<p>The Streptococcus Milleri/Anginosus group (SMG) includes Streptococcus anginosus, Streptococcus constellatus and Streptococcus intermedius. The SMG is found in healthy individuals but these bacteria are most known clinically for being associated with invasive disease and more recently, airway infections including cystic fibrosis (CF). The SMG like many other streptococci are naturally competent, being able to actively bind, uptake and integrate extracellular DNA. Competence regulation involves a competence-stimulating peptide (CSP) derived from the ComC precursor and a two- component signaling system (a histidine kinase ComD and its response regulator ComE). In this study, I examined the distribution of CSP/ComD sequences and competence in 170 SMG clinical isolates from CF airways and invasive disease. Five predicted CSP sequences were observed; one represented a newly predicted CSP and two arose from frameshift mutations in comC and appeared to be non-functional. The three CSPs fall into two functional groups that do not cross-activate due to receptor specificity. In addition, I observed that the Streptococcus constellatus subspecies pharyngis strains could not be transformed. However, I demonstrated that the pharyngis strains possess a functional ComCDE pathway, suggesting that the CSP regulates genes other than those involved in natural transformation. For many strains, I observed high endogenous competence levels that were only marginally induced by added peptide. These strains appear to be constitutively competent during exponential growth. The high basal level of expression and the heterogeneity in the SMG competence systems could impact how the SMG evolve during colonization and infections and specifically acquire antibiotic resistance and virulence genes.</p> / Master of Science (MSc)