Characterization of a Glutamate Transporter Operon, GlnQMPH, in Streptococcus mutans

Background:  In bacteria, intracellular glutamate and glutamine levels are closely linked with nitrogen metabolism of the cell.  Notably, nitrogen metabolism, especially glutamine metabolism, has been strongly linked to virulence in a number of bacterial species. In S. mutans, a chief etiological agent involved in dental caries, the uptake of, and factors participating in, the glutamate/ glutamine metabolic pathway are largely unknown. Objective: To investigate the role of the glnQMHP operon, a putative glutamate transporter, in S. mutans. Methods: A deletion mutant defective in the Gln operon (SmuGLT) was created using PCR ligation mutagenesis and assayed for its ability to transport glutamate, form biofilms and grow under a variety of environmental stressors (e.g. SDS, EtOH, pH).  Results: Uptake of radio-labeled glutamate by S. mutans UA159 wild-type (Wt) and SmuGLT were quantified.  The average maximal rate of glutamate uptake was significantly reduced by over 10-fold in the transporter deficient SmuGLT strain compared to Wt (p<0.001).  Biofilm assays conducted with SmuGLT cells in glucose or sucrose-supplemented medium showed no difference in biofilm biomass relative to the Wt strain.  However, biofilms of both strains formed in the presence of 10mM glutamate showed significantly more biofilm biomass compared with biofilms grown in non-supplemented medium (p<0.001).  Furthermore, analysis of growth in pH 5.5 medium demonstrated that growth of SmuGLT was severely compromised relative to that of the parent strain.  Conclusions:  Findings of this study provide insight into the nitrogen metabolism system of S. mutans, and suggest a possible link between nitrogen metabolism, biofilm formation and resistance to acidic growth conditions.  Acknowledgements: NIH Grant R01DE013230 and CIHR grant MT-15431