Identification and molecular characterisation of a fibrinogen binding protein from Streptococcus iniae

Binding of serum components by surface M-related proteins, encoded by the emm genes, in streptococci constitutes a major virulence factor in this important group of organisms. The present study demonstrates fibrinogen binding by S. iniae, a Lancefield non-typeable pathogen causing devastating fish losses in the aquaculture industry and an opportunistic pathogen of humans, and identifies the proteins involved and their encoding genes. Fibrinogen binding by S. iniae significantly reduced respiratory burst activity of barramundi peritoneal macrophages in primary cultures compared to BSA-treated or untreated controls, indicating a potentially important role for fibrinogen binding cell-surface proteins in avoiding phagocytic attack in fish. We describe a novel emm-like gene, simA, encoding a 57 kDa fibrinogen binding M-like protein in S. iniae. These SiM proteins and their corresponding tetrameric structures from some sequevar types (approximately 230 kDa) bound fibrinogen in Western blots. simA was most closely related (32% identity) to the demA gene of S. dysgalactiae. Genome walking and sequencing determined the genetic organization of the simA region had similarities to the mgrC regulon in GCS and to S. uberis. Moreover, a putative multigene regulator, mgx was orientated in the opposite direction to the simA gene in common with S. uberis, but contrary to findings in GAS and GCS. In GAS, diversity among emm-genes and consequent diversity of their M-related proteins results in substantial antigenic variation. However, an extensive survey of S. iniae isolates from diverse geographic regions and hosts revealed only three variants of the gene, with one sequevar accounting for all but two of the 50 isolates analysed. These proteins play a role in avoiding oxidative attack by phagocytic cells during infection of fish by S. iniae, but genetic diversity amongst these key surface proteins has not yet arisen. This lack of diversity coupled with a functional role in macrophage resistance suggests that these proteins may constitute important targets for future vaccines against S. iniae in fish.