Background. The Streptococcus pneumoniae serotype 19A sequence type (ST) 320 clone, derived from an international Taiwan19F-14 (ST236) clone, has become prevalent in many countries.Methods. The dynamics of invasive pneumococcal disease (IPD) were determined using the database of the National Notifiable Disease Surveillance System in Taiwan. The virulence of 19A ST320 and Taiwan19F-14 (ST236) were assessed in mice. By constructing an isogenic serotype 19F variant of the 19A ST320 strain (19F ST320), we analyzed the role of capsular type and genetic background on the difference in virulence between 19A ST320 and Taiwan19F-14 (ST236).Results. Between 2008 and 2011, IPD due to serotype 19A increased from 2.1 to 10.2 cases per 100 000 population (P <. 001); IPD due to any serotype also significantly increased (P =. 01). Most serotype 19A isolates belonged to ST320. Using competition experiments in a murine model of colonization, we demonstrated that 19A ST320 outcompeted Taiwan19F-14 (ST236; competitive index, 20.3; P =. 001). 19F ST320 was 2-fold less competitive than the 19A ST320 parent (competitive index, 0.47; P =. 04) but remained 14-fold more competitive than Taiwan 19F-14 (ST236; competitive index, 14.7; P <. 001).Conclusions. Genetic evolution of pneumococcal clones from Taiwan19F-14 (ST236) to 19A ST320 has made this pneumococcus better able to colonize of the nasopharynx. This evolution reflects not only a switch in capsular serotype but also changes in other loci.
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