Serotyping of Streptococcus pneumoniae based on capsular genes polymorphisms

Diagnostic primer extension assay to serotype Streptococcus pneumoniae. Assay validation. Background: Monitoring of Streptococcus pneumoniae serotype epidemiology is essential since serotype replacement is a concern when introducing new polysaccharide-conjugate vaccines. To simplify S. pneumoniae serotyping, a novel PCR-based automated microarray assay was developed to assist in the tracking of the serotypes. Results: Autolysin (lytA), pneumolysin (ply) and eight genes located in the capsular operon (cps) were amplified using multiplex PCR. This step was followed by a tagged fluorescent primer extension step targeting serotype-specific polymorphisms. The tagged primers were then hybridized to a microarray. Results were exported to an expert system that transforms genetic typing data into capsular serotype identification. The assay was validated on 166 cultured S. pneumoniae samples from 63 different serotypes as determined by the Quellung method. In addition, the assay was tested on clinical specimens including 43 cerebrospinal fluid samples from patients with meningitidis and 59 nasopharyngeal aspirates from bacterial pneumonia patients. The assay presented with no cross-reactivity for 24 relevant bacterial species found in these types of samples. The limit of detection for serotyping and S. pneumoniae detection was 100 genome equivalent per reaction. Conclusion: This automated assay is amenable to clinical testing and does not require any culturing of the samples. The assay will be useful for the evaluation of serotype prevalence changes after new conjugate vaccines introduction. 166 quellung serotyped strains and two negative controls