BACTERIOPHAGES ISOLATED FOR THE CONTROL OF PATHOGENIC Vibrio harveyi  IN SHRIMP HATCHERIES

Phage therapy has been accepted as alternative to chemotherapy in aquaculture systems especially in shrimp larval production. The uniqueness of bacteriophages as therapeutics is that they are self replicating entities specific to their targets with no side effects. However, concern exists with their use because of their ability to be agents of generalized transduction in the environment and thereby mediate the transfer of genes among bacteria. This is important because phages selected for therapeutic purposes must not carry virulence genes or properties conferring antimicrobial resistance and toxin genes to the host. Moreover, therapeutic phages should not become latent during their life cycle. Using an enrichment method, bacteriophages were isolated from sediment and water samples collected from the Vembanad estuary, Cochin, India using 12 isolates of Vibrio harveyi. By this method four phage lysates were obtained all of which were purified by soft agar over lay method, titres determined and stocked at 4^◦ C. High-titre phage stocks were prepared, stocked at 4^◦ C and their stability checked for an year. Host range of the phage was assessed against 162 bacterial isolates spanning 14 species on which they showed broad spectrum lytic activity. One phage was lytic to strains of V. alginolyticus and V. parahaemolyticus which are potentially pathogenic to shrimp and other aquatic animals in culture systems. Lytic efficiency of all phages was checked on their respective host bacterial species. In all cases the phages were able to arrest the growth of V. harveyi for about 5-6 hours after which phage resistant forms emerged and began to dominate. By electron microscopy, it was found three of the lysates contained Myovirus-like bacteriophages with an icosahedral head, a short collar and a contractile tail. One phage, Viha45 was Siphovirus-like with an icosahedral head and long, flexible, helical non-contractile tail. It was infective to 62 % V. harveyi isolates tested, and to its near taxonomical neighbours V. alginolyticus and V. parahaemolyticus. Development of resistance or lysogenization is an impediment in the development of phage therapy in aquaculture thus necessitating a deeper study of phage diversity and phage-host dynamics in the environment.