MICROBIOLOGY

• Group A Streptococcus (GAS), beta-hemolytic — grows on blood agar. See Streptococcal species or S. pneumoniae modules for other streptococci.
  • Streptococcus pyogenes, Lancefield group A.
  • Gram-positive cocci in chains [Fig 1].
• Ecologic niches are the pharynx, genital mucosa and skin; colonization rates:
  • Adults 2-3%
  • School children 15-20%
• Virulence depends on proteins that represent toxins, mimic host macromolecules and elicit immune responses.
  • M protein gene (emm) ~100 serotypes
    • It may be used to track outbreaks, including invasive infections
    • The invasive infections are associated with certain types (emm1, emm3, emm4, emm6, emm12, emm18, emm89).
      
• All isolates remain susceptible to beta-lactams; however, worrisome and rapid increases in resistance to alternative antibiotics are observed in the U.S. since 2020 (CDC data from 2023). Rates vary considerably by geography (e.g., macrolide resistance in China > 90%).
  • Erythromycin-resistance (macrolides, both inducible and constitutive): 27%
  • Clindamycin resistance: 26.5%