Within-host genetic diversity of pneumococcal serotype 3 during one-year prolonged carriage in a healthy adult.

Streptococcus pneumoniae adapts within hosts through genetic variation and genome plasticity, facilitating persistence under antibiotic and immune pressures. Here, we investigated a prolonged pneumococcal carriage episode (>337 days) in a healthy, HIV-uninfected adult in Malawi. Whole-genome sequencing of single-colony isolates and plate sweep samples confirmed persistent colonisation by a multidrug-resistant serotype 3 strain with a distinct sequence type (GPSC10-ST18362), which maintained stable predominance despite transient acquisition of other serotypes. The sequentially sampled isolates showed 2 to 11 single-nucleotide polymorphism differences, no evidence of recombination, and modest gene loss involving mobile genetic elements. The total genome size decreased from 2.06 Mb to 2.03 Mb across isolates. Intrahost single-nucleotide variants were identified in genes related to metabolism, stress response, and DNA repair, but showed no consistent signatures of positive selection. Capsular locus analysis revealed deletions consistent with GPSC10-related vaccine-escape profiles. These findings highlight the capacity of GPSC10-ST18362 to persist asymptomatically for months with limited within-host genomic diversity.