Identification and characterization of Staphylococcus delphini internalization pathway in non-professional phagocytic cells.

The intracellular lifestyle of bacteria is widely acknowledged to be an important mechanism in chronic and recurring infection. Among the Staphylococcus genus, only Staphylococcus aureus and Staphylococcus pseudintermedius have been clearly identified as intracellular in non-professional phagocytic cells (NPPCs), for which the mechanism is mainly fibronectin-binding dependent. Herein we used bioinformatics tools to search for possible new fibronectin-binding proteins (FnBPs-like) in other Staphylococcus species. We found a protein in Staphylococcus delphini called Staphylococcus delphini surface protein Y (SdsY). This protein shares 68% identity with the Staphylococcus pseudintermedius surface protein D (SpsD), 36% identity with S. aureus FnBPA, and 39% identity with S. aureus FnBPB. The SdsY protein possesses the typical structure of FnBPs-like, including an N-terminal signal sequence, an A domain, a characteristic repeated pattern, and an LPXTG cell wall anchor motif. The level of adhesion to immobilized fibronectin was significantly higher in all S. delphini strains tested as compared with the fibronectin-binding deficient S. aureus DU5883 strain. By using a model of human osteoblast infection, the level of internalization of all strains tested was significantly higher than with the invasive-incompetent S. aureus DU5883. These findings were confirmed by phenotype restoration after transformation of DU5883 by a plasmid expression vector encoding the SdsY repeats. Additionally, using fibronectin-depleted serum and murine osteoblast cell lines deficient for the beta1 integrin, the involvement of fibronectin and beta1 integrin was demonstrated in S. delphini internalization. The present study demonstrates that additional staphylococcal species are able to invade NPPCs and proposes a method to identify FnBPs-like proteins.