A Bacterial Pheromone System Regulates Biofilm Formation and Colonization

Objectives: Streptococcus pneumoniae is a nasopharyngeal colonizer and a major human pathogen. The burden of pneumococcal disease is over 1 million fatalities per year, and the majority of deaths occur in children. Importantly, the pneumococcus is a close relative to Streptococcus mitis, a prevalent biofilm colonizer in the nasopharynx and in the oral cavity. Our aims were to investigate pheromone systems in S. pneumoniae and to assess whether pneumococcal pheromones can activate signaling in S. mitis.
Methods: Genome analysis and RT-PCR were used for gene identification and investigation of autoinducing activity. RNA-sequencing was performed for a comprehensive assessment of differential gene expression. Deletion mutants were constructed by PCR mutagenesis and, subsequently, tested for biofilm formation on epithelial surfaces, and for nasopharyngeal colonization in mice. For analyses between groups, t tests were used.
Results: We identified a pheromone system in S. pneumoniae that is present in almost half of all fully sequenced strains of this species. The system acts through two key components: a regulator that binds to DNA (Rgg), and a peptide that is secreted and reimported (SHP), acting as a signaling molecule. The global transcription profile showed that the Rgg/SHP complex activates a 10-gene operon downstream of the peptide. Further, the knockout strain of Rgg formed less biofilm on eukaryotic cells in vitro. In animal models of nasal carriage, the ΔRgg strain was outcompeted by its parent; therefore, showing a decreased potential to colonize the host. We also identified a similar system in S. mitis that responded to the pneumococcal pheromone.
Conclusions: We characterized a pheromone system in S. pneumoniae involved in biofilm formation and carriage; two highly important traits, considering that colonization is the first step to infection in all cases of pneumococcal disease. Signaling interference in streptococci may lead to novel strategies to prevent and treat biofilm-associated diseases.