Photobacterium damselae is a marine pathogenic bacterium that includes two distinct subspecies, piscicida and damselae. The subspecies damselae is considered a non-clonal and generalist pathogen for a variety of marine animals and also for humans, whereas subsp. piscicida is a more specialized pathogen and only infects fish. Interestingly, most of their known virulence factors are encoded within mobile genetic elements, which include plasmids and pathogenicity islands. Highly virulent subsp. damselae isolates harbor the large conjugative plasmid pPHDD1 that encodes two potent cytotoxins, damselysin (Dly) and phobalysin P (PhlyP), whereas a third hemolysin phobalysin C (PhlyC) is encoded in chromosome I within a highly variable region that resembles a hot spot for recombination of horizontally acquired DNA. In addition, a chromosomal pathogenicity island encoding a vibrioferrin siderophore synthesis and uptake system is a feature of many subsp. damselae strains, and a large virulence plasmid that encodes a type III secretion system has been described in the type strain of this subspecies. Regarding the subspecies piscicida, many isolates harbor a small (pPHDP10) and a large plasmid (pPHDP70) that encode the apoptotic toxin AIP56 and the siderophore piscibactin for host iron scavenging, respectively. Moreover, subsp. piscicida genomes have undergone an expansion of insertion sequence (IS) elements that caused a massive gene decay, a process likely fueled by its host-dependent lifestyle. The repertoire of horizontally acquired DNA within the species is completed with a number of antibiotic resistance plasmids and integrative and conjugative elements (ICEs) that exhibit geographical distribution patterns. Collectively, horizontal gene transfer can be considered as a major driving force that shaped the distinct pathogenic strategies of each subspecies of P. damselae.
