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[Microbiology](https://durgical.com/en/microbiology/) · [Bacteria](https://durgical.com/en/microbiology/#d-bacteria)

# Streptococci and enterococci

- Bacteria

Gram-positive cocci in chains or pairs that cause anything from sore throat and pneumonia to newborn sepsis and endocarditis.

Updated 2026-11-01

## What they are

These are catalase-negative Gram-positive cocci arranged in pairs or chains. They are sorted by haemolysis on blood agar (alpha, beta or gamma) and by Lancefield groups (A, B, D). Enterococci, once group D streptococci, are hardier: they grow in bile and salt and are intrinsically resistant to cephalosporins.

## What they share

- Cocci in chains or pairs, catalase negative
- Beta-haemolytic: S. pyogenes and S. agalactiae
- Alpha-haemolytic: S. pneumoniae and viridans
- Enterococcus: grows on bile esculin and in 6.5 % NaCl
- Enterococcus: intrinsically resistant to cephalosporins

## Key points

- Beta-haemolytic: group A bacitracin sensitive; group B bacitracin resistant and CAMP positive.
- Alpha-haemolytic: pneumococcus optochin sensitive and bile soluble; viridans optochin resistant.
- Group A leaves immune sequelae: rheumatic fever and glomerulonephritis.
- Group B: the classic leading cause of newborn sepsis and meningitis.
- Vancomycin-resistant enterococcus (VRE): D-Ala-D-Ala becomes D-Ala-D-Lac; contact precautions.

## Microorganisms in this group

- [**Streptococcus pyogenes** A throat and skin bacterium that causes strep throat, impetigo and necrotising fasciitis, and can leave rheumatic fever or kidney damage. Exam priority: high](https://durgical.com/en/microbiology/streptococcus-pyogenes/)
- [**Streptococcus pneumoniae** A nose and throat bacterium behind most community pneumonia, ear infections, sinusitis and meningitis; a vaccine is in Colombia's PAI. Exam priority: high](https://durgical.com/en/microbiology/streptococcus-pneumoniae/)
- [**Streptococcus agalactiae** A bacterium that lives in the gut and vagina of many pregnant women and can cause sepsis, pneumonia and meningitis in the newborn. Exam priority: medium](https://durgical.com/en/microbiology/streptococcus-agalactiae/)
- [**Viridans streptococci** Mouth streptococci that cause tooth decay and subacute endocarditis; S. gallolyticus is linked to colon cancer. Exam priority: medium](https://durgical.com/en/microbiology/viridans-streptococci/)
- [**Enterococcus** Gut cocci that cause urinary infection, endocarditis and hospital infections; some strains resist vancomycin (VRE). Exam priority: medium](https://durgical.com/en/microbiology/enterococcus/)

## Test yourself

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

## Which two tests separate the alpha-haemolytic streptococci?

Optochin and bile solubility: pneumococcus is sensitive and soluble; viridans streptococci are resistant and insoluble.

## Which beta-haemolytic streptococcus is bacitracin sensitive?

Streptococcus pyogenes (group A); group B (S. agalactiae) is resistant.

## Why do cephalosporins fail against enterococci?

Their penicillin-binding proteins have low affinity for cephalosporins: an intrinsic resistance.

## What is the mechanism of enterococcal vancomycin resistance?

van genes (such as vanA) change the peptidoglycan end from D-Ala-D-Ala to D-Ala-D-Lac, which vancomycin binds poorly.

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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 Overview of Pneumococcal Disease](https://www.cdc.gov/pneumococcal/hcp/clinical-overview/index.html) (CDC, 2026); [Clinical Overview of Group B Strep Disease](https://www.cdc.gov/group-b-strep/hcp/clinical-overview/index.html) (CDC, 2024); [Vancomycin-resistant Enterococci (VRE) Basics](https://www.cdc.gov/vre/about/index.html) (CDC, 2024).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/groups/streptococci-and-enterococci/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/groups/streptococci-and-enterococci/