Additive attenuation of virulence and cariogenic potential of Streptococcus mutans by simultaneous inactivation of the ComCDE quorum-sensing system and HK/RR11 two-component regulatory system

The genome of Streptococcus mutans harbours 13 two-component signal transduction systems (TCSTSs). Of these, a peptide-mediated quorum-sensing system, ComCDE, and the HK/RR11 two-component system are well known to regulate several virulence-associated traits in in vitro experiments, including genetic competence, bacteriocin production, biofilm formation and stress responses. In this study, we investigated the hypothesis that inactivation of ComCDE, HK/RR11 or both systems would attenuate the virulence and cariogenicity of S. mutans. The results showed that simultaneous inactivation of both signal transduction systems additively attenuated S. mutans virulence and cariogenicity, since inactivation of either of these systems alone did not result in the same degree of effect. The double deletion mutant SMcde-hk11 was defective in genetic competence, had a reduced acid production, was unable to grow at pH 5.0 and formed an abnormal biofilm with reduced biomass. Animal studies showed that this mutant had reduced capabilities for oral colonization, succession and initiation of dental caries. A competitive index (CI) analysis using a mixed-infection animal model revealed that all the mutants, particularly SMcde-hk11, had reduced fitness in their ecological niches and were unable to compete with the wild-type strain for persistence in dental biofilms. The evidence from this study suggests that the ComCDE and HK/RR11 signal transduction systems can be considered to be novel targets for the development of strategies in the prevention and treatment of S. mutans infections.