Application of capsular sequence typing (CST) to serotype non-viable Streptococcus pneumoniae isolates from an old collection.

Serotyping of Streptococcus pneumoniae is essential for monitoring changes in the pneumococcal population and the impact of vaccines. Recently, various DNA-based methods have become available and are increasingly used because they are cheaper and easier to perform than the Quellung reaction. Our aim was to apply a DNA-based method, capsular sequence typing (CST), to a collection of non-viable lyophilized pneumococcal isolates dating from the 1980s to elucidate the serotypes circulating in Italy 30 years ago. As a preliminary evaluation of the method, CST was applied to 68 recent pneumococcal isolates representative of the most common serotypes circulating in Italy in invasive pneumococcal disease (IPD) previously serotyped by the Quellung reaction. CST was then applied to 132 lyophilized non-viable isolates. A serotype-specific polymerase chain reaction (PCR), using primers suggested by the Centers for Disease Control and Prevention (CDC), was performed when CST did not yield a univocal serotype. Considering the control isolates, CST concordance with the Quellung reaction was 95.6 %. For the non-viable lyophilized isolates, CST identified a univocal serotype for 59.4 % of the isolates. This percentage increased to 78.1 % if CST was combined with serotype-specific PCR. The most frequent serotypes in the collection of non-viable strains were: 3 (15.6 %), 14 (11.7 %), 35B (5.5 %), 19A (5.5 %), and 8 (4.7 %). CST proved to be a valid method for serotyping pneumococcal strains and provided information about pneumococcal serotypes present in Italy 30 years ago. The combination of CST with serotype-specific PCR was an effective strategy to identify pneumococcal serotypes that can be suggested also for routine laboratories.