India is blessed with enormous aquatic resources. In fact in a
country like India, providing sustainable livelihood opportunities
and food security for ever increasing rural population is perhaps
the greatest national challenge, where aquaculture plays a pivotal
role in meeting this challenge. As per UNO projection on population
over the next 50 years, India will exceed China as the most populous
country in the world and taking this alarming situation, we should
assume agriculture along with aquaculture and livestock as the most
appropriate farm activities and as a tool for poverty reduction,
food security and overall rural development. So blue revolution can
be a solution to feed this ever increasing population. Although
aquaculture in India has shown a rapid progress during past few
years but some major problems are hindering the progress path and
disease being one of them. Hence for successful aquaculture we must
ensure quality feed, good environment and disease free seeds and
juveniles.
With intensification of aquaculture involving high stocking
densities coupled with external inputs of feed and fertilizer,
sometimes of doubtful quality, the fish farmer is now face to face
with diseases of various kinds from protozoan to helminth parasites
to fungal, bacterial and viral diseases and syndromes. This has
resulted in increased use of chemicals and antibiotics that are
anyway not a welcome addition to the human food through this
important food chain. India suffered the worst disaster in mid
eighties due to epizootic ulcerative syndrome (EUS) which lead to
heavy mortality of fishes in ponds, tanks, rivers and reservoirs.
The country again suffered a major setback again in brackish water
aquaculture when the most valuable shrimp crops meant for export
were lost in almost all maritime states particularly Andhra Pradesh.
This was due to white spot syndrome and due to this a large number
of farms were closed and farmers gave up shrimp farming which lead
to decline in foreign exchange through shrimp farming. Fish and
shellfish management have to be given their dues if the industry has
to be sustainable and the farmers and entrepreneurs have to be
making profits. Antibiotics have sometimes been used to reduce
disease; however indiscriminate use has in some cases led to
increased antibiotic resistance and problem of tissue residues and
trade issues. There is a wide variety of vaccines available for
other sectors but a very few in fisheries sector. The reason is very
little research in fish pathology particularly in our country. So a
question comes how to control disease? However, one measure that
might be of assistance is the use of "probiotics". "Probiotics"
generally includes bacteria, cyanobacteria, micro algae fungi, etc.
Some Chinese researchers translate it into English as "Normal
micro biota" or "Effective micro biota"; it includes
Photosynthetic bacteria, Lactobacillus, Actinomycetes,
Nitrobacteria, Denitrifying
bacteria, Bifidobacterium, yeast, etc. Usually, it does not include
micro algae. In English literature, probiotic bacteria are generally
called the bacteria which can improve the water quality of
aquaculture, and (or) inhibit the pathogens in water there by
increasing production. "Probiotics", "Probiont",
"Probiotic bacteria" or "Beneficial bacteria"
are the terms synonymously used for probiotic bacteria. Probiotics
are a cultured product or live microbial feed supplement, which
beneficially affects the host by improving its intestinal balance
and health of the host. The first probiotic discovered long time ago
was Lactobacillus sp., the lactic acid producing bacteria.
CURRENT RESEARCH STATUS OF PROBIOTICS
Nogami and Maeda (1992) isolated a bacteria strain from a crustacean
culture pond. The bacterial strain was found to improve the growth
of crab (Portunus trituberculatus) larvae and repress the
growth of other pathogenic bacteria, especially Vibrio spp., but
would not kill or inhibit useful micro algae in sea water when it
was added into the culture water. Among the bacteria population
present in the culture water of the crab larvae, the numbers of
Vibrio spp. and pigment bacteria decreased or even became
undetectable when the bacteria was added into culture water. The
production and survival rate of crab larvae were greatly increased
by the addition of the probiotic bacteria into the culture water.
They also suggested that the bacterium might improve the
physiological state of the crab larvae by serving as a nutrient
source during its growth. This bacterium may have a good effect in
the crab larval culture as a biocontrolling agent in the future.
Austin et al (1992) reported a kind of micro algae
(Tetraselmis suecica), which can inhibit pathogenic
bacteria of fish. Teraselmis suecica was observed to
inhibit Aeromonos hydrophila, A. salmonicida,
Serrstia liquefaciens, Vibrio anguillaram, V.
salmonicida and Yersnia ruckeri type I. When used as a
food supplement, the algal cells inhibited laboratory-induced
infection in Atlantic salmon. When used therapeutically, the algal
cells and their extracts reduced mortalities caused by A.
salmonicida, A. liquefaciens, V.
anguillaram, V. salmonicida and Yersnia ruckeri
type I. They suggested that there may be some bioactive compounds in
the algal cells, and there appears to be a significant role for
Tetraselmis in the control of fish diseases.
Smith and Davey (1993) reported that a fluorescent
strain pseudomonad bacteria can competitively inhibit the growth of
fish pathogen A.
salmonicida. Their results
show that the fluorescent pseudomonad is capable of inhibiting the
growth of A. salmonicida in culture media and that this
inhibition is probably due
to competition for free iron. In a challenge test of
the Atlantic salmon by A. salmonicida, a statistically
significant reduction in the frequency of stress-induced infection
in the group of fish bathed in the bacterium fluorescent pseudomonad
compared to the control group was observed.
Austin et al (1995) reported a probiotic strain of
Vibrio alginolyticus, which did not cause any harmful
effect in salmonids. By using the cross-streaking method, the
probiont was observed to inhibit the fish pathogens.
When the freeze-dried culture supernatant was added to the
pathogenic bacteria such as V. ordalii, V. anguillarum,
A. salmonicida and Y. ruckeri, showed a rapid or
steady decline in the number of culturable cells, compared to the
controls. Their results indicated that application of the probiont
to Atlantic salmon culture led to a reduction in mortalities when
challenged with A. salmonicida and to a lesser extent V.
anguillarum and V. ordalii. The observation with this
probiotic Vibrio is encouraging, and it appears that there
is tremendous potential for the use of such probiotics in
aquaculture as part of a disease control strategy.
Maeda and Nagami (1989) reported some aspects of the biocontrolling
method in aquaculture. In their study bacterial strains possessing
vibrio static activity which improved the growth of prawn and crab
larvae were observed. By applying these bacteria in aquaculture, a
biological equilibrium between competing beneficial and deleterious
microorganisms was produced, and results show that the population of
Vibrio spp., which frequently causes large scale damage to
the larval production, was decreased. Survival rate of the
crustacean larvae in these experiments showed much higher than those
without the addition of bacterial strains. They hope that addition
of these strains of bacteria will repress the growth of Vibrio
spp., fungi and other pathogenic microorganisms. Their data
suggest that controlling the aquaculture ecosystem using bacteria
and protozoa is quite possible and if this system is adopted, it
will maintain the aquaculture environment in better condition, which
will increase the production of fish and crustaceans.
Garriques and Arevalo (1995) reported that the use of V.
alginolyticus as a probiotic agent may increase survival and
growth in P. vannamei postlarvae by competitively excluding
potential pathogenic bacteria, and can effectively reduce or
eliminate the need for antibiotic prophylaxis in intensive larvae
culture system. They believe that in nature a very small percentage
of Vibrio sp. is truly pathogenic, and the addition of
potentially pathogenic bacteria to aquaculture system through water,
algae, and/or Artemia was recognized. In their study, the addition
of the bacteria V. alginolyticus as a probiotic to mass
larvae culture tanks resulted in increased survival rates and growth
over the controls and the antibiotic prophylaxes.
Jiravanichpaisal and Chuaychuwong et al (1997) reported the
use of Lactobacillus sp.
as the probiotic bacteria in the giant tiger shrimp (P.
monodon
Fabricius). They designed to investigate an
effective treatment of Lactobacillus sp. against vibriosis
and white spot diseases in P. monodon. They investigated
the growth of some probiotic bacteria, and their survival in the 20
ppt sea water for at least 7 days. Inhibiting activity of two
Lactobacillus sp. against Vibrio sp., E. coli,
Staphylococcus sp. and Bacillus subtilis was
determined.
Direkbusarakom and Yoshimizu et al (1997) reported Vibrio
spp. which dominate in shrimp hatchery against some fish
pathogens. Two isolates of Vibrio spp. which are the
dominant composition of the flora in shrimp hatchery, were studied
for antiviral activity against infectious haematopoietic necrosis
virus (IHNV) and Oncorhynchus masou virus (OMV). Both strains of
bacteria showed the antiviral activities against IHNV and OMV by
reducing the number of plaque.Their results demonstrate the
possibility of using the Vibrio flora against the
pathogenic viruses in shrimp culture.
Sugita and Shibuga (1996) reported the antibacterial abilities of
intestinal bacteria in freshwater cultured fish. They isolated
bacteria from the intestine of 7 kinds of freshwater cultured fish,
and investigated the antibacterial abilities of these bacteria to 18
fish or human common pathogenic bacteria. Their
results indicated that the bacteria isolated from intestine of 7
kinds of freshwater cultured fish possess the antibacterial
abilities, and the presence of the intestinal bacteria can protect
the fish against the infection by pathogenic bacteria.
Maeda and Liao (1992) reported on the effect of bacterial strains
obtained from soil extracts on the growth of prawn larvae of P.
monodon. Higher
survival and molt rates of prawn larvae were observed in the
experiment treated with soil extract, and the
bacterial strain which promoted the growth of prawn larvae was
isolated. They have assumed that if a specific bacterium is cultured
and added to the prawn ecosystem to the level of 10 million cell/ml,
other bacteria may hardly inhibit the same biotype because of
protozoan activity which shall be one of the way to biologically
control the aquaculture water biotype and ecosystem.
Maeda and Nogami et al (1992) have reported the utility of
microbial food assemblages in culturing a crab, Portunus
trituberculatus. Assemblages of microorganisms were produced by
adding several nutrients, urea, glucose and potassium phosphate, to
natural seawater with gentle aeration in which bacteria and yeast
were prevailing. When these cultured microbes were added to sea
water where crab larvae of Portunus trituberculatus were
reared, bacteria numbers decreased very rapidly, followed by the
decrease in flagellated protozoa and diatoms. Their results suggest
that the crab larvae fed on these microorganisms successively. They
found some
strains of bacteria promoted larval growth, although yeasts did not
support its growth. By adopting
these assemblages of microorganisms a high yield was obtained for a
prawn larva P. japonicus, although the success was not
always consistent.
Douillet and Langdon (1994) have reported use of probiotics for the
culture of larvae of the Pacific oyster (Crassostrea gigas
Thunbeerg). They added probiotic bacteria as a food supplement to
xenic larval cultures of the oyster Crassostrea gigas which
consistently enhanced growth of larvae during different seasons of
the year. Probiotic bacteria were added, at 0.1 million cells/ml, to
cultures of algal-fed larvae, the proportion of larvae that are set
to produce spat, and subsequently the number of spat increased.
Manipulation of bacterial
population present in bivalve larval cultures is a potentially
useful strategy for the enhancement of oyster production.
They suggest that the
mechanisms of the action of probiotic bacteria are providing
essential nutrients that are not present in the algal diets or
improving the oyster's digestion by supplying digestive enzymes to
the larvae or removing metabolic substances released by bivalves or
algae.
Maeda and Liao (1994) have reported microbial processes in
aquaculture environment and their importance in increasing
crustacean production. They suggested that based on the
photosynthesis of micro algae mainly, it was clarified that
bacteria, protozoa and other microorganisms from microbial food
assemblages use the organic matter produced by the algae and that
these assemblages play a significant role in the aquatic food chain.
The growth of the larvae and their production were markedly promoted
by the probiotic bacteria. In their paper, they also described the
presence of a bacterial clump, stained with a fluorescent dye,
inside the digestive organ of the crab Portunus trituberculatus.
In China, the studies on probiotics in aquaculture were focused on
the photosynthetic bacteria. Qiao Zhenguo et al (1992) have studied
three strains of photosynthetic bacteria used in prawn (P.
chinensis) diet preparation and their effect. Addition
of the photosynthetic bacteria in the food or culture water was
found to improve the growth of the prawn and the quality of the
water.
Cui Jingjin et al (1997) have reported on the
application of photosynthetic bacteria in the hatchery rearing of P.
chinensis. They used a mixture of several kinds of
photosynthetic bacteria (Rhodomonas sp. ) as water cleaner
and auxiliary food. Their
results showed that the water quality of the pond treated with the
bacteria was remarkably
improved, the fouling on the shell of the larvae was reduced, the
metamorphosis time of the larvae was 1 day or even earlier, and the
production of post-larvae was more than that of the control.
Wang Xianghong et al (1997) have done some research work on
probiotic bacteria in shrimp aquaculture. On the basis of studies on
intestinal micro flora of wild adult shrimp P. chinensis,
they have chosen some probiotic bacteria from shrimp intestinal
flora. When the two probiotic bacterial strains were added to the
larval culture water, the
survival rate, the abilities of disease resistence and low salinity
tolerance were improved; average body length and weight were
increased. In addition, the probiotic bacteria, when added to the
larval culture water was found not to influence the total
bacterial number
and water quality
of the sea water. They also found that some probiotic bacteria can
produce some digestive enzymes; these enzymes may improve the
digestion of shrimp larvae, thus enhancing the ability of stress
resistance and health of the larvae.
Mechanism of action of the probiotic bacteria
The mechanism of action of the probiotic bacteria has not been
studied systematically. According to some recent publications, in
the aquaculture the mechanism of action of the probiotic bacteria
may have several aspects.
1.Probiotic bacteria may
competitively exclude the pathogenic bacteria or produce substances
that inhibit the growth of the pathogenic bacteria.
2. Provide essential
nutrients to enhance the nutrition of the cultured animals.
3. Provide digestive
enzymes to enhance the digestion of the cultured animals.
4. Probiotic bacteria
directly uptake or decompose the organic
matter or toxic material in the water improving the quality of the
water.
Chinese researchers have done some studies on the probiotic bacteria
to improve the shrimp culture water, and achieved remarkable results
(Li Zhuojia et al 1997). For example, when photosynthetic bacteria
was added into the water, it could eliminate the NH3-N, H2S and
organic acids, and other harmful materials rapidly, improve the
water quality and balance the pH. The heterotrophic probiotic
bacteria may have chemical actions such as oxidation,
ammoniafication, nitrification, denitrification, sulphurication and
nitrogen fixation. When these bacteria were added into the water,
they could decompose the
excreta of fish or prawns, remaining food materials, remains of the
plankton and other organic materials to CO2, nitrate and phosphate.
These inorganic salts provide the nutrition for the growth of micro
algae, while the bacteria grow rapidly and become the dominant group
in the water, inhibiting the growth of the pathogenic
microorganisms. The photosynthesis of the micro algae provide
dissolved oxygen for oxidation and decomposition of the organic
materials and for the respiration of the microbes and cultured
animals. This kind of cycle may improve the nutrient cycle, and it
can create a balance between bacteria and micro algae, and
maintaining a good water quality environment for the cultured
animals.
The
feasibility and future of the application of probiotics in
aquaculture
Based on the previous research results on probiotics we
suggest that the use of probiotic bacteria in aquaculture has
tremendous scope and the study of the application of probiotics in
aquaculture has a glorious future. At
present, the probiotics are widely applied in United States of
America, Japan, European countries, Indonesia, India and Thailand,
with commendable results. The probiotics have become commodities in
some countries, for example, the
Alken-Murray Corporation and American
Standard Products company of United States of America and the
company of Japan have their probiotics products. The study may
create a new field of industrial products, like the industrial
fields of aquaculture product processing and Aquacultural food
processing.
China is a large country in aquaculture, but the application and
development of the probiotics in Chinese aquaculture is very meager
when compared to other countries. In recent years, the diseases of
shrimps hindered the development of shrimp culture. The Chinese
government has realized the economic value and potential social
benefits of the application of probiotics in aquaculture, and has,
recently, paid more attention to the study and development of
probiotics in aquaculture. Thus the government has increased the
research funds for it. Probiotics
principally inhibit the growth and decrease the pathogenicity of the
pathogenic bacteria, enhance the nutrition of the aquacultured
animals, improve the quality of the aquaculture water and decrease
the use of antibiotics and other chemicals; thus decreasing
environmental contamination by the residual antibiotics and
chemicals. This benefit of probiotics will be long
lasting, and the application of probiotics will become a major field
in the development of aquaculture in the future.
Application
of Probiotics in Aquaculture
During the past 20 years, aquaculture industry has been growing
tremendously, especially that of marine fish, shrimps and bivalves.
But, as with many other industries, this rapid growth has brought
with it the problem of environmental pollution. Contamination of
coastal waters due to aquaculture is posing serious concerns among
law makers as well as scientists. The coastal environment has been
seriously damaged, often resulting in disease outbreaks. Recently,
shrimp culture all over the world has been frequently affected by
viral and bacterial diseases inflicting huge loss. In China, the
production of shrimps decreased seriously. The production of shrimps
was 200,000 tons in 1992, but was only 55,000 tons in 1994.
Pathogenic microorganisms implicated in these outbreaks were
viruses, bacteria, rickettsia, mycoplasma, algae, fungi and
protozoan parasites. For preventing and controlling diseases, a host
of antibiotics, pesticides and other chemicals were used possibly
creating antibiotic resistant bacteria, persistence of pesticides
and other toxic chemicals in aquatic environment and creating human
health hazards. Thus, how to improve the ecological environment of
aquaculture has become the focus of attention of international
aquaculture.
Now, researchers are trying to use probiotic bacteria in aquaculture
to improve water quality by balancing bacterial population in water
and reducing pathogenic bacterial load. Researchers are increasingly
paying more attention to this new approach (ecological aquaculture),
and have made considerable headway. This review, on the basis of the
new research findings in probiotics applied to aquaculture, analyze
and summarize the mechanism of probiotic action in aquaculture.
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