Update on the evolving landscape of pneumococcal capsule types: new discoveries and way forward

SUMMARYStreptococcus pneumoniae (the "pneumococcus") is a significant human pathogen. The key determinant of pneumococcal fitness and virulence is its ability to produce a protective polysaccharide (PS) capsule, and anti-capsule antibodies mediate serotype-specific opsonophagocytic killing of bacteria. Notably, immunization with pneumococcal conjugate vaccines (PCVs) has effectively reduced the burden of disease caused by serotypes included in vaccines but has also spurred a relative upsurge in the prevalence of non-vaccine serotypes. Recent advancements in serotyping and bioinformatics surveillance tools coupled with high-resolution analytical techniques have enabled the discovery of numerous new capsule types, thereby providing a fresh perspective on the dynamic pneumococcal landscape. This review offers insights into the current pneumococcal seroepidemiology highlighting important serotype shifts in different global regions in the PCV era. It also comprehensively summarizes newly discovered serotypes from 2007 to 2024, alongside updates on revised chemical structures and the de-novo determinations of structures for previously known serotypes. Furthermore, we spotlight emerging evidence on non-pneumococcal Mitis-group strains that express capsular PS that are serologically and biochemically related to the pneumococcal capsule types. We further discuss the implications of these recent findings on capsule nomenclature, pneumococcal carriage detection, and future PCV design. The review maps out the current status and also outlines the course for future research and vaccine strategies, ensuring a continued effective response to the evolving pneumococcal challenge.