## Lancefield Grouping of Streptococcus pyogenes ### The Group A Carbohydrate Antigen **Key Point:** Lancefield group A streptococci are defined by the presence of a cell wall carbohydrate antigen composed of **N-acetyl-β-D-glucosamine (GlcNAc) and rhamnose (L-rhamnose)** in a specific repeating polymer. **Mnemonic:** **GLAR** = **G**lucosamine + **L**-rhamnose = **A**ntigen (Group A) **High-Yield:** This carbohydrate antigen: - Is located in the cell wall peptidoglycan - Is used for serological identification via Lancefield typing - Is distinct from the M protein (which is a separate virulence factor) - Is present in all S. pyogenes strains, making it a universal identifier ### Lancefield Groups: Quick Reference | Group | Carbohydrate Antigen | Clinical Significance | | --- | --- | --- | | **A** | N-acetyl-β-D-glucosamine + rhamnose | S. pyogenes; pyogenic infections, rheumatic fever | | **B** | N-acetyl-β-D-galactosamine | S. agalactiae; neonatal meningitis, sepsis | | **C** | N-acetyl-β-D-galactosamine + glucose | S. equi, S. dysgalactiae; less common in humans | | **D** | Glycerol teichoic acid (not a carbohydrate) | S. bovis; endocarditis | | **G** | N-acetyl-β-D-galactosamine | S. canis; rare in humans | **Clinical Pearl:** Lancefield typing is performed using latex agglutination or precipitin tests with group-specific antisera. It is essential for confirming S. pyogenes in clinical laboratories. ### Why Not the Other Options? - **Galactose + N-acetyl-D-galactosamine:** This is the Group B antigen (S. agalactiae) - **Glucose + mannose:** Not a Lancefield-defining antigen - **Lactose + sucrose:** These are fermentation substrates, not cell wall antigens [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 27e Ch 15]
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