712
named pathogen is accurate. It would sometimes help if authors referred colloquially to a group, e.g. aeromonads, rather than a named species, such as Aer. hydrophila. The dominance of 16S rRNA sequencing should have helped, but evidence
suggest that practitioners do not always appreciate the shortcomings of the technique. 16S rRNA sequencing is a powerful tool, but the data obtained have to be
used in context, and not as the sole means of bacterial taxonomy.
The realization that not every microbial cell is capable of being cultured has
come as a wakening call to science. Does this lack of culturability refl ect an absence
of suitable methods, damage/senescence of the bacterial cells, or the presence of
unique microbial forms that are incapable of growing outside of the host? What
exactly is the status of Candidatus, and will it ever be capable of in vitro growth? It
is speculative how often the presence of unculturable bacteria may have been
masked by secondary invaders or saprophytes that grow on whatever medium has
been used for isolation. Then, there are situations, such as red mark syndrome and
strawberry disease of rainbow trout, that are now considered to be infectious
disease(s), but what is the causal agent? Is rickettsia really responsible or is there
something else involved in the pathology? In the new Millennium, there are many
more fundamental questions to be answered, and so interest in bacterial fi sh diseases is likely to increase rather than contract. So what developments are likely to
be seen in the future:
The Recognition of New and Emerging Conditions
As aquaculture increases both in total production and in the range of species used,
new diseases will continue to occur. In some cases, this will refl ect the movement of
micro-organisms from one host to another. This is likely to happen when a new fi sh
species is introduced into an area for farming, and there is an exchange of organisms. It may be that the new species lacks resistance, and therefore infection develops. Also in large monoculture situations, it is easier for a potential pathogen to enter
a weakened host, initiate a disease cycle, and spread to adjacent animals. New diseases will require research to develop effective control and diagnostic procedures.
Co-Infections
There is a tendency for scientists to want to associate diseases with single species of
pathogens (Fig. 15.1 ). The possibility of two or more organisms working together
or sequentially may seem like heresy. Yet, the current approaches to diagnoses
would be unlikely to spot species succession, and co-infections might be mistaken
for evidence of contamination. With species succession the situation could arise
whereby one organism instigates an infection and others develop/exacerbates the
condition. There have been personal observations of Aer. salmonicida instigating
15 Conclusions
named pathogen is accurate. It would sometimes help if authors referred colloquially to a group, e.g. aeromonads, rather than a named species, such as Aer. hydrophila. The dominance of 16S rRNA sequencing should have helped, but evidence
suggest that practitioners do not always appreciate the shortcomings of the technique. 16S rRNA sequencing is a powerful tool, but the data obtained have to be
used in context, and not as the sole means of bacterial taxonomy.
The realization that not every microbial cell is capable of being cultured has
come as a wakening call to science. Does this lack of culturability refl ect an absence
of suitable methods, damage/senescence of the bacterial cells, or the presence of
unique microbial forms that are incapable of growing outside of the host? What
exactly is the status of Candidatus, and will it ever be capable of in vitro growth? It
is speculative how often the presence of unculturable bacteria may have been
masked by secondary invaders or saprophytes that grow on whatever medium has
been used for isolation. Then, there are situations, such as red mark syndrome and
strawberry disease of rainbow trout, that are now considered to be infectious
disease(s), but what is the causal agent? Is rickettsia really responsible or is there
something else involved in the pathology? In the new Millennium, there are many
more fundamental questions to be answered, and so interest in bacterial fi sh diseases is likely to increase rather than contract. So what developments are likely to
be seen in the future:
The Recognition of New and Emerging Conditions
As aquaculture increases both in total production and in the range of species used,
new diseases will continue to occur. In some cases, this will refl ect the movement of
micro-organisms from one host to another. This is likely to happen when a new fi sh
species is introduced into an area for farming, and there is an exchange of organisms. It may be that the new species lacks resistance, and therefore infection develops. Also in large monoculture situations, it is easier for a potential pathogen to enter
a weakened host, initiate a disease cycle, and spread to adjacent animals. New diseases will require research to develop effective control and diagnostic procedures.
Co-Infections
There is a tendency for scientists to want to associate diseases with single species of
pathogens (Fig. 15.1 ). The possibility of two or more organisms working together
or sequentially may seem like heresy. Yet, the current approaches to diagnoses
would be unlikely to spot species succession, and co-infections might be mistaken
for evidence of contamination. With species succession the situation could arise
whereby one organism instigates an infection and others develop/exacerbates the
condition. There have been personal observations of Aer. salmonicida instigating
15 Conclusions