Isolation and genomic characterization of a lytic siphoviridae phage infecting lactococcus garvieae from rainbow trout ( Oncorhynchus mykiss )

Abstract

During intensive rainbow trout ( Oncorhynchus mykiss ) farming operations conducted between 2021 and 2024 in the Karkamış and Keban Dam Lakes, ten Lactococcus garvieae isolates were recovered from moribund fish. Bacterial identification was performed using classical microbiological procedures, the Biolog GEN III system, and Sanger sequencing. To detect lytic bacteriophages targeting L. garvieae , water samples collected from farming areas were processed using both spot testing and the double agar overlay assay. Host-range assessment revealed that two of the ten sequenced isolates were susceptible in spot tests, while only one produced clear plaques in the double agar method. Transmission electron microscopy characterized the isolated phage possessing an icosahedral capsid with an approximate diameter of 200 nm. Whole-genome sequencing of the phage revealed a genome size of 34,324 bp with a GC content of 58.41%. Metagenomic classification identified the phage as a member of the family Siphoviridae . These findings provide new insights into the diversity of L. garvieae strains circulating in Turkish aquaculture systems and contribute to the development of phage-based alternatives for managing bacterial infections in aquaculture.