Streptococcus intermedius: a primer

A member of the anginosus group, best known for brain and liver abscesses.

At a glance

Group

Streptococcus anginosus group, with S. anginosus and S. constellatus

Gram stain

Gram-positive cocci in chains

Habitat

Normal flora of the mouth, upper airway, gut and genitourinary tract

Known for

Abscesses, especially in the brain and liver

Key toxin

Intermedilysin, which targets human cells through CD59

The organism

Older papers call this group the “Streptococcus milleri group”. Lancefield grouping is unreliable here: isolates can carry group A, C, F or G antigens, or none. On culture, the group is often described as smelling of caramel. Identification now relies on MALDI-TOF or sequencing, because biochemical tests struggle to separate the three species.

Why it matters clinically

All three species can cause invasive infection, but S. intermedius is the one most often linked to brain and liver abscesses. Brain abscesses often have a dental or sinus source.

Intermedilysin

Intermedilysin is a cholesterol-dependent cytolysin, a pore-forming toxin. Most toxins in this family bind cholesterol on any cell, but intermedilysin first binds human CD59, so it damages human cells far more readily than those of other animals.

Resources

• From normal flora to brain abscesses: a review of Streptococcus intermedius (Frontiers in Microbiology, 2020)
• UK Standards for Microbiology Investigations