Bacterium
Streptococcus anginosus group
Abscess-forming viridans (milleri) streptococci
strep-toh-KOK-us an-jih-NOH-sus
High-yield clue
Viridans-group streptococci that form abscesses (brain, liver, lung, dental) are the signature study clue.
Overview
A subgroup of the viridans streptococci (also called the S. milleri group) that includes S. anginosus, S. constellatus, and S. intermedius. They are normal oral, GI, and genital flora notable for a strong tendency to form pus-filled abscesses.
Classification
- Gram-positive
- Cocci in chains
- Viridans (milleri) group
- Facultative anaerobe
- Small colony size
Lab & identification clues
- Variable hemolysis on blood agar
- Small (<0.5 mm) colonies vocabulary
- Inconsistent Lancefield A/C/F/G reaction
- Optochin resistant (unlike S. pneumoniae)
Associations
- Deep-seated abscess formation study association
- Oral/GI/genital normal-flora origin
- Dental and head-and-neck infection vocabulary
- Bacteremia in immunocompromised hosts
Commonly confused with
- Streptococcus pneumoniae
- Streptococcus pyogenes
Frequently asked questions
What is Streptococcus anginosus group?
A subgroup of the viridans streptococci (also called the S. milleri group) that includes S. anginosus, S. constellatus, and S. intermedius. They are normal oral, GI, and genital flora notable for a strong tendency to form pus-filled abscesses.
What is the Gram stain of Streptococcus anginosus group?
Streptococcus anginosus group is Gram positive.
What are the lab clues for Streptococcus anginosus group?
Lab clues for Streptococcus anginosus group include Variable hemolysis on blood agar, Small (<0.5 mm) colonies vocabulary, Inconsistent Lancefield A/C/F/G reaction, and Optochin resistant (unlike S. pneumoniae).
What is Streptococcus anginosus group commonly confused with?
Streptococcus anginosus group is commonly confused with Streptococcus pneumoniae and Streptococcus pyogenes.
Original student-study summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment, dosing, or specimen-handling guidance.