Amyloid Fibrils Produced by Streptococcus sanguinis Contribute to Biofilm Formation and Immune Evasion.

Bacterial surface proteins assembled into amyloids contribute to biofilm formation and host immune evasion. Streptococcus sanguinis, a pioneer colonizer of teeth commonly involved in cardiovascular infections, expresses about thirty-three proteins anchored to the cell wall by sortase A. Here, we characterized the production of amyloid in S. sanguinis strains differing in biofilm and immune evasion phenotypes and investigated the role of sortase A in amyloidogenesis. Amyloid was identified in biofilms formed by nine strains, using Congo red (CR) staining and cross-polarized light microscopy. Additionally, EGCG, an amyloid inhibitor, impaired biofilm maturation in a strain-specific fashion. The amounts of amyloid-like components quantified in culture fluids of nine strains using thioflavin T and fluorimetry negatively correlated with bacterial binding to complement-activating proteins (SAP, C1q), C3b deposition and rates of opsonophagocytosis in PMNs, implying amyloid production in immune evasion. The deletion of the sortase A gene (srtA) in strain SK36 compromised amyloid production and sucrose-independent biofilm maturation. The srtA mutant further showed increased susceptibility to C3b deposition and altered interactions with PMNs as well as reduced persistence in human blood. These findings highlight the contribution of amyloids to biofilm formation and host immune evasion in S. sanguinis strains, further indicating the participation of sortase A substrates in amyloidogenesis.

Oral streptococci show diversity in resistance to complement immunity.

PURPOSE: Mechanisms underlying systemic infections by oral species of Mitis (Streptococcus mitis, Streptococcus oralis) and Sanguinis (Streptococcus gordonii, Streptococcus sanguinis) commensal streptococci are poorly understood. This study investigates profiles of susceptibility to complement-mediated host immunity in representative strains of these four species, which were isolated from oral sites or from the bloodstream. METHODOLOGY: Deposition of complement opsonins (C3b/iC3b), and surface binding to C-reactive protein (CRP) and to IgG antibodies were quantified by flow cytometry in 34 strains treated with human serum (HS), and compared to rates of opsonophagocytosis by human PMN mediated by complement (CR1/3) and/or IgG Fc (FcγRII/III) receptors. RESULTS: S. sanguinis strains showed reduced susceptibility to complement opsonization and low binding to CRP and to IgG compared to other species. Surface levels of C3b/iC3b in S. sanguinis strains were 4.5- and 7.8-fold lower than that observed in S. gordonii and Mitis strains, respectively. Diversity in C3b/iC3b deposition was evident among Mitis species, in which C3b/iC3b deposition was significantly associated with CR/FcγR-dependent opsonophagocytosis by PMN (P<0.05). Importantly, S. gordonii and Mitis group strains isolated from systemic infections showed resistance to complement opsonization when compared to oral isolates of the respective species (P<0.05). CONCLUSIONS: This study establishes species-specific profiles of susceptibility to complement immunity in Mitis and Sanguinis streptococci, and indicates that strains associated with systemic infections have increased capacity to evade complement immunity. These findings highlight the need for studies identifying molecular functions involved in complement evasion in oral streptococci.