Emergence of a piliated and multidrug-resistant <i>Streptococcus pneumoniae</i> serotype 35B-ST156 clone in Japan

<i>Streptococcus pneumoniae</i> serotype 35B, a nonvaccine serotype, is common among clinical pneumococcal isolates in Japan after vaccination. We report the emergence of serotype 35B sequence type 156 (35B-ST156), a rare type previously reported in Japan, in isolates from patients at a hospital. Multilocus sequence typing of serotype 35B isolates from 2014 to 2023 revealed that 35B-ST156 appeared in 2019 and was the most common sequence type of 35B in 2023. All 17 isolates of 35B-ST156 had the type 1 pilus, an adhesion factor, and were nonsusceptible to β-lactams, erythromycin, and co-trimoxazole. Whole-genome sequencing analysis demonstrated that the isolates harbored type 1 pilus and exhibited resistance profiles for β-lactam (<i>pbp1a</i>: 4, <i>pbp2b</i>: 12, and <i>pbp2x</i>: 7), macrolide (<i>mefE</i>), and co-trimoxazole (<i>folA</i> mutation and <i>folP</i> insertion). The analysis, along with global ST156 isolates in the database, suggested that the 35B-ST156 isolates in the present study were closely related to the strains prevalent in the United States, which had been generated by capsular switching, and clonally spread in the region through repeated small mutations. An increase in the proportion of 35B-ST156 influenced the antimicrobial susceptibility pattern of serotype 35B. The increased prevalence of ST156 in nonvaccine serotypes may alter antimicrobial efficacy against pneumococcal infections in Japan. Therefore, this clone should be monitored for effective treatment and prevention of pneumococcal infections.IMPORTANCEAlthough an increase in the prevalence of <i>Streptococcus pneumoniae</i> 35B-ST156 has not been reported in Japan, we found that ST156 was the most common sequence type of 35B among clinical isolates at a hospital in 2023. All 35B-ST156 isolates had a type 1 pilus and were multidrug resistant. Whole-genome sequencing analysis of the 35B-ST156 isolates showed that these isolates were closely related to the strains in the United States, generated by capsular switching and prevalent after 13-valent pneumococcal conjugate vaccine implementation, and clonally spread in the region. Furthermore, an increase in the proportion of 35B-ST156 influenced the antimicrobial susceptibility pattern of serotype 35B. The results provide useful information for guiding the treatment and prevention of pneumococcal infections.