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Evelien T M Berends1, Johanna F Dekkers, Reindert Nijland

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Complement activation on Gram-positive bacteria leads to specific Membrane Attack Complex (MAC) deposition. This previously unrecognized interaction occurs at specialized bacterial regions, challenging existing models of MAC assembly and function.

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Area of Science:

  • Immunology
  • Microbiology
  • Molecular Biology

Background:

  • The complement system's Membrane Attack Complex (MAC or C5b-9) is crucial for immune defense, directly killing bacteria.
  • Gram-positive bacteria possess a thick peptidoglycan layer that typically protects them from MAC-mediated lysis.
  • Some Gram-positive pathogens secrete proteins that inhibit MAC formation, adding complexity to this interaction.

Purpose of the Study:

  • To investigate the deposition patterns of the Membrane Attack Complex (MAC or C5b-9) on Gram-positive bacterial surfaces.
  • To understand the implications of MAC deposition on Gram-positive bacteria, particularly in light of their known resistance mechanisms.
  • To explore potential new models for MAC assembly and function based on observed interactions.

Main Methods:

  • Complement activation was induced on Gram-positive bacteria using serum.
  • Immunoblotting was employed to detect and characterize C9 in both monomeric and polymeric forms, indicating C5b-9 complex formation.
  • Confocal microscopy was utilized to visualize the spatial distribution of C5b-9 deposition on bacterial cells.

Main Results:

  • Complement activation resulted in specific surface deposition of C5b-9 complexes on Gram-positive bacteria.
  • SDS-stable C9 polymers confirmed the presence of ring-structured C5b-9 complexes.
  • Confocal microscopy revealed non-random C5b-9 deposition at specialized regions: the division septum in Streptococcus pyogenes and the poles in Bacillus subtilis.
  • C5b-9 deposition patterns contrasted with the random surface distribution of C3b.

Conclusions:

  • A previously unrecognized interaction between the C5b-9 complex and Gram-positive bacteria was identified.
  • MAC deposition on Gram-positive bacteria occurs at specific cellular locations, not randomly.
  • These findings suggest a potential need for revised models of MAC assembly and function in the context of Gram-positive pathogens.