PfbA, a novel plasmin- and fibronectin-binding protein of Streptococcus pneumoniae, contributes to fibronectin-dependent adhesion and antiphagocytosis.

Streptococcus pneumoniae is a major causative agent of mortality throughout the world. The initial event in invasive pneumococcal disease is the attachment of pneumococci to epithelial cells in the upper respiratory tract. Several bacterial proteins can bind to host extracellular matrix proteins and function as adhesins and invasins. To identify adhesins or invasins on the pneumococcal cell surface, we searched for several proteins with an LPXTG anchoring motif in the whole-genome sequence of Streptococcus pneumoniae and identified one, which we called PfbA (plasmin- and fibronectin-binding protein A), that bound to human serum proteins. Immunofluorescence microscopy and fluorescence-activated cell sorter analysis revealed that PfbA was expressed on the pneumococcal cell surface. A DeltapfbA mutant strain was only half as competent as the wild-type strain at adhering to and invading lung and laryngeal epithelial cells. In addition, epithelial cells infected with DeltapfbA showed morphological changes, including cell flattening and a loss of microvilli, that did not occur in cells infected with the wild-type strain. The mutant strain also exhibited a weaker antiphagocytotic activity than wild type in human peripheral blood. Moreover, the growth of wild-type bacteria in human whole blood containing anti-PfbA antibodies was reduced by approximately 50% after 3 h compared with its growth without the antibody. These results suggest that PfbA is an important factor in the development of pneumococcal infections.