The requirement for minimized use of antibiotics and the ban on antibiotic-supplemented feeds calls for alternative disease control approaches, like the use of bacteriophages (viruses of bacteria). Currently, there are no locally characterized bacteriophages against Edwardsiella tarda as a cheap alternative for the management of “Edwardsiellosis” on fish farms in Uganda. This study characterised in vitro-lytic activity and genotypic properties of selected bacteriophages against E. tarda.
Edwardsiella tarda-specific phages from the gut contents of market fresh tilapia fish were isolated and purified by the double agar overlay plaque technique. Spot assay was used to determine in vitro-lytic activity, pH and temperature stability; as well as one-step growth curve and antibiofilm activity. Oxford Nanopore Technology and Illumina technology were used to sequence the genomes of the host bacteria and one phage strain, respectively. A de novo genome assembly was done using various bioinformatic tools. Presence of resistance and virulence genes was determined using bact_hybv4 and online public bioinformatics websites. Phylogenetic tree for both phage and host genomes were constructed using Mega11 and edited using FigTreeV1.4.4 software.
Results: Three lytic phages against E. tarda, viz USF_EDP_P1, USF_EDP_P2, and USF_EDP_W10 were identified. Plaque sizes ranged from 1-2mm; efficiency of plating (EOP) of 1, Multiplicity of Infection 0.4 - 0.8. Only one phage USF_EDP_W10 demonstrated high host range with a burst size of 400 PFU/infected bacterial cell and, burst time of 10 minutes (Fig 1A). Highest phage stability was recorded between -20°C and 30°C and pH of 7-7.5 (Fig 1B and C). The phage exhibited antibiofilm activity. Predicted genes of phage genome included holin, endolysin, and spanin but not AMR and virulence genes. Phylogenetic tree structure clustered the phage USF_EDP_W10 together with the genus Moonvirus.