Streptococcus mitis response to the competence pheromone

Objectives: Pheromones, known as quorum-sensing signals, orchestrate the transition of bacteria from an individual to a social lifestyle. In several streptococci, pheromone sensing mediated by competence-stimulating peptides (CSP) is associated with development of competence for transformation and biofilm formation. This system has been characterized in detail in Streptococcus pneumoniae, a close relative of the predominant oral colonizer Streptococcus mitis. It is not known, however, the extent to which the S. mitis response to CSP, a human commensal, may resemble that in the human pathogen S. pneumoniae.
To characterize the global transcriptional response to CSP pheromone in the oral commensal bacterium Streptococcus mitis strain NCTC12261.
Methods: Streptococcus mitis transcriptome analysis using RNA sequencing was performed for a thorough assessment of differential gene expression in response to CSP. After analyzing RNA sequencing results, we searched for homologues to competence genes in S. pneumoniae. RT-PCR was conducted to establish whether the upregulated genes in response to CSP, with no homologues in S. pneumoniae, were early or late CSP-induced genes.
Results: S. mitis transcriptome analysis revealed that 100 genes were upregulated at least two-fold by CSP. Homologues of S. pneumoniae genes acting in transport and fratricide were identified, as well as essential genes involved in foreign DNA uptake and recombination. Among these, we could find 13 genes that did not present any homology with S. pneumoniae. All of them showed a time-response to CSP indicative of a late CSP response.
Conclusions: This is the first transcriptome analysis of S. mitis showing the effect of the CSP pheromone. The results showed that the induced genes corresponded to approximately 6% of the S. mitis genome, and revealed upregulated sequences not previously identified in S. pneumoniae. Elucidating mechanisms used by S. mitis to orchestrate group behavior may lead to novel strategies to control dental biofilm associated diseases.