Emergence of serotype 23F among levofloxacin-resistant invasive Streptococcus pneumoniae in the Republic of Korea: Association with the expansion of GPSC6.

BACKGROUND: Although serotype 23F is included in the current pneumococcal conjugate vaccines (PCVs), its recent contribution to the emergence of levofloxacin-resistant invasive Streptococcus pneumoniae in South Korea remains unclear. Despite the inclusion of serotype 23F in PCVs, recent increases in the abundance of levofloxacin-resistant isolates have been observed in South Korea. METHODS: Temporal trends, antimicrobial resistance profiles, and genomic characteristics of serotype 23F isolates collected through the Infectious Diseases Surveillance System operated by the Korea Disease Control and Prevention Agency (KDCA) from 2015 to 2024 were analyzed. Whole-genome sequencing was performed to characterize lineage distribution, quinolone resistance-determining region (QRDR) mutations, antimicrobial resistance determinants, and penicillin-binding protein (PBP) allele combinations. RESULTS: Among the 1885 invasive pneumococcal isolates, 90 were resistant to levofloxacin. Of these, 29 were serotype 23F, and 68.9% (20/29) of the serotype 23F isolates were levofloxacin resistant. Serotype 23F was infrequently detected before 2020, but its contribution to levofloxacin-resistant invasive isolates became more apparent after 2021. WGS revealed that GPSC6 was first identified after 2020 and became the predominant lineage among levofloxacin-resistant serotype 23F isolates by 2022. GPSC6 isolates exhibited concurrent gyrA and parC mutations and carried multiple resistance genes, including erm(B), tet(M), folA, and folP, and harbored uncharacterized PBP allele combinations. CONCLUSIONS: These findings suggest that the recent increase in the levofloxacin-resistant serotype 23F in South Korea is associated with the expansion of GPSC6 and highlight the value of lineage-level genomic surveillance within vaccine-type serotypes.