How complement kills E. coli. II. The apparent two-hit nature of the lethal event

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• The complement system is crucial for innate immunity.
• Understanding the mechanism of complement-mediated cell damage is vital.
• The number of complement 'hits' required for cell lysis varies by target.

Purpose of the Study:

• To investigate the hit kinetics of complement action on red blood cells and E. coli.
• To differentiate between single-hit and multiple-hit mechanisms of complement-mediated damage.
• To analyze the dose-response curves of complement's terminal complex.

Main Methods:

• Constructing dose-response curves by adding purified C7 or C8 to depleted serum.
• Analyzing the shape of dose-response curves to determine hit kinetics.
• Measuring complement-mediated lysis, permeation, membrane damage, and bacterial killing.

Main Results:

• Complement acts on red blood cells via a single-hit mechanism for lysis and permeation.
• E. coli requires multiple complement hits for outer membrane damage, inner membrane damage, and killing.
• A nonlethal single-hit response in E. coli was identified, aiding multiple-hit analysis.
• Complement-mediated membrane damage and killing of E. coli were determined to be a 2-hit process.

Conclusions:

• Complement exhibits distinct hit kinetics for different biological targets.
• E. coli's susceptibility to complement involves a multi-hit mechanism, unlike red blood cells.
• The identified single-hit phenomenon in E. coli provides a novel tool for studying complement interactions.