Understanding the role of potassium and its transport systems in Streptococcus mutans

Streptococcus mutans is a cariogenic bacterium found in the human dental plaque biofilm. Amongst its various attributes, the ability to metabolize a variety of dietary sugars to produce acid, resistance to pH stress by activating an acid tolerance response, ability to produce sticky glucan polymers and the ability to naturally take up genetic material are important properties of S. mutans. One way bacteria withstand external environmental stress is by accumulating intracellular potassium. In the genome of S. mutans, four putative potassium transport systems were identified, namely, the Trk1 (trkB/trk/pacL) system, Trk2 (trkA/trkH ) system, GlnQHMP, and Kch (SMU_1848). The hypothesis of this study was that S. mutans transports potassium using specific transport systems, and relies on K+ homeostasis for regulation of its multiple physiological and virulence attributes. To investigate the role of these systems, strains with deletions of each system were constructed. The most profound effects on the growth, pH tolerance, genetic transformation and biofilm formation were observed when the Trk2 system was absent compared to wild type UA159 and trk1-, glnQHMP- or kch deficient strains. Furthermore, deletion of the Trk2 transport system resulted in reduced intracellular potassium accumulation. To confirm the role of Trk2 as a potassium uptake system, this system was cloned, expressed and functionally characterized in the Escherichia coli TK2420 mutant strain lacking constitutive potassium uptake systems. Collectively, this study identified Trk2 as the key potassium transport system in S. mutans, and demonstrated its importance and the role of potassium homeostasis with regard to biofilm formation, stress tolerance, growth and genetic transformation. This work constitutes the first characterization of the Trk systems in oral streptococci.