Carried pneumococci in Massachusetts children: The contribution of clonal expansion and serotype switching

The introduction of conjugate vaccination against seven serotypes of Streptococcus pneumoniae (the pneumococcus) has had profound consequences for the population structure of this pathogen, both in asymptomatic carriage and invasive disease. The prevalence of vaccine serotypes has, in vaccinated populations, fallen to the point that they are barely detectable (1). Nevertheless, the prevalence of overall pneumococcal carriage has shown no such decrease, remaining roughly constant. This is believed to be because the removal of the vaccine serotypes (4, 6B, 9V, 14, 18C, 19F and 23F) has enabled non-vaccine serotypes (of which more than 80 are known) to take their place: this phenomenon is known as serotype replacement. Along with serotype replacement in carriage, an increase in invasive disease due to non-vaccine serotypes has been observed. Thus far in the US this has been small, and the net benefits of vaccination in terms of preventing disease have not been eroded to any great extent (2). This may be because of the serotypes which have to date been involved in replacement in carriage, only 19A is associated with a comparatively higher propensity to cause invasive disease (3). We do not know if the increased carriage prevalence of other serotypes will lead to their emergence as a cause of invasive disease. The pneumococcus is a naturally transformable organism, with the ability to take up DNA from other strains (and species) and incorporate it into its own genome. This has been documented in many cases at the cps locus which contains the genes which determine serotype (4). Pneumococci which previously expressed vaccine serotypes can gain a non-vaccine serotype by this method (5). Uncertainty remains about the population level impact of this ‘serotype switching’ following vaccination; while it has been noted in surveillance of IPD (5, 6) the importance of clones produced by serotype switching in the population as a whole is not known. In the pre-vaccine era, antibiotic resistance was concentrated among the serotypes selected for inclusion in the vaccine. One of the advantages of vaccination has been a marked reduction in high level penicillin resistance, as a result of the removal of these highly drug resistant vaccine serotype strains (7). There are concerns that serotype switching could lead to the re-emergence of resistant clones in association with non-vaccine serotypes. Possible cases of serotype switching are readily detected by a combination of serotyping and multi-locus sequence typing (MLST), a method for typing of bacterial strains which collects sequence data at seven housekeeping loci (8). Evidence of serotype switching is identified when isolates are identical at all MLST loci, yet have different serotypes. MLST also allows for the identification of clones associated with resistance or particular disease manifestations. We have applied this technique to a large sample of pneumococci isolated from carriage in generally healthy Massachusetts children during the fall and winter of 2006/2007.