Staphylococcal virulence factors – Protein A localization Protein A, which binds the Fc portion of IgG and helps the bacterium evade opsonization, is a cell wall–associated component of which organism?

Introduction / Context:
Protein A is a hallmark virulence factor of Staphylococcus aureus. By binding the Fc region of IgG, it orients antibodies incorrectly on the bacterial surface, reducing opsonization and phagocytosis. Detection of Protein A contributes to rapid identification in clinical laboratories.

Given Data / Assumptions:

• Protein A is a surface-anchored protein (via sortase-mediated LPXTG motif linkage).
• It is characteristic of S. aureus, not broadly present in CoNS.
• Protein A contributes to immune evasion and virulence.

Concept / Approach:
Because Protein A binds IgG Fc, it interferes with complement activation and Fc receptor–mediated phagocytosis. Diagnostic latex agglutination kits often detect clumping factor and Protein A together to identify S. aureus rapidly from colonies.

Step-by-Step Solution:
Recall organism-specific virulence repertoire.Match Protein A with S. aureus cell wall localization.Select “Staphylococcus aureus.”

Verification / Alternative check:
Phenotypic tests (coagulase, clumping factor, Protein A) cluster with S. aureus identity; genomic analyses (spa gene) encode Protein A.

Why Other Options Are Wrong:

• CoNS and micrococci lack canonical Protein A expression.
• “None” is incorrect because S. aureus is a definitive source.
• Enterococcus species are unrelated Gram-positive cocci lacking Protein A.

Common Pitfalls:
Assuming all staphylococci share identical virulence determinants; Protein A is a distinguishing factor of S. aureus.

Final Answer:
Staphylococcus aureus.