AbstractStreptococcus mutans (SM) is a common bacterial species of the oral cavity of humans involved in the pathogenesis of dental caries, which can also promote infectious endocarditis after accessing the bloodstream from oral niches. However, little is known about the virulence factors involved in SM capacity to survive in the bloodstream and to cause systemic infections. During the process of host colonization, SM undergoes several changes in its transcriptome in response to environmental stimuli, which are in part controlled by two-component transduction systems (TCS). SM strains have 12 to 13 TCS, which are typically comprised of a histidine kinase sensor protein (K), and a cognate cytoplasmic response regulator (R), encoded by genes organized in operons. In addition, SM expresses the orphan R, CovR. In other streptococcal species, CovR is phosphorylated by the cognate K protein (CovS), and represses the transcription of cell surface and secreted proteins involved in evasion to host immune components, e.g., opsonins. In SM, CovR is phosphorylated by the VicK protein of the TCS VicRK. Recently, we established that deletion of the gene encoding CovR (covR) in the SM strain UA159 impairs deposition of C3b/iC3b of the complement system, which are major blood opsonins. Additionally, the covR mutant showed low susceptibility to phagocytosis by human PMN and increased survival in blood. Comparative analyses of the intensity of C3b deposition between SM clinical isolates indicate large variability in the susceptibility to C3b deposition in this species. Strains isolated from blood showed lower susceptibility to C3b deposition compared to oral isolates. The goal of this project is to investigate the role of the CovR and VicRK regulators in the resistance to C3b deposition in SM strains isolated from blood. To that purpose, a total of four blood strains identified by our group as having low susceptibility to C3b deposition will be compared with other four strains isolated from the oral cavity, which are susceptible to C3b deposition, and with the reference strain UA159. Polymorphisms in the covR and vicRK chromosome loci will be investigated by sequencing the amplicons obtained with different set of primers. Transcriptional activities of covR, vicK e vicR will be also analyzed by RT-qPCR and compared between strains and between eventually found polymorphic strains. In addition, the protein profiles of cell extracts and culture fluids will be compared between the knockout mutants of covR (UAcov) and vicK (UAvic), and the other studied strains. The results of this study may provide novel insights about the roles of CovR and/or VicRK in the SM susceptibility to complement immunity.
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