Streptococcus intermedius is an oral commensal organism belonging to the Streptococcus anginosus group (SAG). S. intermedius causes periodontitis as well as invasive, pyogenic infection of the central nervous system, pleural space or liver. Compared with other SAG organisms, S. intermedius has a higher mortality as well as a predilection for intracranial infection, suggesting it is likely to possess virulence factors that mediate specific interactions with the host resulting in bacteria reaching the brain. The mechanisms involved are not well described. Intracranial suppuration (ICS) due to S. intermedius infection can manifest as an abscess within the brain parenchyma, or a collection of pus (empyema) in the sub- or extra-dural space. These infections necessitate neurosurgery and prolonged antibiotic treatment and are associated with a considerable burden of morbidity and mortality. The incidence of ICS is increasing in several settings, with SAG species accounting for an increasing proportion of cases. There is a paucity of published literature regarding S. intermedius pathogenesis as well as few published genomes, hampering molecular epidemiological research. This perspective evaluates what is known about the clinical features and pathogenesis of ICS due to S. intermedius and explores hypothetical explanations why the incidence of these infections may be increasing.
Keywords: Streptococcus anginosus group; Streptococcus intermedius; Streptococcus milleri group; Viridans streptococci; brain abscess; extradural empyema; streptococcal infections; subdural empyema.
Streptococcus intermedius is a type of bacteria that normally lives in the mouth. Most of the time it does not cause disease but it can cause mild infections, for example of the gums. Sometimes, it can cause infections deeper in the body, for example the brain, lungs or liver. Brain infection with Streptococcus intermedius results in the build-up of pus in the brain or between the skull and the brain. People with these infections need both brain surgery and long courses of antibiotics. Many are left with lasting damage resulting in disability and a significant proportion die from their infection. Several studies have suggested that these infections are becoming more common and that Streptococcus intermedius is causing an increasing number of brain infections. Compared with other bacteria that cause these sorts of infections, not much is known about Streptococcus intermedius – we do not have many genome sequences or many scientific studies exploring how it causes disease. This piece of work describes what is known about brain infection with Streptococcus intermedius and proposes possible explanations for why these infections may be becoming more common.