Isolation and characterization of alpha-enolase, a novel fibronectin-binding protein from Streptococcus suis

Streptococcus suis is an important swine pathogen that causes meningitis, endocarditis, arthritis and septicaemia. As a zoonotic agent, S. suis also causes similar diseases in humans. Binding of pathogenic bacteria to extracellular matrix components enhances their adhesion to and invasion of host cells. In the present study we isolated and identified a novel fibronectin-binding protein from S. suis. The native protein (designated SsEno) possessed not only high homology with other bacterial enolases but also enolase activity. We cloned, expressed and purified SsEno and showed that it is ubiquitously expressed by all S. suis serotypes and we identified its surface localization using immunoelectron microscopy. ELISA demonstrated that SsEno binds specifically to fibronectin and plasminogen in a lysine-dependent manner. Additional surface plasmon resonance assays demonstrated that SsEno binds to fibronectin or plasminogen with low nanomolar affinity. Inhibition experiments with anti-SsEno antibodies also showed that bacterial SsEno is important for the adhesion to and invasion of brain microvascular endothelial cells by S. suis. Overall, the present work is the first study, to our knowledge, to demonstrate a fibronectin-binding activity of a bacterial enolase, and shows that, similar to other bacterial fibronectin-binding proteins, SsEno may contribute to the virulence of S. suis.