What is Streptococcus anginosus?
Streptococcus anginosus is a member of the viridans group streptococci (VGS) that is uniquely characterized by its tendency to form abscesses and cause hematogenously disseminated infections, distinguishing it from other α-hemolytic streptococcal species. 1
Microbiological Classification
S. anginosus belongs to the S. anginosus group (also historically called the S. milleri group), which includes three species: S. anginosus, S. intermedius, and S. constellatus 1, 2
This group is part of the normal human flora and commonly colonizes the gastrointestinal tract, oral cavity, and genitourinary tract 3, 4
The organism is typically α-hemolytic on blood agar, though this taxonomy has caused some historical confusion 1
Unique Clinical Characteristics: The Abscess-Former
The defining feature that sets S. anginosus apart from other viridans streptococci is its propensity to form deep-seated abscesses and cause metastatic infections. 1, 2
Sites of Abscess Formation:
- Brain abscesses 2, 5, 6
- Myocardial abscesses 1, 2
- Visceral abscesses (liver, spleen) 1, 2, 4, 5
- Muscle abscesses (pyomyositis) 7
- Septic arthritis 1, 2
- Vertebral osteomyelitis 1, 2
Antimicrobial Susceptibility
The S. anginosus group is usually sensitive to penicillin, but some strains may exhibit variable penicillin resistance 1, 2
This variable resistance pattern is clinically important and distinguishes it from other α-hemolytic streptococci that are more predictably penicillin-susceptible 1
Treatment typically involves β-lactams (penicillin G or cephalosporins), often combined with surgical drainage 5
Critical Clinical Associations
Infective Endocarditis
S. anginosus causes infective endocarditis as part of the viridans streptococci group 2, 3
When S. anginosus endocarditis is diagnosed, a systematic search for metastatic abscesses should be conducted by imaging the brain, liver, spleen, and myocardium 2
The duration of antimicrobial treatment may need to be longer than for other α-hemolytic streptococci due to its abscess-forming tendency 2
Colorectal Malignancy
S. anginosus bacteremia has an association with colorectal cancer, similar to but less well-established than the association with Streptococcus gallolyticus (bovis) 4
Clinicians should consider screening for colorectal cancer in patients presenting with S. anginosus bacteremia, particularly when accompanied by liver abscesses 4
Important Clinical Pitfalls
Don't Dismiss as a Contaminant
Even a single positive blood culture growing S. anginosus should prompt careful evaluation for a pyogenic focus of infection 7
Because it is part of normal flora, it may be mistakenly dismissed as a contaminant when labeled generically as "viridans streptococci" 7
Disseminated Infection Recognition
S. anginosus can cause highly disseminated pyogenic infections affecting multiple organ systems simultaneously 5
Concomitant brain-liver involvement occurs in 50% of disseminated cases, brain-lung in 42%, and brain-spleen in 8% 5
Underlying conditions such as dental infections, malignancy, and gastrointestinal disease are present in 42% of cases 5
Surgical Intervention is Often Necessary
Surgical drainage is required in approximately 67% of S. anginosus infections to achieve full clinical recovery 5
Antimicrobial therapy alone is often insufficient due to the abscess-forming nature of this organism 5, 6
Delayed diagnosis and treatment can be fatal, particularly in central nervous system infections 6