254
reduction of tetrazoliums to insoluble formazans. There was not any alteration in
the LPS composition of the cells, but an alteration in the protein composition was
recorded, with a reduction in some (15, 17, 22, 30 and 70 kDa proteins) and an
increase in a 49 kDa protein. This was accompanied by a loss in DNA. That these
cells were still alive was indicated by the development of large bizarre shapes following the addition of yeast extract and nalidixic acid (after Kogure et al. 1979 ).
To summarise, the question as to whether Aer. salmonicida is able to persist in a
free-living form outwith a fi sh host is still outstanding. An equally important corollary concerns the pathogenicity of such forms, assuming that they exist, i.e. can they
retain the ability to infect fi sh? Traditionally, Aer. salmonicida has been defi ned as
an obligate fi sh pathogen. However, there appears to be an increasing trend in ecological studies to at least consider the possibility that this defi nition may no longer
hold true at all times. On the basis of the available data obtained from numerous
survival studies, it may be stated that Aer. salmonicida has the ability to persist in
the aquatic environment for protracted periods. It is the mechanism of this survival
and its effects on the organism in the natural environment around which the debate
now centres. It is possible that Aer. salmonicida could exist in a so-called nonculturable stages (perhaps due to the presence of an altered morphological state,
such as L-forms). After all, the reason for the occurrence of explosive outbreaks of
furunculosis among fi sh populations, particularly salmonids, which have not been
previously expose to the disease, has yet to be explained. The answer to this important question of facultative versus obligate pathogen still awaits the development of
methods for the more effi cient and refi ned detection of the organism in the natural
environment. As one astute scientist has said, ‘absence of evidence is not evidence
of absence’; hence further efforts to breach this gap in our understanding of Aer.
salmonicida epizootiology are essential.
Aeromonas salmonicida – Transmission by Fish Fish undoubtedly play a major
role in the transmission of disease among themselves. One early study Blake and
Clark ( 1931 ) stated that furunculosis was only spread by infected fi sh or by material
which has come into contact with them. Fortunately, the recovery of Aer. salmonicida from fi sh tissues is generally less troublesome than seeking it in other environmental sources. Fish may act as a source of infection in two ways: those which have
died or are ill with furunculosis are heavily contaminated with the pathogen, alternatively they may be carriers which, although appearing healthy, harbour the pathogen in their organs where it can be released if the fi sh eventually succumb to the
disease. Both these aspects have received attention. McCarthy ( 1980 ) established
that material from a furuncle could contain up to 10
8 viable cells/ml of necrotic tissue, and was interested to assess the viability of Aer. salmonicida in dead fi sh and
the degree to which they could contaminate pond water. He found that Aer. salmonicida remained viable in fi sh (muscle) tissue for 32 days, and for 40 days in the tank
water where the dead fi sh had been kept, thus providing a possible source of infection for healthy fi sh. Another study showed that Aer. salmonicida remained viable
in infected trout tissues and internal organs (heart, liver, spleen and kidney) for 49
days when the fi sh were stored at −10 °C (Cornick et al. 1969 ). However, the patho5 Aeromonadaceae Representative (Aeromonas salmonicida)
reduction of tetrazoliums to insoluble formazans. There was not any alteration in
the LPS composition of the cells, but an alteration in the protein composition was
recorded, with a reduction in some (15, 17, 22, 30 and 70 kDa proteins) and an
increase in a 49 kDa protein. This was accompanied by a loss in DNA. That these
cells were still alive was indicated by the development of large bizarre shapes following the addition of yeast extract and nalidixic acid (after Kogure et al. 1979 ).
To summarise, the question as to whether Aer. salmonicida is able to persist in a
free-living form outwith a fi sh host is still outstanding. An equally important corollary concerns the pathogenicity of such forms, assuming that they exist, i.e. can they
retain the ability to infect fi sh? Traditionally, Aer. salmonicida has been defi ned as
an obligate fi sh pathogen. However, there appears to be an increasing trend in ecological studies to at least consider the possibility that this defi nition may no longer
hold true at all times. On the basis of the available data obtained from numerous
survival studies, it may be stated that Aer. salmonicida has the ability to persist in
the aquatic environment for protracted periods. It is the mechanism of this survival
and its effects on the organism in the natural environment around which the debate
now centres. It is possible that Aer. salmonicida could exist in a so-called nonculturable stages (perhaps due to the presence of an altered morphological state,
such as L-forms). After all, the reason for the occurrence of explosive outbreaks of
furunculosis among fi sh populations, particularly salmonids, which have not been
previously expose to the disease, has yet to be explained. The answer to this important question of facultative versus obligate pathogen still awaits the development of
methods for the more effi cient and refi ned detection of the organism in the natural
environment. As one astute scientist has said, ‘absence of evidence is not evidence
of absence’; hence further efforts to breach this gap in our understanding of Aer.
salmonicida epizootiology are essential.
Aeromonas salmonicida – Transmission by Fish Fish undoubtedly play a major
role in the transmission of disease among themselves. One early study Blake and
Clark ( 1931 ) stated that furunculosis was only spread by infected fi sh or by material
which has come into contact with them. Fortunately, the recovery of Aer. salmonicida from fi sh tissues is generally less troublesome than seeking it in other environmental sources. Fish may act as a source of infection in two ways: those which have
died or are ill with furunculosis are heavily contaminated with the pathogen, alternatively they may be carriers which, although appearing healthy, harbour the pathogen in their organs where it can be released if the fi sh eventually succumb to the
disease. Both these aspects have received attention. McCarthy ( 1980 ) established
that material from a furuncle could contain up to 10
8 viable cells/ml of necrotic tissue, and was interested to assess the viability of Aer. salmonicida in dead fi sh and
the degree to which they could contaminate pond water. He found that Aer. salmonicida remained viable in fi sh (muscle) tissue for 32 days, and for 40 days in the tank
water where the dead fi sh had been kept, thus providing a possible source of infection for healthy fi sh. Another study showed that Aer. salmonicida remained viable
in infected trout tissues and internal organs (heart, liver, spleen and kidney) for 49
days when the fi sh were stored at −10 °C (Cornick et al. 1969 ). However, the patho5 Aeromonadaceae Representative (Aeromonas salmonicida)