CovR Regulates Streptococcus mutans Susceptibility To Complement Immunity and Survival in Blood

Abstract

b Streptococcus mutans, a major pathogen of dental caries, may promote systemic infections after accessing the bloodstream from oral niches. In this study, we investigate pathways of complement immunity against S. mutans and show that the orphan regulator CovR (CovR Sm ) modulates susceptibility to complement opsonization and survival in blood. S. mutans blood isolates showed reduced susceptibility to C3b deposition compared to oral isolates. Reduced expression of covR Sm in blood strains was associated with increased transcription of CovR Sm -repressed genes required for S. mutans interactions with glucans (gbpC, gbpB, and epsC), sucrose-derived exopolysaccharides (EPS). Consistently, blood strains showed an increased capacity to bind glucan in vitro. Deletion of covR Sm in strain UA159 (UAcov) impaired C3b deposition and binding to serum IgG and C-reactive protein (CRP) as well as phagocytosis through C3b/iC3b receptors and killing by neutrophils. Opposite effects were observed in mutants of gbpC, epsC, or gtfBCD (required for glucan synthesis). C3b deposition on UA159 was abolished in C1q-depleted serum, implying that the classical pathway is essential for complement activation on S. mutans. Growth in sucrose-containing medium impaired the binding of C3b and IgG to UA159, UAcov, and blood isolates but had absent or reduced effects on C3b deposition in gtfBCD, gbpC, and epsC mutants. UAcov further showed increased ex vivo survival in human blood in an EPSdependent way. Consistently, reduced survival was observed for the gbpC and epsC mutants. Finally, UAcov showed an increased ability to cause bacteremia in a rat model. These results reveal that CovR Sm modulates systemic virulence by regulating functions affecting S. mutans susceptibility to complement opsonization.

Two-component signal transduction systems in oral bacteria

“…Some of these gene functions are also associated with the ability of S. mutans to avoid host immune functions in systemic infections [95]. However, expression of these genes is not constitutive, and expression under different conditions varies between strains [95–97].…”

“…Increased expression of virulence functions was observed in strains associated with cariogenicity and systemic diseases (e.g. bacteraemia and infectious endocarditis) compared with strains isolated under healthy conditions [95,97]. Therefore, it is necessary to define the TCS regulatory circuits in S. mutans , as well as in other streptococcal species in oral biofilms in order to understand the environmental signals that trigger expression of virulence phenotypes.…”

“…GtfB and GtfC are not expressed by commensal S. sanguinis or S. gordonii , although these species express Gtfs that synthesize different types of glucan [125]. The expression of glucan-binding proteins allows S. mutans to bind to glucan in biofilms, a mode of growth that affords evasion of complement [95], an important host-defence system present in saliva, GCF, and blood [126,127]. In S. mutans , VicR Sm negatively regulates pepO and SMU _ 399 , which encode proteases implicated  in evasion of complement system [118].…”

“…In EMSA assays, it was observed that VicR Sm VicR and CovR co-bind the promoter regions of lysM , gbpB , and gtfC [107]. CovR is also a regulator of functions required for evasion to host immunity [95], and VicR Sm /CovR Sm co-regulatd genes include those required for evasion of complement and opsonophagocytosis, i.e. pepO and smu.399 [118,143].…”

“…CovR is also a regulator of functions required for evasion to host immunity [95], and VicR Sm /CovR Sm co-regulatd genes include those required for evasion of complement and opsonophagocytosis, i.e. pepO and smu.399 [118,143]. These findings indicate that VicR/CovR cooperate to more precisely coordinate functions of cell division with biofilm formation and evasion of host immunity, two essential functions for S. mutans persistence in oral niches.…”