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Identification and characterization of novel quorum sensing systems through the scope of interspecies interaction in Streptococcal species
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Quorum sensing (QS) is a chemical-based intercellular communication method related to population density used by single-celled organisms to coordinate gene regulation and resultant group behavior. These behaviors include biofilm formation, competence, motility, virulence, pathogenicity, et al., and have been associated with human infections causing endocarditis, oral caries, pulmonary disease, orthopedic implant infection, et al. The Streptococcus genus provides insight as a model organism for studying peptide-based control of QS in Gram positive bacteria. By observing interspecies interactions and investigating their chemical origins, the Streptococcus’s own molecular biology aids in the elucidation of crucial elements of the system. For instance, both Competence Stimulating Peptide (CSP) and sigX-Inducing Peptide (XIP) promote the transformation of streptococci by inducing competence through distinct and shared biochemical pathways. This allows for the identification of critical structural features of associated enzymes and peptides through the creation of peptide-based probes and use of reporter strains. The ComABCDE pathway, using CSP, and the ComRS pathway, using XIP, present opportunities to develop non-bactericidal treatments for infections dependent on QS systems, avoiding the selective pressure exerted by antibiotics. Bacteriocins, CSPs/XIPs, proteases, transport proteins, histidine kinases, and other transcription factors all present useful targets for academic and therapeutic study. In this work, I set out to identify QS systems in previously unstudied streptococcal strains through the scope of interspecies interactions and begin their characterization. To this end, I used a range of clinically relevant species to observe single- species growth tendencies, as well as combinations of species for growth assays to establish baseline growth expectations. Interspecies interaction patterns assisted in the identification of potential QS systems, and after homology-based searches and gene sequencing, an ComRS with an atypical XIP were observed in S. sobrinus W1703. Time-dependent protein extraction were unable to confirm native XIP’s or CSP’s presence in liquid colonies of the strains studied, and synthesis of the presumed most biochemically active form of XIP from this strain was attempted. The elucidation of the ComRS system in S. sobrinus could allow for the development of a model reporter strain for ComRS in the Streptococcus genus, including establishing a cross-species interactive view of clinically relevant bacterial communities. Future XIP synthesis, optimization of conditions for native QS signal production, and phenotypic assays (particularly competence, bacteriocin production, and hemolysis) using XIP collectively will contribute to an overarching understanding of QS communication systems and can lead to the advancement of target selection and therapeutic design.