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A Fishy Business
The pathogenicity and control of pathogens bedeviling aquaculture are
becoming increasingly urgent research agendas for fish microbiologists
Bernard Dixon
I think I have detected an increase in the frequency of papers appearing in wide-ranging microbiology journals on fish infections and methods of combating them. If reports on piscine
pathogens are indeed spilling over from their
appropriate specialized publications into the
general literature, there is a highly plausible
clutch of linked explanations.
Firstly, the traditional toil of hauling fish out
of the seas in nets has, like cattle ranching on
land, reached its productive limit. Secondly, this
shift has triggered a massive expansion of fish
farming, which is now the fastest-growing sector of the world’s food economy. Having risen
by an average of 11% per year over the past
decade, aquaculture is expected to overtake beef
production as a source of food by 2010. Thirdly,
these seismic changes are underlining the pressing
need to deal more effectively with the largest
causes of financial losses in aquaculture: the
moulds and bacteria responsible for fish diseases.
Saprolegnia, an oomycete (water mould) related to the Phytophthora infestans which precipitated the Irish potato famine at the end of the
19th century, is attracting particular concern.
As pointed out by Pieter van West of Aberdeen
University in Scotland, the dangers posed by this
ever-present threat to fish farms have been exacerbated by a decision six years ago to prohibit the
agent formerly deployed against the organism.
“Up until 2002, Saprolegnia infections in
aquaculture were kept under control with malachite green, an organic dye that is very efficient
in killing the pathogen,” van West writes in
Mycologist (20:99 –104, 2006). “However, the
use of malachite green has been banned worldwide due to its carcinogenic and toxicological
effects. . . This has resulted in a dramatic reemergence of Saprolegnia infections in aquaculture. As a consequence Saprolegnia parasitica
108 Y Microbe / Volume 3, Number 3, 2008
is now, economically, a very important fish
pathogen, especially on catfish, salmon and
trout species.”
Enthusiasts with fish tanks at home are familiar with Saprolegnia as an occasional nuisance in the form of white or grey patches of
filamentous mycelium on the body or fins of
freshwater species. On a world scale, however, it is a formidable foe. As van West emphasizes, it has not only contributed to crashes
in natural populations of salmonids throughout the world. It also causes losses of millions
of dollars annually for the aquaculture businesses of the United States, Canada, Scotland,
Scandinavia, Chile, and Japan.
In the absence of malachite green, the few
alternative chemicals that do staunch the disease
in salmonid eggs do not give adequate protection after hatching. Hopes are now pinned on
future advances in understanding the basic molecular processes of this ferocious water mould,
its interactions with susceptible fish, and the
identification of the relevant genes and proteins.
It’s possible, perhaps likely, that such developments will spawn new strategies to help the
ailing aquaculture industry.
Meanwhile, one recent development is already being harnessed in the battle against bacterial fish diseases. This is the use of naturally
antagonistic organisms, thought to function either by competitive inhibition, by nonspecifically enhancing immunity, or by modifying the
environment. Categorized as probiotics (and associated with a similar mixture of genuine science and hocus-pocus as that which surrounds
probiotics in humans), they are already being
incorporated into fish feed.
While some of these organisms are lactic acid
bacteria, other genera and species are showing a
greater degree of genuine efficacy. One recent
example is the Bacillus subtilis AB1 which Brian
Austin and coworkers at Heriot-Watt University, Edinburgh, and the University of West Indies in St. Augustine, Trinidad, have developed
as a putative agent to combat species of Aeromonas which attack rainbow trout.
As described in the Journal of Applied Microbiology (103:1699 –1706, 2007), the organism
was one of several obtained from the digestive
tract of euthanized rainbow trout and selected
for its inhibitory activity against a pathogenic
aeromonad recovered from diseased tilapia.
When healthy rainbow trout received the B.
subtilis AB1 in their feed for 14 days, they
survived challenge with the pathogen. In immunological assays, the bacillus also specifically stimulated respiratory burst, serum and
gut peroxidase activities, phagocytic killing,
total and alpha-1 antiprotease activities, and
lymphocyte populations.
“The fact that B. subtilis AB1 was isolated
from the gut of apparently healthy rainbow
trout confirms the potential role of gut microorganisms in exerting an important role in the
wellbeing of the host fish,” Austin and his group
conclude. The organism “stimulated both cellular and humoral immune responses, which may
have provided the rainbow trout with adequate
protection to survive the challenge by the highly
virulent Aeromonas sp.” Studies conducted elsewhere have demonstrated that a closely related
B. subtilis shows antibiosis against pathogenic
vibrios. It has also been used to improve pond
water quality, leading to increased survival of
black tiger prawns.
An alternative approach to the control of fish
pathogens is, of course, immunization. But the
difficulties inherent in their development can be
gauged from recent, painstaking progress in
studying a genus, Moritella, that is taxonomically remote from Aeromonas and other bacterial fish pathogens. The psychrophile M. viscosa
is a particular target as the cause of winter ulcers
in fish raised in sea cages below 10oC. Endemic
in farmed salmonids in North Atlantic countries, the disease causes significant mortality and
thus financial losses. Cod is vulnerable, as is
turbot, though halibut is relatively resistant.
A polyvalent vaccine has been on the market
for several years, and has reduced the frequency
of the disease. However, there is still considerable room for improvement. Yet the route towards a more potent and specific vaccine has
been bedeviled by an elementary difficulty—the
sensitivity of the bacterium to lysis—that reminds us how far fish microbiology has lagged
behind other sectors of the subject until comparatively recently. Whereas the fragility of Neisseria gonorrhoeae and Streptococcus pneumoniae
has been recognized for many years, and their
autolytic enzymes thus intensively investigated,
far less is known concerning those enzymes in
marine bacteria.
Two researchers who are now tackling this
issue are Eva Benediktsdottir and Karen Heidarsdottir at the University of Iceland in Reykjavik. According to a recent report in Letters in
Applied Microbiology (45:115–120, 2007),
their work so far should not only facilitate the
manufacture of an effective vaccine but also
throw light on the pathogenicity of M. viscosa.
Their meticulous studies have confirmed the environmental factors responsible for the premature lysis of the bacterial cells, and defined the
composition of the medium and the temperature
which minimize lysis and guarantee successful
culture.
Probably the most important single advance
in fish pathology over the past year has been the
delineation of the complete genome sequence
of Flavobacterium psychrophilum by Eric
Duchaud and colleagues in several different
laboratories in France. Causing extensive necrosis known as “cold-water disease” in mature fish and a lethal hemorrhagic septicemia
in young fish, F. psychrophilium is responsible
for considerable economic losses in all major
areas of salmonid aquaculture. There is no
specific vaccine, and antibiotics do not offer a
satisfactory method of control. However, as
described in Nature Biotechnology (25:763–
769, 2007), the French research is expected to
lead to major advances in understanding the
molecular pathogenesis of the infection and
thus the development of efficient disease control strategies.
Though the tiny crustaceans called sea lice
have been in the news recently for their attacks
on wild and farmed salmon in British Columbia,
it is bacteria, closely followed by oomycetes
such as Saprolegnia, that inflict the greatest economic harm on aquaculture worldwide. Notwithstanding progress in biological control and
other techniques, genome sequencing seems
likely to provide the wherewithal for substantial
advances in the future.
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