## Lancefield Classification of Streptococci ### Streptococcus pyogenes — Group A Streptococcus (GAS) **Key Point:** *Streptococcus pyogenes* is the Group A Streptococcus (GAS) and is classified based on the presence of the Group A carbohydrate antigen in its cell wall. This antigen is composed of **N-acetyl-β-D-glucosamine (GlcNAc)** linked to rhamnose. **High-Yield:** The Group A antigen is a cell wall polysaccharide that is used for serological identification and classification. It is distinct from the M protein (which is used for serotyping within Group A) and the hyaluronic acid capsule. ### Lancefield Grouping System | Lancefield Group | Species | Hemolysis | Group Antigen | Clinical Significance | |---|---|---|---|---| | A | *S. pyogenes* | β | Carbohydrate (GlcNAc) | Pharyngitis, impetigo, rheumatic fever, PSGN | | B | *S. agalactiae* | β | Carbohydrate | Neonatal meningitis, maternal colonization | | C | *S. equi*, *S. zooepidemicus* | β | Carbohydrate | Animal pathogens, rare in humans | | D | *Enterococcus* spp. | Variable | Glycerol teichoic acid | UTI, endocarditis | | F, G | Various | β | Carbohydrate | Rare human pathogens | **Clinical Pearl:** The Group A antigen is used in rapid diagnostic tests (latex agglutination, immunochromatographic assays) to identify *S. pyogenes* directly from throat swabs. This is distinct from M protein typing, which identifies >200 serotypes of GAS. **Mnemonic:** **GAS = Group A Streptococcus** — β-hemolytic, carbohydrate antigen (GlcNAc), M protein serotypes, causes acute pharyngitis and sequelae (ARF, PSGN). [cite:Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases 9e Ch 195]
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