Methods: Comparative genomic hybridization studies using S. pneumoniae DNA microarrays were performed on 1 S. oralis, 1 S. pseudopneumoniae and 5 S. mitis isolates. Microarray studies were carried out using the S. pneumoniae SPv1.1.0 microarrays, designed and produced by the Bacterial Microarray Group, St. Georges, University of London (BµG@S). Slides were scanned using ScanArray Express (Packard Biosciences), feature extraction and normalisation carried out using BlueFuse for Microarray 3.5 software (Bluegnome Ltd) and gene lists created using GeneSpring GX 7.3.1 (Agilent Technologies) software. Validation of comparative genomic hybridizations was by PCR using primers used to design the microarray probes.
Results: Homologues to 312/337 pneumococcal virulence genes were present in at least one of the S. mitis group isolates. The five S. mitis isolates tested had homologues to between 72 and 83% of pneumococcal genes, determined by hybridisation to the pneumococcal gene probe. Furthermore, the S. oralis isolate hybridised to 84% of the genes, and the S. pseudopneumoniae isolate to 91.4%. Of interest was the absence of hybridisation to the transcriptional regulator rlrA, present in the RlrA islet encoding the pneumococcal pilus and genes in the PsrP islet.
Conclusions: The presence of pneumococcal virulence factor homologues in closely related commensal species not only furthers understanding of the relationships between these species, but also offers insight of importance for future pneumococcal vaccine design.