Species-specific clinical characteristics of Streptococcus anginosus group bacteraemia: a five-year retrospective cohort study

Purpose The Streptococcus anginosus group (SAG) is comprised of S. anginosus, constellatus, and S. intermedius which can cause invasive infection with a particular predilection for abscess formation. We aimed to describe the clinical characteristics, infectious sources, abscess formation and outcomes of SAG bacteraemia across NHS Greater Glasgow and Clyde, Scotland, UK, and compare clinical manifestations between species. Methods We retrospectively identified all blood culture sets isolating SAG bacteria in NHS GG&C from 1st January 2020 to 31st December 2024 using our electronic laboratory information management system. Electronic clinical records were reviewed. Results Among 200 positive blood culture sets, 180 patients with clinically significant SAG bacteraemia were identified. The overall annual incidence was 3.0 per 100,000 population-years (95% CI 2.6–3.5). S. anginosus was the most common species (56%), followed by S. constellatus (29%), and S. intermedius (15%). An infectious focus was identified in 138/180 (77%) patients, most commonly hepatobiliary. Abscesses were identified in 71/180 (39%) patients with the highest proportion among patients with S. intermedius bacteraemia (18/27, 67%). Infective endocarditis (IE) occurred in 6/180 (3%) patients and was caused exclusively by S. anginosus. Thirty-day mortality was 18% (33/180) and was not significantly different between species. Out of 42 patients with a SAG bacteraemia without an identified focus, previously unrecognised malignancy was identified in 11 (26%) patients. Conclusion SAG bacteraemia was associated with substantial mortality and species-specific differences in abscess identification. Previously unrecognised malignancy was identified relatively frequently among patients without an identified infectious focus, but this finding is subject to ascertainment bias and requires prospective evaluation.