Biological and genomic characterization of an antibiofilm lytic bacteriophage infecting Photobacterium damselae subsp. piscicida

Photobacterium damselae subsp. piscicida (PDP), the causative agent of photobacteriosis, causes substantial economic losses in marine aquaculture worldwide. Although bacteriophages (phages) have emerged as promising alternatives to antibiotics for controlling bacterial fish pathogens, phages infecting PDP remain poorly characterized. In this study, we isolated a novel lytic phage, vB_PdP-TS25, from shellfish collected from the west coast of Korea and characterized its biological, genomic, and antibiofilm properties. Transmission electron microscopy showed that vB_PdP-TS25 has an icosahedral capsid and a short non-contractile tail, consistent with a podovirus-like morphology. The phage exhibited efficient lytic activity against PDP, with a latent period of 60 min and a burst size of approximately 125 PFU per infected cell. It remained stable across a broad pH range of 5–10 and at temperatures up to 37 °C. Its genome comprises 69,954 bp of linear dsDNA encoding 88 predicted open reading frames and one tRNA gene. No genes associated with antimicrobial resistance, virulence, or lysogeny were identified. Comparative genomic, phylogenetic, and gene-sharing network analyses consistently showed that vB_PdP-TS25 belongs to a divergent lineage of unclassified Caudoviricetes , with less than 30% intergenomic similarity to its closest known relatives. The phage also significantly inhibited PDP biofilm formation and disrupted established mature biofilms, possibly through the activity of two predicted depolymerase-associated proteins. These findings expand current knowledge of PDP-infecting phages and provide a foundation for developing phage-based strategies to control photobacteriosis in aquaculture.