Johns Hopkins Antibiotic (ABX) GuidePathogensBacteria

Streptococcus species

Michael Melia, M.D.
Streptococcus species is a topic covered in the Johns Hopkins Antibiotic (ABX) Guide.

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  • Nomenclature and taxonomy of streptococci confusing because of many historical efforts at describing the class.
  • Often described by 5% sheep blood agar hemolysis (1902) or Lancefield carbohydrate group antigens (1933).
  • Broadly organized into two groups by hemolysis, Lancefield and phenotype testing (1937):
    1. Pyogenic (beta-hemolytic) including Groups A, B, C, E, F & G.
      • Group A streptococci: cause complete hemolysis/lysis of red cells in blood agar media around/under colonies caused by streptolysin (exotoxin), so called β-hemolysis. See separate module for details on this organism.
    2. Viridans group
      • Usually yield α-hemolysis on blood agar (green surrounding colony - hence name, viridans); may be β-hemolytic or non-hemolytic (latter sometimes termed γ-hemolysis).
  • Other organisms previously classified as Streptococci are now separate genera:
    • Lactococci: generally not human pathogens.
    • Enterococci: see separate module.
    • Streptococcal-like, catalase-negative, Gram-positive cocci.
      • e.g., Leuconostoc, Pediococcus
    • Formerly nutritionally-variant Streptococci
      • Abiotrophia, Granulicatella
  • 16S rRNA gene sequencing (1990’s) yield true phylogenetic relationships. Facklam classification presented here (see references).

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Last updated: August 3, 2016