Based on PFGE profile analyses, no capsular switch events were detected and thus no evidence was found in our study of vaccine escape recombinant isolates as reported by Bruegemann et al. in 2007 [40]. However, LY2157299 in vivo it should be noted that the failure to detect capsular switch events could be linked to the relatively small sample size of 174 PFGE profiles. In the present

study, besides the pneumococcal prevalence comparisons that allowed detection of the known serotype replacement phenomenon between VT and NVT isolates (Table 2 and Table 3), we actually identified the mechanism of the vaccine’s effect in our setting. We show that within a month, in children aged between 12 and 24 months, a single dose of PCV7 decreases VT colonization as it prevents de novo acquisition, and conversely increases NVT colonization, namely by enhancing NVT unmasking ( Table 4). Our data is in accordance with previous studies, which suggest that conjugate

vaccines reduce VT carriage by preventing de novo acquisition rather than clearance [19], [41], [42] and [43]. Besides this major mechanism of the vaccine’s effect we propose that an additional one is the enhancement of NVT unmasking ( Table 4). Assessment of this last mechanism was only possible due to the study of multiple colonization. As a result of the paucity of multiple carriers, we were unable to conclude about a specific Levetiracetam tendency Wnt signaling of serotype associations before and after a single vaccine dose. Nevertheless, we found that 13 serotypes (6A, 6B, 7F, 11A, 14, 16F, 17F, 19A, 19F, 23B, 23F, 33F, and 38) and non-typeable isolates were able to co-colonize, associating with other serotypes in the children’s nasopharynx. In the vaccinated group, serotype 6A was the most common serotype observed among multiple carriers. Worthy of note is the fact that in the PCV7 era, the nasopharynx of multiple carriers can constitute

a reservoir for VT isolates. Some VTs (e.g. 6B, 14 and 19F) prevailed as minor serotypes “masked” by the dominant NVT isolates, in opposition to what occurred in the control. Whether or not the preferred co-existence of some serotypes reflects similarity of their chemical structures, similar nutritional requirements and/or bacteriocin compatibility [44] of the particular isolates remains to be determined. In summary, the present study demonstrates that, as early as 1 month after vaccination with a single dose, PCV7 causes serotype replacement of VT by NVT isolates in single and multiple carriers, with the mechanisms of the vaccine’s effect being the prevention of VT de novo acquisition and enhancement of NVT unmasking.