Objectives: Pneumococci are common colonizer, especially of children, and cocolonization of different serotypes is an important factor for intraspecies genetic exchange. The aim of this study was to analyze pneumococcal carriage and serotype distribution in unvaccinated healthy children in Iceland and compare conventional culture methods and molecular methods using DNA extracted directly from the samples.
Methods: Nasopharyngeal swabs were obtained from 514 children aged 2-6 year attending day care centers in Reykjavik in 2009. The swabs were selectively cultured for pneumococci and the isolates serotyped using latex agglutination. DNA was also extracted directly from the swabs and serotyped using a multiplex PCR panel designed to detect vaccine serotypes and the most commonly carried non-vaccine serotypes.
Result: Pneumococcal carriage was detected in 391 (76.1%) of the children using polymerase chain reaction (PCR) and in 371 (72.2%) using conventional methods. Cocolonization was detected in 92 (23.5%) of the carriers when PCR method was used and in 30 (8.1%) when conventional methods were used, detecting 500 and 401 strains, respectively (P < 0.0001). The most common serotypes were 23F, 19A, 6B, 6A and 19F, rates 13-8%. The number of isolates of serotypes included in the 10-valent and 13-valent vaccines and detected by PCR were 234 (58.4%) and 363 (90.5%), respectively and by conventional methods 186 (46.4%) and 293 (73.1%), respectively.
Conclusion: Cocolonization was detected in a fourth of the children carrying pneumococci using DNA extracted directly from nasopharyngeal swabs. The rate of carriage was very high, but no serotype dominated, and the children were commonly colonized by vaccine serotypes, especially cocolonized children.