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    PROBIOTIC

    PROBIOTIC IN AQUACULTURE

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    INTRODUCTION

    Aquaculture has become an important economic activity in many countries. In large-scale

    production facilities, where aquatic animals are exposed to stressful conditions, problems relatedto diseases and deterioration of environmental conditions often occur and result in serious

    economic losses. Prevention and control of diseases have led during recent decades to a

    substantial increase in the use of veterinary medicines. These amounts of antibiotics have exerted

    a very strong selection pressure towards resistance among bacteria, which have adapted to thissituation, mainly by a horizontal and promiscuous flow of resistance genes (SCAN, 2003).

    Resistance mechanisms can arise one of two ways: chromosomal mutation or acquisition of

    plasmids. The use of probiotics or beneficial bacteria, which control pathogens through a varietyof mechanisms, is increasingly viewed as an alternative to antibiotic treatment.

    WHAT IS A PROBIOTICS?

    The term probiotics is generally used to denote bacteria that promote the health of otherorganisms. Lilley and Stillwell (1965) described them as substances secreted by onemicroorganism, which stimulated the growth of another.

    An expert with the Joint Food and Agriculture Organization of the United Nations/World Health

    Organization (FAO/WHO), stated that probiotics are live microorganisms, which whenconsumed in adequate amounts, confer a health benefit for the host (FAO/WHO, 2001).

    Verschuere et al. (2000) suggested the definition a live microbial adjunct which has a beneficial

    effect on the host by modifying the host-associated or ambient microbial community, byensuring improved use of the feed or enhancing its nutritional value, by enhancing the hostresponse towards disease, or by improving the quality of its ambient environment

    Irianto and Austin (2002) thus a probiotic is an entire or components of a micro-organism that isbeneficial to the health of the host.

    MECHANISM OF ACTION

    Probiotic strains have been shown to inhibit pathogenic bacteria both in vitro and in vivo

    through several different mechanisms.

    some possible benefits linked to the administering of probiotics have been suggested as:

    1. Competitive exclusion of pathogenic bacteria2. Source of nutrients and enzymatic contribution to digestion3. Direct uptake of dissolved organic material mediated by the bacteria4. Enhancement of the immune response against pathogenic microorganisms

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    5. Antiviral effects.6. Interaction with phytoplankton

    1. Competitive exclusionBacterial antagonism is a common phenomenon in nature, therefore, microbial interactions play

    a major role in the equilibrium between competing beneficial and potentially pathogenicmicroorganisms.

    Probiotics competive in three ways:

    A. Competition for Adhesion sitesB. Competition for NutrientsC. Production of Inhibitory compounds.

    A. COMPETITION FOR ADHESION SITESProbiotics Organisms compete with the pathogens for the adhesion sites and food in the gut

    epithelial surface and finally prevent their colonization. Adhesion capacity and growth on or inintestinal or external mucous has been demonstrated in vitro for fish pathogens like Vibrioanguillarum andAeromonashydrophila ( Krovacek, 1987).

    B. COMPETITION FOR NUTRIENTS:Probiotics utilizes nutrients which may be consumed by pathogens microbes. Competition fornutrients can play an important role in the composition of microbiota of the intestinal tract orambient environment of the cultured aquatic organism.

    C. PRODUCTION OF INHIBITORY COMPOUNDSProbiotic bacteria releases a variety of chemical compounds that are inhibitory to both gram positive and gram negative bacteria. These includes bacteriocines, sideropheres, lysozymes,

    proteases hydrogen peroxides etc. lactic acid bacteria are known to produce compounds such asbacteriocins that are inhibitory to other microbes.

    The first report of the existence in seawater of bacteria with an inhibitory effect against a Vibriosp. has been attributed to Gaixa (1889).

    In aquaculture, Thalassobacter utilis, has shown inhibitory effects against Vibrio anguillarum.The use ofVibrio alginolyticus strains as a probiotics has been recommended to increase survivaland growth of white shrimp (Litopenaeus vannamei) post-larvae in Ecuadorian hatcheries.

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    So competitive exclusion of potential pathogenic bacteria effectively reduces or eliminates theneed for antibiotic prophylaxis in intensive larviculture systems.

    2. Source of nutrients and enzymatic contribution to digestionIt has been suggested that microorganisms have a beneficial effect in the digestive processesof aquatic animals. In fish, it has been reported that Bacteroides and Clostridium sp. havecontributed to the host's nutrition, especially by supplying fatty acids and vitamins (Sakata,

    1990).

    Prieur et al (1990) observed that some bacteria may participate in the digestion processes ofbivalves by producing extracellular enzymes, such as proteases, lipases, as well as providing

    necessary growth factors.

    Dall and Moriarty (1983) reported that microbiota may serve as a supplementary source of

    food and microbial activity in the tract digestive may be a source of vitamins or essentialamino acids.

    3. Influence on water qualityIt has been reported that gram positive bacteria especially Bacillus sp. Improve water quality by converting the organic matter back to carbon dioxide into the environment. During the production cycle, high levels of gram-positive bacteria can be minimize the buildup ofdissolved and particulate organic carbon.Dalmin et al (2001) reported that use ofBacillus sp. improved water quality, survival andgrowth rates and increased the health status of juvenile Penaeus monodon and reduced thepathogenic vibrios.

    4. Enhancement of the immune responseProbiotic stimulate the hosts specific immunity and non specific immunity. Some of the

    bacteria like Lactic acid bacteria (LAB) increase the resistance to enteric infection. The

    mechanism of immune stimulation is yet not known, but specific cell wall components or cell

    layers may act as adjuvants and increase humoral immune response.Sakai et al(1995) demonstrated that oral administration ofClostridium butyricum bacteria to

    rainbow trout enhanced the resistance of fish to vibriosis, by increasing the phagocytic

    activity of leucocytes.

    Nikoskelainen et al. (2003) showed that administration of a lactic acid bacterium

    Lactobacillus rhamnosus (strain ATCC 53103) at a level of 105 cfu g1 feed, stimulated the

    respiratory burst in rainbow trout (Oncorhynchus mykiss).

    5. Antiviral effectsSome bacteria have antiviral effect, but the mechanism which these bacteria do is not known.

    Kamei et al (1988) reported that strains ofPseudomonas sp., Vibrios sp.,Aeromonas sp., andgroups of coryneforms isolated from salmonid hatcheries, showed antiviral activity against

    infectious hematopoietic necrosis virus (IHNV) with more than 50% plaque reduction.

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    6. Interaction with phytoplanktonProbiotics bacteria have a significant algicidal effect on many species of microalgae

    particularly of red tide plankton. Bacteria antagonistic towards algae would be undesirable in

    green water larval rearing technique in hatchery where unicellular algae are cultured and

    added, but would be advantageous when undesired algae species developed in the cultureperiod.

    SILENT FEATURE OF PROBIOTICS

    1. The probiotic should be non pathogenic and non toxic to the host ,

    2. It should be accepted by the host, e.g. through ingestion and potential colonization andreplication within the host,

    3. It should reach the location where the effect is required to take place,

    4. It should actually workin vivo as opposed to in vitro findings,

    5. It should preferably not contain virulence resistance genes or AB resistance genes.

    6. It should be capable of exerting beneficial influence on the host organism ( increase growthand / or disease resistance)

    7. it should be capable of surviving and metabolizing in the gut environment ( resistant to low pH

    and organic acids)8. It should be capable of remaining viable for a longer period under storage and field conditions.

    9. It should be capable of stimulating the immune system.

    10. It should be able to modify the microbial component of water such that the pathogenicmembers are reduced through competitive exclusion.

    11. It should be able to efficiently breakdown the organic load with least consumption of

    dissolved oxygen.

    12. It should help to reduce the BOD/COD levels and other obnoxious gases in the sediment.

    13. It should be cheap, efficient and easily available.

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    SELECTION CRITERIA FOR PROBIOTICS

    Method to select probiotic bacteria for the use in culture system should include the followingsteps:

    yCOLLECTION OF BACKGROUND INFORMATIONBefore the start of research on the development of probiotics, the activities about the

    culture practices and economics of the development should be studied. A close

    knowledge of the rearing practices used in an aquaculture farm is necessary to determine

    whether a probiotic application would be feasible or not.

    y ACQUISITION OF PUTATIVE PROBIOTICSThe acquisition of good pool of candidate probiotic is of major importance in this

    process. It is a vital in his