence alone would cause all the remaining diffi culties concerning ecological work on Aer. salmonicida to evaporate. It should be noted, however, that
although a selective medium for Aer. salmonicida has not yet been formulated, CBB
serves well as a differential growth medium, and is increasingly employed for this
purpose. This medium is especially useful in the detection of Aer. salmonicida in
fi sh tissues. For instance using CBB, Cipriano et al. ( 1992 ) recovered the pathogen
from 56 % of mucus samples, but interestingly from only 6 % of kidney material,
taken from salmonids. This was reinforced by a later study, which pointed to the
presence of the organism in gills and well as mucus, i.e. external carriage (Cipriano
et al. 1996a , b ). The data has been summarised, as follows:
Aeromonas salmonicida
Chromobacterium, Esch. coli, Flavobacterium and Pseudomonas, and their metabolites, to inhibit the survival of Aer. salmonicida in non-sterile seawater was also
reported by Effendi and Austin ( 1991 ). Thus, because the isolation and study of the
viability of the pathogen is complicated by competition and inhibition by other
organisms, it is not surprising that investigations into survival of the pathogen,
which could help to establish whether or not it is capable of a free-living existence
outside of fi sh, have invariably retreated to laboratory-based experiments using sterilized water. A fi ltration method tested by Maheshkumar et al. ( 1990 ) attempted,
with some degree of success, to overcome the diffi culties of isolation of Aer. salmonicida posed by the presence of other bacteria, i.e. the possible overgrowth by
small numbers of cells of the pathogen. In their studies, up to 5 l of hatchery water
was seeded with Aer. salmonicida and passed through 1
-MDS electropositive fi lters.
This technique was used in combination with removal of the fi lters after backwashing, soaking them in a small volume of 3 % beef extract solution, and followed by
the scraping of the fi lters to remove trapped bacteria. When all the eluates from the
fi lters were combined, recovery of Aer. salmonicida was determined to be 35 %.
Thus, Maheshkumar et al. ( 1990 ) concluded that the fi ltration technique demonstrated greater sensitivity than the direct examination of water. Also, the enumeration of Aer. salmonicida was not overly effected by the presence of other bacteria
because in the water samples the pathogen retained the ability to produce a brown
pigment. However, these authors noted that biochemical/serological tests would be
necessary for the detection of atypical non-pigmenting isolates. Nonetheless the
classic dilemma of many ecological studies persists, i.e. how does the response of
an organism in laboratory-based experiments relate to its performance in the natural
environment where it needs to interact in a diverse and heterogeneous community?
It is a problem still in search of a reliable solution.
It has often been stated that ecological investigations of Aer. salmonicida are
hampered by the lack of an effective selective isolation medium specifi cally
formulated for the pathogen. It was, for example, the opinion of Cornick et al.
( 1969 ) that the development of a selective medium for Aer. salmonicida could quite
possibly change the present views on the habitat and viability of the pathogen.
McCarthy ( 1980 ) also believed a selective medium would greatly assist ecological
work. No doubt such a medium would be extremely useful; however, it is unlikely
that its existence alone would cause all the remaining diffi culties concerning ecological work on Aer. salmonicida to evaporate. It should be noted, however, that
although a selective medium for Aer. salmonicida has not yet been formulated, CBB
serves well as a differential growth medium, and is increasingly employed for this
purpose. This medium is especially useful in the detection of Aer. salmonicida in
fi sh tissues. For instance using CBB, Cipriano et al. ( 1992 ) recovered the pathogen
from 56 % of mucus samples, but interestingly from only 6 % of kidney material,
taken from salmonids. This was reinforced by a later study, which pointed to the
presence of the organism in gills and well as mucus, i.e. external carriage (Cipriano
et al. 1996a , b ). The data has been summarised, as follows:
Aeromonas salmonicida