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
Your notes
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.