The First Genome-Based Characterization of a Photobacterium damselae subsp. piscicida Strain KLC25 from Türkiye

Photobacterium damselae subsp. piscicida (Pdp) is one of the major bacterial pathogens affecting marine aquaculture worldwide. This study presents, to our knowledge, the first comparative genomic characterization of a Pdp strain (KLC25) isolated in Türkiye in the context of publicly available genomes from different geographic regions. Average nucleotide identity (ANI) and digital DNA–DNA hybridization (dDDH) analyses supported the taxonomic assignment of KLC25 to P. damselae. Whole-genome phylogeny positioned KLC25 closest to the isolates MT1415 (Italian) and DI21 (Spanish), whereas core genome phylogeny clustered it with the Cypriot strain BIC8 and the Greek strain GalP1, revealing differences in clustering between whole- and core-genome approaches. Plasmid-associated sequences homologous to pPHDP10, pPHDP70 and pPHDPT3 were also detected in KLC25. Comparative analysis of the prophage regions predicted in KLC25 revealed similarities to phages from different Pdp strains, with most regions showing their highest sequence similarity to phages associated with different strains. While virulence-related determinants were highly conserved across the analyzed genomes, anti-phage defense systems and acquired antimicrobial resistance genes exhibited greater variability. Overall, this study expands our understanding of the genomic diversity and evolution of Pdp and provides a valuable resource for future investigations into its pathogenicity and epidemiology.