Streptococci

Morphology

  • Gram Positive Cocci
  • Arranged in chains
    • Think Streptococcus throat like necklace
  • May elaborate capsular polysaccharide that impedes phagocytosis » Slime
  • Pili project from capsule

Classification

GroupOrganismsNotes
ß-haemolytic group AStreptococcus pyogenesProduce haemolysins. Majority of pathogens
ß-haemolytic group B (B = Baby)Streptococcus agalactiaeNormal flora of female genital tract. Important in neonatal infections
ß-haemolytic group G (G = Gut)Enterococcus, Streptococcus faecalis, Streptococcus faeciumNormal enteric flora
Non-ß-haemolyticStreptococcus pneumoniaeCause pneumonia
Non-ß-haemolyticStreptococcus viridansNormal respiratory flora
Non-ß-haemolyticPeptostreptococciGut anaerobes

Toxins/ Enzymes

  • Haemolysins
    • Haemolyse RBC
      • Incomplete disruption – Alpha haemolysis
      • Complete disruption – Beta haemolysis
    • Group A – ß-Haemolytic Strep produce
      • Streptolysin O
        • Streptolysin S
  • Streptokinase
    • Converts Plasma Plasminogen to Plasmin
    • Proteolytic enzyme that digests fibrin
  • Hyaluronidase
    • Splits Hyaluronic acid
    • Aids in spreading infection
  • Erythrogenic Toxin
    • Causes rash of Scarlet Fever

Pathology

Streptococci, gram stain of streptococcus pyogenes
Gram stain of Streptococcus pyogenes. Image by Gary E. Kaiser, Ph.D., Wikimedia Commons, CC BY 3.0.
  • ß-Haemolytic Streptococcus
  • Commonly cause
    • Erysipelas
    • Strep Throat
    • Impetigo
    • Infective Endocarditis (SBE)
    • Rheumatic Fever
    • Acute Glomerulonephritis

Treatment

  • All sensitive to Penicillin G

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.