| Summary: | <p>Abstract</p> <p>Background</p> <p>Binding of serum components by surface M-related proteins, encoded by the <it>emm </it>genes, in streptococci constitutes a major virulence factor in this important group of organisms. The present study demonstrates fibrinogen binding by <it>S. iniae</it>, 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.</p> <p>Results</p> <p>Fibrinogen binding by <it>S. iniae </it>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 <it>emm</it>-like gene, <it>simA</it>, encoding a 57 kDa fibrinogen binding M-like protein in <it>S. iniae</it>. These SiM proteins and their corresponding tetrameric structures from some sequevar types (~230 kDa) bound fibrinogen in Western blots. <it>simA </it>was most closely related (32% identity) to the <it>demA </it>gene of <it>S. dysgalactiae</it>. Genome walking and sequencing determined the genetic organization of the <it>simA </it>region had similarities to the <it>mgrC </it>regulon in GCS and to <it>S. uberis</it>. Moreover, a putative multigene regulator, <it>mgx </it>was orientated in the opposite direction to the <it>simA </it>gene in common with <it>S. uberis</it>, but contrary to findings in GAS and GCS. In GAS, diversity among <it>emm</it>-genes and consequent diversity of their M-related proteins results in substantial antigenic variation. However, an extensive survey of <it>S. iniae </it>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.</p> <p>Conclusion</p> <p>These proteins play a role in avoiding oxidative attack by phagocytic cells during infection of fish by <it>S. iniae</it>, 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 <it>S. iniae </it>in fish.</p>
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