Which of the following is the primary mechanism by which Staphylococcus aureus evades the complement system?

Explanation

## Complement Evasion by S. aureus **Key Point:** The **primary** mechanism by which *Staphylococcus aureus* evades the complement system is through **Protein A**, which binds the Fc region of IgG antibodies in the wrong orientation, thereby blocking complement activation via the classical pathway and preventing opsonophagocytosis. ### Mechanism of Protein A-Mediated Complement Evasion Protein A is a surface-anchored virulence factor that: - Binds the **Fc region of IgG** (subclasses IgG1, IgG2, and IgG4) with high affinity, leaving the Fab region pointing away from the bacterial surface - Prevents IgG from acting as an opsonin (Fc region is unavailable for FcγR on phagocytes) - Blocks **classical pathway** complement activation by preventing C1q from binding to the Fc region of surface-bound IgG - Effectively "cloaks" the bacterium from antibody-mediated immune clearance - Also scavenges free IgG in serum, further depleting functional opsonizing antibody This makes Protein A the **primary anti-complement and anti-opsonization** mechanism of *S. aureus*, as recognized in standard microbiology references (Jawetz, Murray's Medical Microbiology, and Mandell's Principles of Infectious Diseases). ### Other S. aureus Virulence Factors (Not Primary Complement Evasion) | Factor | Function | Role in Pathogenesis | |--------|----------|---------------------| | Capsular polysaccharide | Masks PAMPs | Contributes to complement evasion but is secondary; non-encapsulated strains still highly virulent | | Staphylokinase | Degrades fibrin clots | Promotes dissemination; not a complement evasion strategy | | Alpha-toxin (α-hemolysin) | Forms pores in cell membranes | Causes cytolysis; not a complement evasion strategy | | Coagulase | Converts fibrinogen to fibrin | Cloak formation; indirect immune evasion | **High-Yield:** Protein A is the **primary** mechanism of complement and antibody evasion in *S. aureus*. It is encoded by the *spa* gene and is present in virtually all clinical isolates, making it a major virulence determinant. **Clinical Pearl:** Protein A is so central to *S. aureus* immune evasion that it is used in laboratory coagglutination tests — its Fc-binding property allows antibody-coated bacteria to agglutinate antigens efficiently. [cite: Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology, 9th ed., Ch 13; Mandell GL et al. Principles and Practice of Infectious Diseases, 9th ed.]