We investigated impact of culture-enrichment on the rank of serotypes when detected quantitatively in nasopharyngeal samples from toddlers co-carrying multiple pneumococcal strains.
Pneumococcal serotypes were detected in nasopharyngeal swabs (collected in a cross-sectional study from children aged <5 years) using conventional culture method followed by testing in serotype-specific qPCRs the DNA extracted from all bacterial growth harvested from culture-plates. Next, we quantified with qPCR serotypes in uncultured samples and compared serotype ranks before and after culture-enrichment.
Co-presence of multiple serotypes has been detected in 37 (5.6%) of 658 nasopharyngeal samples. The number of serotype carriage events detected by any method in these 37 samples was 83 (2-5 per sample) while the number of strains cultured was 42 (1-2 per sample). For the majority of samples (n=27 or 73%) the serotype ranks in uncultured and culture-enriched samples matched. A shift in serotype rank was observed in 11% of the samples (4 of 37) whereas in another 16% (6 of 37) the discordance was due to fewer serotypes detected in uncultured samples.
While increasing sensitivity of carriage detection, culture-enrichment has limited impact on the rank of serotypes present in nasopharyngeal swabs and can therefore be used to provide insight into co-carriage dynamics.