Variation in transformation frequency and competence gene expression among serotype 3 Streptococcus pneumoniae

BackgroundStreptococcus pneumoniae is a human commensal and the causative agent of pneumococcal disease. Pneumococci are naturally competent - able to uptake exogenous DNA from the environment and incorporate it into their genome through homologous and non- homologous recombination. Recombination has significantly shaped the evolutionary history of S. pneumoniae, as it allows pneumococci to rapidly adapt. Recombination frequencies vary considerably across pneumococcal populations, yet underlying mechanisms for these variations are not well understood. Entry and exit into competence, a state in which the cell can uptake DNA, is tightly regulated through transcriptional changes. MethodsTo elucidate differences in transformation frequency among strains as well as the underlying genetic mechanisms, we carried out in-vitro competence assays and measured gene expression changes during the competent state using RNA sequencing of representative strains belonging to Serotype 3 clonal complex (CC) 180 and a non-CC180 comparison. ResultsWe observed consistent differences in transformation frequencies among groups, which correlated with variation in differentially expressed genes during competence. While all strains exhibited a similar response to competence stimulating peptide (CSP) for early competence genes, we observed variation in expression of late competence genes, which encode the DNA uptake apparatus, DNA repair and recombination proteins. We also observed differences in expression of genes linked to bacteriocin production, which may partially explain observed population-level differences. Further genomic analysis suggests variation in promoter sequences governing late competence genes may be slowing transition from early to late components of the competence pathway. ConclusionsWe show that there is considerable variation in competence even among closely related strains belonging to the same CC and that this variation may be the result of subtle genomic differences. Additional studies are needed to assess the phenotypic impact of genomic variations.