Evaluation of serotype prediction by cpsA-cpsB gene polymorphism in Streptococcus pneumoniae

Abstract

New pneumococcal conjugate vaccines covering a limited number of serotypes are likely to come into widespread use over the next few years. It is unknown what effect this will have on the relative importance of different serotypes as causes of pneumococcal infection. Hence, it will be important to monitor serotype prevalence before, during, and after the introduction of new vaccines. We have investigated the ability of a PCR method based on polymorphisms in two genes common to the different capsule loci to predict the serotype of 93 clinical isolates of Streptococcus pneumoniae submitted to the Central Public Health Laboratory in 1997. Of 70 isolates with vaccine serotypes, 65 were predicted to belong to the correct serotype; this number was improved to 69 with the inclusion of two additional patterns to the database. Of 23 isolates with other serotypes, 19 were correctly predicted as non-vaccine serotypes, the discrepancy lying with four isolates of 6A (non-vaccine serotype) that were indistinguishable from isolates of 6B (vaccine serotype). In situations in which culture of the organism is not feasible, this method could potentially be applicable directly to clinical specimens and could be a valuable aid to the surveillance of pneumococcal serotypes.

FIG. 1

Comparison by GelCompar of AluI restriction digests representing all pattern types defined for the 11 vaccine serotypes. Normalized band patterns are represented on the right, and a dendrogram of relatedness of patterns by use of the Dice coefficient is on the left. Pattern matches of 100% (tolerance, 1.0 + 1.75%) are demonstrated between the 4/5A patterns produced by isolates of serotypes 4 and 5 and the 18c/19fA patterns from the 18C and 19F isolates.