Streptococcus pneumoniae Serotype 9A Isolates Contain Diverse Mutations to wcjE That Result in Variable Expression of Serotype 9V-specific Epitope

Background. Streptococcus pneumoniae is a significant pathogen capable of expressing protective and antigenically diverse capsules. To better understand the molecular basis of capsular antigenic diversity, we investigated the hypothetical serological role of wcjE, which encodes a capsule O-acetyltransferase, in the vaccine-targeted serotype 9V and related serotype 9A. Methods. We inactivated wcjE by recombination in a serotype 9V strain and determined wcjE sequences of 11 serotype 9A clinical isolates. We determined the antigenic phenotypes of these pneumococcal strains with serogroup 9-specific antibodies and flow cytometry. Results. Inactivation of wcjE in a serotype 9V strain resulted in expression of the 9A phenotype. Each serotype 9A clinical isolate contained a distinct mutation to wcjE. Flow cytometry showed that some 9A isolates (herein named 9A alpha) expressed trace amounts of 9V-specific epitopes whereas others (named 9A beta) did not express any. Recombination with 9A alpha wcjE alleles into a 9A beta strain conferred partial expression of 9V-specific epitopes. Conclusions. Each serotype 9A strain independently arose from a serotype 9V strain. Furthermore, clinical isolates identified as 9A can contain mutations to wcjE that are either partially functional or completely nonfunctional, demonstrating a previously unidentified antigenic heterogeneity of serotype 9A isolates.