Streptococcus species

Streptococcus species is a topic covered in the Johns Hopkins ABX Guide.

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MICROBIOLOGY

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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