Plasminogen- and fibronectin-binding protein B is involved in the adherence of Streptococcus pneumoniae to human epithelial cells

Streptococcus pneumoniae is a major cause of morbidity andmortality worldwide. The ability of this bacterium to adhere toepithelial cells is considered as an essential early step in colonization and infection. By screening a whole genome phage display library with sera from infected patients, we previously identified three antigenic fragments matching open reading framespr0075 of the strain R6 genome. This locus encodes for an120-kDa protein, herein referred to as plasminogen- andfibronectin-binding protein B (PfbB), which displays an LPXTGcell wall anchoring motif and six repetitive domains. In thisstudy, by using isogenic pfbB-deleted mutants of the encapsulatedD39 and of the unencapsulated DP1004 type 2 pneumococcalstrains, we show that PfbB is involved in S. pneumoniaeadherence to various epithelial respiratory tract cell lines. Ourdata suggest that PfbB directly mediates bacterial adhesion,because fluorescent beads coated with the recombinant PfbBsp17 fragment (encompassing one of the six repetitive domainsand the C-terminal region) efficiently bound to epithelial cells.Mutants lacking PfbB bound to fibronectin and plasminogenconsiderably less efficiently than wild type bacteria, whereassp17-coated beads specifically bound to both of these substrates.Taken together, our data suggest that, by directly interactingwith fibronectin, PfbB significantly increases the abilityof S. pneumoniae to adhere to human epithelial cells.