Accurate serotype identification of <i>Streptococcus pneumoniae</i> using nanopore Cas9-targeted serotype identification (nCATSerotyping)

<i>Streptococcus pneumoniae</i> (pneumococcus) is a leading cause of community-acquired pneumonia and invasive diseases, particularly among children and the elderly. The introduction of pneumococcal conjugate vaccines has significantly reduced invasive pneumococcal disease, but the prevalence of non-vaccine serotypes and newly emerging serotypes is increasing globally. Thus, accurate serotyping is essential for epidemiological surveillance and the development of next-generation multivalent pneumococcal vaccines. Conventional serotyping methods, including multiplex polymerase chain reaction (mPCR), monoclonal antibody (mAb) assays, and Quellung reaction using rabbit antisera, are limited by serotype coverage and cross-reactivity, making the detection of new or emerging serotypes challenging. In this study, we developed a nanopore Cas9-targeted serotyping (nCATSerotyping) platform, which employs Cas9-mediated enrichment of the capsular polysaccharide synthesis locus followed by Oxford Nanopore sequencing. Applying this method to 276 clinical pneumococcal isolates collected in South Korea (2018-2020), we achieved a serotyping success rate of 97.10% (268/276), significantly outperforming conventional methods such as mAb and mPCR, which identified only 76.45% (211/276) of isolates. Whole-genome sequencing of the remaining eight non-typeable isolates revealed them to be non-pneumococcal (oral streptococci), confirming 100% accuracy for <i>S. pneumoniae</i> serotyping. Importantly, our method identified emerging and underrepresented serotypes, including serotype 13 and null capsule clade strains. nCATSerotyping offers a rapid, accurate, and comprehensive solution for pneumococcal serotyping, with significant advantages in identifying novel and non-typeable strains. This scalable platform will be a valuable tool for global serotype surveillance and next-generation multivalent pneumococcal vaccine development.IMPORTANCEAccurate pneumococcal serotyping is critical for vaccine development and epidemiological surveillance, particularly as non-vaccine serotypes emerge following widespread pneumococcal conjugate vaccine implementation. Current serotyping methods face significant limitations in coverage and accuracy, identifying around 76% of pneumococcal isolates and failing to detect emerging serotypes like serotype 13 and null capsule clades. The nanopore Cas9-targeted serotyping platform addresses these critical gaps by achieving 100% serotyping accuracy for confirmed <i>Streptococcus pneumoniae</i> isolates while identifying previously undetectable strains that conventional methods missed. This comprehensive approach is essential for monitoring vaccine effectiveness, understanding serotype replacement patterns, and informing next-generation vaccine development strategies. Furthermore, the identification of misclassified oral streptococci highlights the diagnostic precision needed for accurate pneumococcal surveillance, ensuring that epidemiological data accurately reflect true pneumococcal disease burden and serotype distribution patterns.