title: Carried pneumococci in Massachusetts children: the contribution of clonal expansion and serotype switching.
abstract: BACKGROUND
  Vaccination against 7 serotypes of Streptococcus pneumoniae has led to the near extinction of vaccine serotypes in both disease and asymptomatic carriage. In carriage, vaccine serotypes have been replaced by nonvaccine serotypes.
  
  
  METHODS
  We used multilocus sequence typing to analyze a sample of 294 isolates of S. pneumoniae carried by Massachusetts children (aged, 3 months-7 years) and examine the results for serotype switching and association with antimicrobial resistance.
  
  
  RESULTS
  Eighty-six distinct sequence types (STs) were found, 10 of which exhibited a serotype other than that which would be expected from previous carriage samples. We interpret this as evidence of past or recent serotype switching. Switched variants include ST 320, which is a common and increasing source of multidrug resistance in this community. Switching events within serogroups were more common than expected by chance (P = 0.043 by a Monte Carlo approach). Using multilocus sequence typing data and eBURST analysis, we also describe clonal dynamics within the important replacement serotypes 19A, 15B/C, 35B, and the recently described 6C.
  
  
  CONCLUSIONS
  Some strains generated by serotype switching are increasingly important parts of the carriage population. In the case of 19A, it appears that the majority of increase is due to ST 320, a recently reported switched variant. This may have consequences for the STs causing invasive pneumococcal disease.
authors.affiliation: Department of Infectious Disease Epidemiology, Imperial College London, London, UK. whanage@hsph.harvard.edu
authors.forename: William P
authors.initials: WP
authors.lastname: Hanage
authors.forename: Cynthia J
authors.initials: CJ
authors.lastname: Bishop
authors.forename: Susan S
authors.initials: SS
authors.lastname: Huang
authors.forename: Abbie E
authors.initials: AE
authors.lastname: Stevenson
authors.forename: Stephen I
authors.initials: SI
authors.lastname: Pelton
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Finkelstein
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D015373:Bacterial Typing Techniques; D002353:Carrier State / Q000453:epidemiology* / Q000382:microbiology*; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D005260:Female; D005838:Genotype; D000069443:Heptavalent Pneumococcal Conjugate Vaccine; D006801:Humans; D007223:Infant; D008297:Male; D008404:Massachusetts / Q000453:epidemiology; D008826:Microbial Sensitivity Tests; D058885:Multilocus Sequence Typing; D011008:Pneumococcal Infections / Q000453:epidemiology* / Q000382:microbiology*; D022242:Pneumococcal Vaccines / Q000008:administration & dosage / Q000276:immunology*; D015995:Prevalence; D012703:Serotyping; D013296:Streptococcus pneumoniae / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification*
chemical_list: D000900:Anti-Bacterial Agents; D000069443:Heptavalent Pneumococcal Conjugate Vaccine; D022242:Pneumococcal Vaccines