Analysis of the functions of pepO protein in systemic virulence of Streptococcus mutans

Abstract

Streptococcus mutans (SM), a major pathogen of dental caries, can promote infective endocarditis and other systemic infections after accessing the bloodstream. Previously, we showed that mutants defective in CovR, a transcriptional repressor of genes for oral virulence, are resistant to phagocytosis by human PMN in a blood-dependent way. In the PhD fellowship project associated with this BEPE request (proc. 2015/07237), we found that SM isolates from systemic infections have reduced susceptibility to complement immunity compared to oral isolates. Deletion of CovR-encoding gene (covR) in the oral strain UA159 impaired C3b-mediated phagocytosis by PMN and increased bacterial survival in blood. We further identified that CovR represses a gene encoding a putative endopeptidase, PepO, and established that deletion of pepO in UA159 increases susceptibility to C3b deposition. In S. pneumoniae, PepO plays multiple functions for evasion to complement immunity, including inactivation of C1q, binding to the complement inhibitor C4BP and cleavage of C3b through PepO-binding to plasminogen. PepO was also involved in degradation of proteins of the extracellular matrix (ECM), and in invasion of endothelial cells. Some of these functions are currently attributed to the collagen-binding protein Cnm, which is expressed by a limited number of SM serotypes. The aim of this BEPE project is to assess individual and cooperative contributions of PepO and Cnm for systemic virulence of SM. To that purpose, we will compare the transcriptional activities of pepO in different SM serotypes (c, e and f) isolated from systemic infections, which produce (cnm+) or not (cnm-) Cnm. PepO production will be also analyzed using antibodies against a PepO recombinant protein. The capacity to bind plasminogen or other ECM components will be also assessed in pepO-isogenic mutants obtained from different SM serotypes, including cnm+ and cnm- strains. These mutants will be also compared regarding their capacity to invade endothelial cells in vitro and their virulence in a Galleria mellonella in vivo model of infection. Therefore, this BEPE project may provide important information on the functions of PepO and Cnm for systemic virulence of SM, which will be required for the selection of the best panel of therapeutic targets to control systemic infections by SM serotypes. (AU)