Comparison of traditional culture and molecular qPCR for detection of simultaneous carriage of multiple pneumococcal serotypes in African children

S. pneumoniae is a common colonizer of the human nasopharynx in high income and low-middle income countries. Due to limitations of standard culture methods, the prevalence of concurrent colonization with multiple serotypes is unclear. We evaluated the use of multiplex quantitative PCR (qPCR) to detect multiple pneumococcal serotypes/group colonization in archived nasopharyngeal swabs of pneumococcal conjugate vaccine naive children who had previously been investigated by traditional culture methods. Overall the detection of pneumococcal colonization was higher by qPCR (82%) compared to standard culture (71%; p < 0.001), with a high concordance (kappa = 0.73) of serotypes/groups identified by culture also being identified by qPCR. Also, qPCR was more sensitive in detecting multiple serotype/groups among colonized cases (28.7%) compared to culture (4.5%; p < 0.001). Of the additional serotypes detected only by qPCR, the majority were of lower density (< 104 CFU/ml) than the dominant colonizing serotype, with serotype/group 6A/B, 19B/F and 23F being the highest density colonizers, followed by serotype 5 and serogroup 9A/L/N/V being the most common second and third colonizers respectively. The ability of qPCR to detect multiple pneumococcal serotypes at a low carriage density might provide better insight into underlying mechanism for changes in serotype colonization in PCV vaccinated children.