Characterization of Streptococcus mutans SMU.787 Locus

Streptococcus mutans, the primary causative agent of dental caries, contains two paralogous genes that are annotated as cell envelope transcriptional regulators, brpA and SMU.787.  Recent studies have shown that BrpA plays a critical role in regulation of the cell envelope biogenesis and homeostasis and affects the ability of S. mutans to survive environmental stresses and accumulate biofilms, traits critical to cariogenicity of this bacterium.  Objectives: The objective of this study is to characterize the role of SMU.787 in S. mutans stress tolerance responses and biofilm formation. Methods:   PCR-Ligation-Mutagenesis was used to delete and replace the coding sequence of SMU.787 with a non-polar kanamycin resistance cassette. Mutants deficient of SMU.787 were isolated and further analyzed using functional assays, including acid killing, hydrogen peroxide challenge, and biofilm formation. Discussions and Results: Results showed that deficiency of SMU.787 does not have any major impact on cell morphology of the deficient mutant, TW263 and its growth rate in BHI.  When subjected to hydrogen peroxide challenge, however, TW263 was 10-fold more susceptible to hydrogen peroxide than the parent strain, UA159.  A slight decrease in survivability of TW263 to acid killing at pH 2.8 was also observed.  However, no major differences were identified in susceptibility against several cell envelope active antimicrobials.  As expected, complementation using a wild-type copy of the coding sequence plus its cognate promoter region in multi-copy plasmid restored the phenotypes to those of UA159.  Preliminary results also suggest that SMU.787-deficiency affects biofilm formation in S. mutans.  When stained with fluorescence-conjugated ConA, TW263 biofilms contained significantly less glucans than those of UA159.  Conclusion: These results suggest that SMU.787 plays a role in S. mutans stress tolerance responses and biofilms formation.  Currently, effort is being made to create and characterize the brpA/SMU.787­ double mutant.