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Antibacterial activity of the contact and complement systems is blocked by SIC, a protein secreted by Streptococcus

Inga-Maria Frick1, Oonagh Shannon, Per Åkesson

  • 1Division of Infection Medicine, Department of Clinical Sciences, Lund University, SE-221 84 Lund, Sweden. Inga-Maria.Frick@med.lu.se

The Journal of Biological Chemistry
|November 12, 2010
PubMed

Insights

Streptococcus pyogenes M1 serotype infections are severe because the secreted SIC protein blocks antibacterial peptides. This allows the bacteria to grow and spread, explaining why M1 infections are often life-threatening.

Area of Science:

  • Immunology
  • Microbiology
  • Molecular Biology

Background:

  • Complement and contact systems generate antibacterial peptides.
  • Streptococcus pyogenes has over 100 serotypes, with M1 isolates linked to severe infections.
  • M1 isolates uniquely secrete the SIC protein.

Purpose of the Study:

  • To investigate the role of SIC protein in S. pyogenes pathogenesis.
  • To determine how SIC affects the host immune response, specifically complement and contact systems.

Main Methods:

  • In vitro studies of contact system activation and antibacterial peptide activity.
  • In vivo mouse model of S. pyogenes sepsis.
  • Analysis of bacterial growth in human plasma.

Main Results:

  • SIC protein inhibits contact system activation.
  • SIC blocks the activity of antibacterial peptides.
  • SIC promotes S. pyogenes growth in human plasma.
  • SIC enhances bacterial dissemination in a mouse sepsis model.

Conclusions:

  • SIC protein is a virulence factor for S. pyogenes M1 serotype.
  • SIC contributes to the severity of S. pyogenes infections by evading host defenses.
  • Targeting SIC may offer a strategy to combat severe S. pyogenes infections.

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