[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/) › Viridans streptococci

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

# Viridans streptococci

- Bacterium
- Gram-positive
- Exam priority: medium

Mouth streptococci that cause tooth decay and subacute endocarditis; S. gallolyticus is linked to colon cancer.

Updated 2026-11-01

Also called: S. mutans, S. sanguinis, S. mitis, S. gallolyticus (formerly S. bovis)

## Key points

- Alpha-haemolytic and optochin resistant (unlike pneumococcus).
- Subacute endocarditis on previously damaged valves.
- S. mutans: caries.
- S. gallolyticus in the blood = look for colorectal cancer.

## At a glance

**Gram**
Gram-positive

**Shape**
Cocci in chains

**Oxygen**
Facultative anaerobe

**Family**
Streptococcaceae

**Stain**
Gram positive; catalase negative; alpha-haemolytic

## Morphology

- Gram-positive cocci in chains
- Alpha (green) haemolysis
- Optochin resistant and bile insoluble

## Transmission

- Normal flora of the mouth and gut (not contagious)
- Enter the blood after dental procedures or through inflamed gums

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- Dextrans: adhesion to enamel and to fibrin-platelet clots on damaged valves
- S. mutans ferments sugars into acid that demineralises enamel

## Diseases it causes

- Subacute endocarditis on damaged valves [Infective endocarditis →](https://durgical.com/en/conditions/infective-endocarditis/)
- Dental caries (S. mutans) [Dental caries →](https://durgical.com/en/conditions/dental-caries/)
- S. gallolyticus bacteraemia linked to colorectal cancer [Colorectal cancer →](https://durgical.com/en/conditions/colorectal-cancer/)

## Clinical picture

- Prolonged fever, a new murmur, fatigue and weight loss (subacute endocarditis)

## Diagnosis

- Serial blood cultures
- Optochin resistant, bile insoluble
- Echocardiogram when endocarditis is suspected
- Colonoscopy if S. gallolyticus is isolated

## Treatment

They are usually susceptible to penicillin or ceftriaxone; susceptibility guides endocarditis treatment. Prophylaxis before dental procedures is reserved for high-risk patients, such as those with prosthetic valves.

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/)
- [Ceftriaxone](https://durgical.com/en/pharmacology/ceftriaxone/)

## Prevention

- Oral hygiene and caries control
- Dental antibiotic prophylaxis only for high-risk cardiac patients

## Memory aid

The viridans are the green residents of the mouth: quiet on the teeth until they ride the bloodstream to a valve that is already bruised.

## Test yourself

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

## Which test distinguishes viridans streptococci from pneumococcus?

Optochin: viridans are resistant and pneumococcus is sensitive.

## Which test is indicated for a patient with S. gallolyticus bacteraemia?

A colonoscopy, because of the link with colorectal cancer.

## Which factor lets viridans streptococci stick to damaged valves?

Dextrans, which adhere to fibrin and platelet deposits.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/microbiology/viridans-streptococci/)

## Related

- [Streptococcus pneumoniae](https://durgical.com/en/microbiology/streptococcus-pneumoniae/)
- [Enterococcus](https://durgical.com/en/microbiology/enterococcus/)

## Related conditions

- [Infective endocarditis](https://durgical.com/en/conditions/infective-endocarditis/)
- [Dental caries](https://durgical.com/en/conditions/dental-caries/)
- [Colorectal cancer](https://durgical.com/en/conditions/colorectal-cancer/)

## 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.

[← 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: [Endocarditis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001098.htm) (MedlinePlus (NLM), 2024); [Dental cavities: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001055.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.

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