Capsule production and growth phase influence binding of complement to Staphylococcus aureus

Area of Science:

• Immunology
• Microbiology
• Infectious Diseases

Background:

• Complement-mediated opsonization via C3 binding is crucial for innate immunity.
• Staphylococcus aureus strains with capsule types 5 and 8 cause most human infections, yet their complement interaction is poorly understood.

Purpose of the Study:

• To investigate the role of the complement pathway in experimental Staphylococcus aureus infection.
• To examine C3 binding to Staphylococcus aureus strains with varying capsule expression and growth phases.

Main Methods:

• Experimental staphylococcal infection model using control and C3-depleted mice.
• In vitro analysis of C3 binding to Staphylococcus aureus strains with different capsule phenotypes (CP++, CP+, CP-) and growth phases (mid-log vs. stationary).

Main Results:

• C3-depleted mice showed significantly higher mortality (64%) compared to control mice (8%) after Staphylococcus aureus challenge.
• Heavily encapsulated Staphylococcus aureus strains bound less C3, with capsule presence and growth phase (mid-log preferred) critically impacting C3 deposition.
• The alternative complement pathway was the primary driver of C3 binding in higher serum concentrations.

Conclusions:

• An intact complement pathway is essential for host defense against Staphylococcus aureus infection.
• Bacterial capsule expression and growth phase significantly influence complement opsonization, impacting bacterial survival.
• Understanding these interactions is key to developing strategies against staphylococcal infections.