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Photobacterium damselae subsp.damselae is a marine bacterium that causes septicemia in marine animals and in humans. Previously, we had determined a major role of pPHDD1 plasmid-encoded Dly (damselysin) and HlyA (HlyApl ) and the chromosome-encoded HlyA (HlyAch ) hemolysins in virulence. However, the mechanisms by which these toxins are secreted remain unknown. In this study, we found that a mini-Tn10 transposon mutant in a plasmidless strain showing an impaired hemolytic phenotype contained an insertion inepsL , a component of a type II secretion system (T2SS). Reconstruction of the mutant by allelic exchange confirmed the specific involvement ofepsL in HlyAch secretion. In addition, mutation ofepsL in a pPHDD1-harboring strain caused an almost complete abolition of hemolytic activity against sheep erythrocytes, indicating thatepsL plays a major role in secretion of the plasmid-encoded HlyApl and Dly. This was further demonstrated by analysis of different combinations of hemolysin gene mutants and by strain-strain complementation assays. We also found that mutation of the putative prepilin peptidase genepilD severely affected hemolysis, which dropped at levels inferior to those ofepsL mutants. Promoter expression analyses suggested that impairment of hemolysin secretion inepsL andpilD mutants might constitute a signal that affects hemolysin and T2SS gene expression at the transcriptional level. In addition, singleepsL andpilD mutations caused a drastic decrease in virulence for mice, demonstrating a major role of T2SS andpilD inP. damselae subsp.damselae virulence.

infection and immunity

Photobacterium damselae subsp. damselae Major Virulence Factors Dly, Plasmid-Encoded HlyA, and Chromosome-Encoded HlyA Are Secreted via the Type II Secretion System