All of the TRUE about M proteins of Streptococci, EXCEPT:
**Core Concept**
M protein is a virulence factor of Streptococcus pyogenes (Group A Streptococcus), playing a crucial role in evading the host's immune response. It is a surface protein that inhibits phagocytosis by binding to complement component C3b and preventing its deposition on the bacterial surface.
**Why the Correct Answer is Right**
M protein is composed of a repetitive, coiled-coil structure that provides structural integrity to the molecule. The protein is anchored to the cell wall through a glycolipid anchor, allowing it to extend away from the bacterial surface. This structure enables M protein to interact with host immune cells and evade phagocytosis.
**Why Each Wrong Option is Incorrect**
**Option A:** M protein is indeed involved in the pathogenesis of Streptococcus pyogenes, making it a true statement.
**Option B:** M protein does inhibit the deposition of C3b on the bacterial surface, which is a key mechanism of its virulence, making it a true statement.
**Option C:** M protein is not a peptidoglycan component, it is a surface protein, making this statement incorrect.
**Clinical Pearl / High-Yield Fact**
M protein is a key virulence factor of Streptococcus pyogenes, and its presence is often used as a diagnostic marker for invasive streptococcal infections.
**Correct Answer: C. M protein is not a peptidoglycan component.**