Clonal spread of serotype 12F ST4846 .

In 2016-2017, there was an increase in the number of paediatric invasive pneumococcal disease (IPD) cases caused by serotype 12F in Chiba Prefecture, Japan. Serotype 12F is one of the major causative serotypes of IPD following the introduction of pneumococcal conjugate vaccine 13 (PCV13), and outbreaks of IPD caused by serotype 12F have recently been reported in several countries. Our goal here was to clarify the relationship among local outbreak strains and the outbreak strains in other countries, and for this we analysed clinical isolates of serotype 12F using several genetic identification methods. All reported IPD cases caused by serotype 12F were reviewed and bacterial strains were collected and analysed. We also analysed serotype 12F strains isolated from other time periods, geographical areas, cases of adult IPD and respiratory specimens as control strains. Multi-locus sequence typing, PFGE and multi-locus variable number tandem repeat analysis (MLVA) were conducted on all isolates. All 26 . serotype 12F isolates, including control strains, belonged to a single sequence type (ST4846) that was the specific ST in Japan. All tested strains demonstrated five MLVA patterns and two PFGE patterns. We determined that the 2016-2017 outbreak of IPD in Chiba Prefecture was caused by clonally related isolates of serotype 12F. The continuous monitoring of IPD caused by serotype 12F is important for evaluating the impact of re-emerging pneumococcal serotypes following the PCV13 introduction era, and MLVA could be a useful tool for identification of outbreak strains.