Photobacterium damselae subsp. damselae, a marine bacterium that causes
infections in marine animals and in humans, produces up to three different
haemolysins involved in virulence, which include the pPHDD1 plasmidencoded
damselysin (Dly) and HlyApl, and the chromosome-encoded HlyAch.
We screened 45 isolates from different origins, and found a correlation between
their haemolytic phenotypes and the differential haemolysin gene content. All
highly and medium haemolytic strains harboured pPHDD1, with amino acid
substitutions in HlyApl and HlyAch being the cause of the medium haemolytic
phenotypes in some pPHDD1-harbouring strains. Weakly haemolytic strains
contained only hlyAch, whereas nonhaemolytic isolates, in addition to lacking
pPHDD1, either lacked hlyAch or contained a hlyAch pseudogene. Sequence
analysis of the genomic context of hlyAch uncovered an unexpected genetic
diversity, suggesting that hlyAch is located in an unstable chromosomal region.
Phylogenetic analysis suggested that hlyApl and hlyAch originated by gene duplication
within P. damselae subsp. damselae following acquisition by horizontal
transfer. These observations together with the differential distribution of
pPHDD1 plasmid among strains suggest that horizontal gene transfer has
played a main role in shaping the haemolysin gene baggage in this pathogen.
