Streptococcus anginosus group
- Bacterium
Gram-positive
Thick peptidoglycan wall: stains purple.- Exam priority: medium
Mouth and gut streptococci with a talent for abscesses: brain, liver, lung and neck.
Also called: Streptococcus milleri group, S. anginosus, S. constellatus, S. intermedius
Quick facts
- Type
- Bacterium · Streptococci and enterococci
- Gram / shape
- Gram-positive · Cocci in chains
- Spread
- Endogenous · Spread follows a breach
- Causes
- Brain abscess · Pyogenic liver abscess
- Treatment
- Drainage is the key
At a glance
- Gram
- Gram-positive
- Shape
- Cocci in chains
- Oxygen
- Facultative anaerobes; many grow best with extra CO2
- Family
- Streptococcaceae
- Size
- Small (pinpoint) colonies
Morphology
- Gram-positive cocci in chains; catalase-negative
- Haemolysis varies: alpha, beta or none
- Lancefield antigen varies (A, C, F, G or none), so grouping does not identify them
- Cultures often have a sweet caramel-like smell
Transmission
- Endogenous: from the mouth, throat, gut or genital tract
- Spread follows a breach: dental infection, sinusitis, bowel disease or surgery
- Not spread from person to person
Reservoir: Human mouth (dental plaque), oropharynx, gut and genitourinary tract
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Enzymes such as hyaluronidase and DNase break down tissue and help form pus
- S. intermedius makes intermedilysin, a toxin that targets human cells
- Synergy with anaerobes in mixed abscesses
Diseases it causes
- Brain abscess, often from sinus or dental infection (especially S. intermedius) Brain abscess →
- Pyogenic liver abscess Pyogenic liver abscess →
- Empyema and lung abscess (especially S. constellatus) Parapneumonic effusion and empyema →
- Deep neck and peritonsillar abscess Deep neck infection and Ludwig angina →
- Intra-abdominal and pelvic abscesses (especially S. anginosus) Intra-abdominal abscess →
Clinical picture
- Fever with a space-occupying lesion; symptoms depend on the abscess site
- When it grows in blood, look for a hidden abscess
- Often part of a mixed infection with Fusobacterium, Prevotella or anaerobic cocci
Diagnosis
- Culture of pus obtained by drainage or aspiration, plus blood cultures
- Imaging (CT, MRI, ultrasound) to find and drain the abscess
- MALDI-TOF mass spectrometry identifies the species; grouping antigens are unreliable
- Brain abscess cultures may be negative after antibiotics; molecular tests help
Treatment
Drainage is the key, plus beta-lactams (penicillin, ampicillin or ceftriaxone), to which most strains are susceptible; add metronidazole when anaerobes are likely. Reduced penicillin susceptibility occurs, so test isolates; vancomycin is used for beta-lactam allergy.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Good dental care and treatment of dental and sinus infections
- No vaccine
Test yourself
Think of the answer, then tap the question to see it.
Which group of streptococci is known for forming abscesses?
The Streptococcus anginosus group: S. anginosus, S. constellatus and S. intermedius.
What should be done when the S. anginosus group grows in a blood culture?
Look for a hidden abscess, for example in the liver, brain, lung or abdomen.
Why is Lancefield grouping unhelpful for this group?
Its members carry different antigens (A, C, F, G) or none at all.
Which member is most linked to brain abscess?
Streptococcus intermedius.
Connections
← 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: The Clinical View on Streptococcus anginosus Group – Opportunistic Pathogens Coming Out of Hiding (Frontiers in Microbiology, 2022); Infections Caused by Group C and G Streptococcus (Streptococcus dysgalactiae subsp. equisimilis and Others): Epidemiological and Clinical Aspects (Microbiology Spectrum (ASM), 2019); Streptococcal Infections - Infectious Disease - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025).
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Representative model, for educational purposes only.