Isolation of Lytic Phages against Edwardsiella tarda from fish gastrointestinal tract

Abstract

The high occurrence of antimicrobial resistance and emergence of multidrug resistant bacterial pathogens including Edwardsiella species strains has been reported in several studies around the globe. This study was conducted from June to October, 2022 aimed at providing a safe, environmentally friendly, abundant and naturally occurring bio-control remedies to antimicrobial resistance observed amongst Edwardsiella species. These pathogens cause infections in fish leading to massive mortalities of fish in ponds, swamps and other fish habitats. The study involved collection of fresh fish gastrointestinal samples which were processed for isolation of bacteriophages specific to Edwardsiella species. The double agar overlay method and spot assay were employed for phage isolation, purification, enumeration and characterization based on host range, multiplicity of infection and stability at different temperatures and pH. A total of four bacteriophages were isolated but only four were characterized. Host range characterization was determined using tentatively identified Edwardsiella sspeciewhere five Edw. tarda strains were all susceptible to the phages, Edw.anguillarium showed resistance and Edw.hoshinae had limited susceptibility. For stability at various temperatures, there was no significant changes in phage concentration on exposure to 30°C to 40°C for 60minutes. However, there was a rapid decrease observed at 50°C while there was no infectivity at 60°C and 70°C. The phages remained stable over a wide pH range (4 - 11) and lost infectivity at extreme alkaline pH 12. Characterization of phages with a diverse host range could aid in the development of effective bio-control strategies for fish pathogens in aquaculture. The phage stability at different thermal and pH conditions are likely to be applied in management of Edwardsiellosis and Enteric Septicemia in fish. The occurrence of multidrug resistance in Edwardsiella species suggests the potential for usage of phages for integrated fish health management. Further studies concerning the safety of phage application, classification and identification are recommended.