Identification and characterization of trimeric autotransporter adhesins from Photobacterium damselae subsp. piscicida

Background Photobacterium damselae subsp. piscicida ( Phdp ) causes severe septicemic infections in marine fish and substantial economic losses in aquaculture. However, the bacterial factors required for host colonization remain poorly characterized. Methods Comparative SDS-PAGE protein profiling of Phdp field isolates followed by mass spectrometry led to the identification of candidate trimeric autotransporter adhesins (TAAs), which were subsequently characterized using molecular, functional and comparative genomic approaches. Results Two previously unrecognized plasmid-encoded TAAs, PadA and PadB, were identified in Phdp field isolates and were shown to be expressed during experimental infection of European sea bass ( Dicentrarchus labrax ). Functional assays showed that PadA and PadB increased the resistance of bacterial pellets to disaggregation and bound fish mucus and extracellular matrix components but did not significantly affect biofilm formation. Expression of PadA or PadB in a PadA- and PadB-negative strain did not increase virulence in sea bass. Comparative genomic analysis further identified a third putative TAA, designated PadC, encoded by chromosomal genes in both P. damselae subspecies. Conclusions By providing the first characterization of TAAs in Phdp , this work expands our understanding of surface-associated factors potentially involved in host-pathogen interactions, and establishes a foundation for future studies on the role of these adhesins in photobacteriosis.