Evaluation of the susceptibility of oral Streptococcus ssp. to the marking by the complement system

Abstract

Bacteria of the human microbiota are in balance with their host; however, under specific conditions, they are able to overcome the host immune defenses and cause infection. The oral cavity represents one of the human body sites with the biggest and most diverse microbiota, and the oral tissues are intimately connected to the bloodstream. The virulence potential of its several microorganisms is partly determined by its capability of resisting to defense components of the host. There is evidence that the predominant species of Streptococcus in the oral cavity can cause systemic diseases in both healthy and immunodeficient subjects, once reaching the bloodstream. Species of Streptococcus express cell wall proteins that are involved in evasions of innate immunity components, such as the complement system, which has an essential role in opsonization and inflammatory responses to microorganisms in blood and tissues. However, little is known about the mechanisms of bacterial evasion of the immune system among oral streptococcal species. Recently, we showed that the oral species Streptococcus mutants is able to escape from phagocytosis by neutrophils in human blood, if two of its gene transcription regulatory systems are inactivated, the Two-Component System VicRK and CovR. (Negrini et al., 2012). Vick and CovR regulate the expression of several genes which encode proteins involved in cell wall biogenesis (Stipp et al., 2013) and our hypothesis is that some of these genes are involved in the evasion from opsonization by the complement system. Cell wall biogenesis proteins have been characterized by our research group in strains of oral species of Streptococcus isolated from blood and oral cavity, which include Streptococcus mitis, Streptococcus oralis, Streptococcus Gordonii e Streptococcus sanguine, isolated by Dr. Mogens Kilian's research group from the University of Aarhus, Denmark (Proc. 2009/50547-0). Our working hypothesis is that strains with a high ability to evade the complement system are more capable of infecting and surviving in the bloodstream and, therefore, cause infection. The aim of this project is to compare the efficiency of complement C3 opsonization in strains isolated from human blood and oral cavity in the major species of oral streptococci, Streptococcus mitis, Streptococcus oralis, Streptococcus Gordonii, and Streptococcus sanguinis. (AU)