## Virulence Factors of Streptococcus pyogenes ### Overview of Correct Statements All three incorrect options are well-established virulence mechanisms of *S. pyogenes*: **Key Point:** M protein is the primary antiphagocytic factor and exhibits molecular mimicry with human tropomyosin, leading to autoimmune sequelae in rheumatic fever. **Key Point:** Hyaluronic acid capsule is a "stealth" molecule — it is chemically identical to the hyaluronic acid in human connective tissue, making it invisible to the immune system and preventing antibody formation. **Key Point:** Protein G binds to the Fc region of IgG, blocking opsonization and complement activation — a classic immune evasion strategy. ### Why Streptolysin S is the Incorrect Statement **High-Yield:** Streptolysin S (SLS) is **oxygen-independent** (anaerobic), not oxygen-dependent. It is a small peptide toxin that can function in anaerobic environments. **High-Yield:** Streptolysin O (SLO) is the **oxygen-labile** (oxygen-sensitive) toxin — it is inactivated by atmospheric oxygen and is best detected in anaerobic conditions. SLO is a cholesterol-dependent cytolysin (CDC) that forms pores in cell membranes. ### Comparison Table: Streptolysins | Feature | Streptolysin O (SLO) | Streptolysin S (SLS) | | --- | --- | --- | | **Oxygen sensitivity** | Oxygen-labile (inactivated by O₂) | Oxygen-independent | | **Molecular class** | Cholesterol-dependent cytolysin (CDC) | Small peptide toxin | | **Detection method** | Best in anaerobic conditions | Aerobic or anaerobic | | **ASO titer** | Elevated in post-streptococcal sequelae | Not used diagnostically | | **Clinical significance** | Marker of recent GAS infection | Direct tissue damage | **Clinical Pearl:** Anti-streptolysin O (ASO) titers are elevated in acute rheumatic fever and post-streptococcal glomerulonephritis, making SLO a useful serological marker of invasive GAS disease. **Mnemonic:** **SLO = Labile** (oxygen-sensitive, anaerobic detection); **SLS = Stable** (oxygen-independent, works aerobically).
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.