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Microbiology · Streptococci and enterococci

Streptococcus pyogenes !

  • Bacterium
  • Gram-positive
  • Droplet
  • Contact
  • Exam priority: high

A throat and skin bacterium that causes strep throat, impetigo and necrotising fasciitis, and can leave rheumatic fever or kidney damage.

Updated 2026-11-01

Also called: group A streptococcus, GAS

At a glance

Gram
Gram-positive
Shape
Cocci in chains
Oxygen
Facultative anaerobe
Family
Streptococcaceae
Stain
Gram positive; catalase negative; beta-haemolytic

Morphology

  • Gram-positive cocci in chains
  • Complete beta-haemolysis on blood agar
  • Bacitracin sensitive and PYR positive

Transmission

  • Respiratory droplets from talking, coughing or sneezing
  • Contact with skin sores or their fluid
  • Shared unwashed utensils; rarely food

Isolation precautions

What the care team adds to standard precautions.

Droplet
Surgical mask within about a metre; single room.
Contact
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

Virulence factors

  • M protein: antiphagocytic; its likeness to heart proteins explains rheumatic fever
  • Streptolysin O: immunogenic haemolysin (ASO antibodies)
  • Pyrogenic exotoxins (superantigens): scarlet fever and streptococcal toxic shock
  • Streptokinase, hyaluronidase and DNase: spread through tissue
  • Hyaluronic acid capsule

Diseases it causes

Clinical picture

  • Fever, sore throat, tonsillar exudate and neck nodes without cough
  • Scarlet fever: sandpaper rash and strawberry tongue
  • Pain out of proportion to what the skin shows (necrotising fasciitis)

Diagnosis

  • Rapid antigen test or throat culture
  • Beta-haemolysis, bacitracin sensitivity, PYR positive
  • Antistreptolysin O (ASO) and anti-DNase B for sequelae
  • Rheumatic fever is diagnosed with the Jones criteria

Treatment

Penicillins (penicillin, amoxicillin) remain effective: penicillin resistance has not been documented. Macrolide resistance is growing. In necrotising fasciitis: urgent surgery plus a beta-lactam and clindamycin to stop toxin production. Secondary penicillin prophylaxis prevents recurrent rheumatic fever.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Timely treatment of strep throat to prevent rheumatic fever
  • Hand and cough hygiene
  • Secondary penicillin prophylaxis after rheumatic fever
  • Droplet precautions (and contact for extensive wounds) through the first hours of antibiotics

Test yourself

Think of the answer, then tap the question to see it.

Which group A strep sequela follows only pharyngitis and not impetigo?

Rheumatic fever; post-streptococcal glomerulonephritis can follow either.

Which virulence factor explains rheumatic fever?

M protein, whose likeness to heart proteins triggers an autoimmune response (molecular mimicry).

Why is clindamycin added in group A necrotising fasciitis?

It inhibits protein synthesis, which cuts toxin production, and it works even when bacteria are very numerous.

Which lab test distinguishes group A from group B?

Bacitracin: group A is sensitive and group B is resistant (group B is also CAMP positive).

Add to my cards (4)

← Back to the group: Streptococci and enterococci

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: About Group A Strep Infection (CDC, 2024); Clinical Guidance for Acute Rheumatic Fever (CDC, 2025); Toxic shock syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.