Streptococcus species

MICROBIOLOGY

• Clinical labs classify streptococci by hemolytic characteristics on 5% sheep blood agar (eg beta-hemolysis), Lancefield Group antigens and other biochemical tests.
• Nomenclature and taxonomy of streptococci confusing because of many historical efforts at describing the class; taxonomic revisions continue as new phylogenetic information becomes available.
• Often described by blood agar hemolysis (1902) or Lancefield carbohydrate group antigens (1933). Organization into 4 groups by hemolysis, Lancefield and phenotype testing (1937):
  1. Pyogenic (beta-hemolytic) including Groups A, B, C, E, F & G
  2. Viridans group
  3. Lactococci (generally not human pathogens)
  4. Enterococci
• 16S rRNA gene sequencing (1990’s) yield true phylogenetic relationships. Facklam classification presented here (see references).
• Viridans streptococci produce alpha (green-hence viridans) hemolysis on blood agar.