A Novel Competence Pathway in the Oral Pathogen Streptococcus sobrinus

Streptococcus sobrinus is an etiologic cause of dental caries (tooth decay) in humans. Our knowledge of S. sobrinus is scant despite the organism's important role in oral health. It is widely believed that S. sobrinus lacks the natural competence pathways that are used by other streptococci to regulate growth, virulence, and quorum sensing. The lack of natural competence has also prevented genetic manipulation of S. sobrinus, limiting our knowledge of its pathogenicity. We discovered that most strains of S. sobrinus contain a new class of the ComRS competence system. Although S. sobrinus is typically placed among the mutans group streptococci, the S. sobrinus ComRS system is most similar to the competence pathways in the salivarius group. Unlike all other ComRS systems, the S. sobrinus pathway contains 2 copies of the transcriptional regulator ComR and has a peptide pheromone (XIP) that lacks any aromatic amino acids. Synthetic XIP enables transformation of S. sobrinus with plasmid or linear DNA, and we leverage this newfound genetic tractability to confirm that only 1 of the ComR homologs is required for induced competence while the other appears to suppress competence. Exogenous XIP increases the expression of bacteriocin gene clusters and produces an antimicrobial response that inhibits growth of S. mutans. We also identified 2 strains of S. sobrinus that appear to be "cheaters" by either not responding to or not producing XIP. We show how a recombination event in the nonresponsive strain could restore function of the ComRS pathway but delete the gene encoding XIP. Thus, the S. sobrinus ComRS pathway provides new tools for studying this pathogen and offers a lens into the evolution of ecological cheaters.