[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Streptococci and enterococci](https://durgical.com/en/microbiology/groups/streptococci-and-enterococci/) › Streptococcus pyogenes

[Microbiology](https://durgical.com/en/microbiology/) · [Streptococci and enterococci](https://durgical.com/en/microbiology/groups/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

## Key points

- Beta-haemolytic, bacitracin sensitive, PYR positive.
- Rheumatic fever only after pharyngitis; glomerulonephritis after throat or skin.
- M protein = molecular mimicry with myocardium.
- A raised ASO means recent infection.
- Always penicillin sensitive; clindamycin added in toxin-driven infection.

## 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

- Streptococcal pharyngitis [Streptococcal pharyngitis →](https://durgical.com/en/conditions/streptococcal-pharyngitis/)
- Rheumatic fever and rheumatic heart disease (after pharyngitis only) [Rheumatic heart disease →](https://durgical.com/en/conditions/rheumatic-heart-disease/)
- Impetigo [Impetigo →](https://durgical.com/en/conditions/impetigo/)
- Necrotising fasciitis [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene) →](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- Post-streptococcal glomerulonephritis (after throat or skin infection) [Post-streptococcal glomerulonephritis →](https://durgical.com/en/conditions/post-streptococcal-glomerulonephritis/)
- Cellulitis and erysipelas [Cellulitis of the leg →](https://durgical.com/en/conditions/cellulitis-of-the-leg/)
- Streptococcal toxic shock syndrome [Toxic shock syndrome →](https://durgical.com/en/conditions/toxic-shock-syndrome/)

## 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:

- [Benzylpenicillin (penicillin G)](https://durgical.com/en/pharmacology/benzylpenicillin/)
- [Amoxicillin](https://durgical.com/en/pharmacology/amoxicillin/)
- [Clindamycin](https://durgical.com/en/pharmacology/clindamycin/)

## 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

## In Colombia

Rheumatic heart disease is still present in Colombia and Latin America, above all where access to care is limited; it is an important cause of mitral valve disease in young adults.

## Memory aid

Group A sounds the Alarm in the throat today; the heart and kidneys pay the bill weeks later.

## 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)](https://durgical.app/estudiar/?add=/en/microbiology/streptococcus-pyogenes/)

## Related

- [Staphylococcus aureus](https://durgical.com/en/microbiology/staphylococcus-aureus/)
- [Streptococcus agalactiae](https://durgical.com/en/microbiology/streptococcus-agalactiae/)

## Related conditions

- [Streptococcal pharyngitis](https://durgical.com/en/conditions/streptococcal-pharyngitis/)
- [Rheumatic heart disease](https://durgical.com/en/conditions/rheumatic-heart-disease/)
- [Impetigo](https://durgical.com/en/conditions/impetigo/)
- [Necrotizing soft tissue infection (necrotizing fasciitis, gas gangrene)](https://durgical.com/en/conditions/necrotizing-soft-tissue-infection/)
- [Post-streptococcal glomerulonephritis](https://durgical.com/en/conditions/post-streptococcal-glomerulonephritis/)
- [Cellulitis of the leg](https://durgical.com/en/conditions/cellulitis-of-the-leg/)
- [Toxic shock syndrome](https://durgical.com/en/conditions/toxic-shock-syndrome/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

**Contact**
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

**Droplet**
Surgical mask within about a metre; single room.

**Airborne**
N95 respirator and a negative-pressure room.

[← Back to the group: Streptococci and enterococci](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/group-a-strep/about/index.html) (CDC, 2024); [Clinical Guidance for Acute Rheumatic Fever](https://www.cdc.gov/group-a-strep/hcp/clinical-guidance/acute-rheumatic-fever.html) (CDC, 2025); [Toxic shock syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000653.htm) (MedlinePlus (NLM), 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/streptococcus-pyogenes/