The Importance of Bacteriophage in Aquaculture

*K. PAU BIAK LUN AND JAYA NAIK. Ph.D. Research Scholar
COLLEGE OF FISHERIES, MANGALORE-575002

Introduction:

Bacteriophages were the last of the three major classes of viruses to be discovered in the year 1892 by Ivanowski and in 1897 by Beijernisk discovered the plant viruses. Animal viruses were discovered in 1902 by Loeffter and Froesch, while phages were discovered independently by Frederick and Twort in England in 1915 and by Felin D. Herelle at the Pasteur Institute in Paris in 1917.

Fig: Bacteriophage

Bacteriophages are the viruses which infects bacteria. Bacterial virus are widely distributed in nature and found in water, in soil, in humans or in the cell of microbes. Bacteriophages like all viruses are composed of nucleic acid surrounded by a protein coat. There are two main type of bacterial virus:-

i). Lytic or virulent phages: -

These phages infect the bacteria and the cells respond by producing large number of virus by lysing or killing the bacterial cells.

Bacteriophage by their nature seems to be very good candidates for the antibacterial therapy. Phages are highly most specific to one or another bacterial species. They are known to be non-toxic to plants and animals

including fishes and usually increases in titer as they infect, multiply in and kill their target microbes.

Lytic and lysogenic cycles of a bacteriophage

Lysogen phages: -

These phages incorporate their genome into the host genome and divide along with the host bacterial cells and no cell lyses occurs in this type of phages. In this addition there are some filamentous phages which simply leak out of cells without killing them.

Use in Aquaculture:

Advantages of Bacteriophages over Antibiotics:

In aquaculture, antibiotics have been the best choice to control or treat bacteria disease but frequent used and long term exposure to antibiotic in an uncontrolled and unfashionable manner both as prophylactic and therapeutics agent in hatcheries and farms has lead to the emergences of antibiotic resistant strains of bacteria in the aquatic environment (Resangpan and Kitao, 1992 and Karunasagar, et al., 1994). This result in major out break of disease in aquaculture and lead to huge economical lost to the fish farmers. In order to combat and counter these problems, there is a search for better, efficient solution to treat bacterial infection. This give rise to the application of bacteriophages in aquaculture.

The application of phages in aquaculture has been good sound advantages over the used of antibiotics. This includes the narrow root ranges of phages, no serious side effect; phages resistant bacteria remain susceptible to other phages having a similar target range. Above all selecting new phages is a relatively fast process that can frequently be accomplished in days or weeks. On the contrary, antibiotics target both pathogen and normal micro-floral, which have multiple side effects; resistant to antibiotics is not limited to target bacteria only and besides this developing a new antibiotic against antibiotics resistant bacteria is a lengthy process and may take several years. The used of phages for the biological control of pathogens of cultured fish and shrimp has developed interest in recent years since no drugs residues, drug toxicity are associated with this type of therapy(Wu and choa,1981; Nakai et al ., 1999. The used of bacteriophages in aquaculture system seem to be very promising and challenging. It gained importances due to the limited number of licensed antibiotics that are available for use in aquaculture.

Presently in aquaculture, antibiotic like oxytetracycline potentiated sulphonamides and fluroquinolones are administered primarily through the feed may not always be successful because of the poor fed consumption by diseased fish, environmental factors and the emergences of antibiotics resistant strain.(Wu and Chao,1982).

Disadvantages

However, there are also some disadvantages with phages-therapy, in case of disease outbreak with unknown bacteria , high specificity may be a problems, since the causative agent has to be stabilized to identify the phages which can effectively infect this bacteria.

Phages therapy can't be used for intracellular bacteria because the phages are continuously cleared by the spleen, liver and other filtering organs (Recticulo-endothelial system).

Some Applications

Hitherto, only few attempts have been made to use bacteriophages to treat bacterial disease in aquaculture. The earliest attempt or the potential application phages to aquaculture was reported by Wu and Chao in 1982. They examined the effect of phages ET-1 which is isolated from a pond water in Taiwan on Edwardsiella tarda. It was reported that in an in-vitro experiment, this phages kills 25 of 27 E. tarda strains and reduced the bacterial count to less than 0.15% when the bacterial suspension of 1.2x10¹²cells /ml was infected with phages ET-1. The studies done by Park et al (1997) here shown that bacteriophages can be used to control Lactococcus garvieae infection of yellow tail (Seriola quinquerdiata). The research group of Park and Nakia continued their interest to bacteriophages of Pseudomonas plecoglossidae, the causative agent of bacterial hemorrhagic disease in cultured Ayu fish. They reported that oral administration of phages impregnated feed to Ayu resulted in protection against experimental infection of Pseudomonas plecoglossidae. Successful phages treatment by oral administration in fish is of practical value as a route for therapeutics administration of phages to a large number of fish. (Nakai et al 1999).

The easy movement of phages from alimentary tract to the blood circulation system was observed in case of fish and human (Slopet et al., 1987). There is also report on the application of bacteriophages against Aeromonas salmonicide causing furanculosis in trout.

Conclusion

With the result obtained from the above, it can be concluded that there is potential for phages control of bacterial diseases in aquaculture. The emergence of antibiotics and the multiple drug resistant bacteria provides the alternatives route for application of phages to treat bacterial infections. This phages treatment has intended from medical field to other like agriculture, food industry and waste water treatment. The application of phages in aquaculture has some good advantages, besides killing the bacteria in the infected fish, phages also reduce the environmental load of pathogen as they era as effective in the environment as they are in the fish.

References:-

NAKAI, T., SUGIMOTO, R., PARK, K. H., MATSUOKA, S., MORI, K., NISHIOKA, T. and MARUYAMA, K., 1999. Protective effects of bacteriophage on experimental Lactococcus graviae infection in yellow tail. Dis. Aquat. Org., 37: 33-41.

PARK, S. C. and NAKAI, T., 2003. Bacteriophage control of Pseudomonas plecoglossicida infection in ayu Plecoglossus altivelis. Dis. Aquat. Org., 53: 33-39

WU, J. L. and CHAO, W. J., 1982. Isolation and application of a new bacteriophage, ET-1, which infect Edwardsiella tarda, the pathogen of Edwardsiellosis. CAPD Fisheries Series No. 8, Fish Dis. Res., (IV), pp 8-17.