Poster Presentation Lancefield International Symposium for Streptococci and Streptococcal Diseases 2025

Pneumococcal-like capsule polysaccharide loci among Streptococcus mitis bloodstream isolates from infective endocarditis cases in the UK and Ireland from 2001-2016 (#264)

Akuzike Kalizang'oma 1 2 3 4 , Brenda Kwambana 1 2 3 4 5 , Juliana Coelho 6 , Karen Broughton 6 , Bruno Pichon 6 , Victoria Chalker 7 , Stephen D Bentley 8 , Chrispin Chaguza 3 8 9 10 11 12 , Robert S Heyderman 3 4
  1. Malawi Liverpool Wellcome Programme, Blantyre, Malawi
  2. Department of Pathology, School of Medicine and Oral Health, Kamuzu University of Health Sciences, Blantyre, Malawi
  3. Department of Infection, Division of Infection and Immunity, University College London, London, United Kingdom
  4. NIHR Global Health Research Group of Vaccines to Prevent Respiratory Pathogens and AMR, Division of Infection and Immunity, University College London, London, United Kingdom
  5. Department of Clinical Sciences, Liverpool School of Tropical Medicine, Liverpool, United Kingdom
  6. Public Health Microbiology Division, UK Health Security Agency, Colindale, London, United Kingdom
  7. NHS England, London, United Kingdom
  8. Parasites and Microbes Programme, Wellcome Sanger Institute, Wellcome Genome Campus, Cambridge, United Kingdom
  9. Darwin College, University of Cambridge, Silver Street, Cambridge, United Kingdom
  10. Department of Clinical Infection, Microbiology and Immunology, Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, Liverpool, United Kingdom
  11. Department of Epidemiology of Microbial Diseases, Yale School of Public Health, Yale University, New Haven, Connecticut, United States
  12. Department of Host-Microbe Interactions, St Jude Children’s Research Hospital, Memphis, Tennessee, United States

Background
Streptococcus mitis is a major cause of infective endocarditis (IE), a life-threatening infection of the heart valves. S. mitis may harbour capsule biosynthesis loci (cps), a key virulence factor for Streptococcus pneumoniae (the pneumococcus). However, the prevalence and diversity of S. mitis strains carrying pneumococcal-like capsules remain poorly characterised. We investigated the occurrence of invasive S. mitis strains with pneumococcal-like serotype capsules.

Methods
We conducted retrospective whole-genome sequencing of 129 S. mitis isolates collected from patients with bloodstream infections and clinically diagnosed IE in the UK and Ireland from 2001-2016. Isolates were obtained from the British Society of Antimicrobial Chemotherapy and the UK Health Security Agency. Population structure and cps were analysed using comparative genomics and novel in silico tools.

Results
We identified 127/129 (98.4%) unique multilocus sequence types and Global Sequence Clusters (GSCs) among the isolates. Complete cps loci were identified in 116/129 (89.9%) isolates, with sequence lengths ranging from 6,628-32,574bp (median: 21,782bp). Pneumococcal-like vaccine serotypes 5 and multiple non-vaccine serotypes including 19C and 23B were detected among 26/116 (22.4%) cps using the pneumococcal SeroCall tool, with nucleotide-BLAST coverage and identity ranging from 82–100% and 86–95.76%, respectively. Novel cps loci were identified in 90/116 (77.6%) of the isolates.

Conclusion
The genomic overlap between S. mitis and pneumococcal cps loci highlights the close evolutionary relationship between the species. Extensive S. mitis cps diversity has the potential to drive pneumococcal vaccine escape facilitated by interspecies horizontal gene transfer at the cps locus.