Streptococcus mutans is a commensal of the human oral microbiome that can promote dental caries under conditions of dysbiosis. This study investigates metalloregulators and their involvement in the S. mutans oxidative stress response. Oxidative stress in the human mouth can derive from temporal increases in reactive oxygen species (ROS) after meal consumption and from endogenous bacterial ROS-producers that colonize the dentition. We hypothesize that the S. mutans PerR (SMU.593) and SloR (SMU.186) metalloregulatory proteins contribute to the regulation of oxidative stress genes and their products. Expression assays with S. mutans UA159 wild type cultures exposed to H2O2 reveal that H2O2 upregulates perR, and that PerR represses sloR transcription upon binding directly to Fur and PerR consensus sequences within the sloR operator. In addition, the results of Western blot experiments implicate the Clp proteolytic system in SloR degradation under conditions of H2O2-stress. To reveal a potential role for SloR in the H2O2-resistant phenotype of S. mutans GMS802 (a perR-deficient strain), we generated a sloR/perR double knockout mutant, GMS1386, where we observed upregulation of the tpx and dpr antioxidant genes. These results are consistent with GMS802 H2O2 resistance and with a role for PerR as a transcriptional repressor. Cumulatively, these findings support a reciprocal relationship between PerR and SloR during the S. mutans oxidative stress response and begin to elucidate the fitness strategies that evolved to foster S. mutans persistence in the transient environments of the human oral cavity. IMPORTANCE In 2020, untreated dental caries, especially in the permanent dentition, ranked among the most prevalent infectious diseases worldwide, disproportionately impacting individuals of low socioeconomic status. Untreated caries can lead to systemic health problems and has been associated with extended school and work absences, inappropriate use of emergency departments, and an inability for military forces to deploy. Together with public health policy, research aimed at alleviating S. mutans-induced tooth decay is important because it can improve oral health (and overall health), especially in underserved populations. This research, focused on S. mutans metalloregulatory proteins and their gene targets, is significant because it can promote virulence gene control in an important oral pathogen, and contribute to the development of an anti-caries therapeutic that can reduce tooth decay.

Streptococcus mutans is a commensal of the human oral microbiome that can instigate dental caries under conditions of dysbiosis. This study investigates S. mutans metalloregulators and their involvement in mediating a response to oxidative stress. Oxidative stress in the oral cavity can derive from temporal increases in reactive oxygen species (ROS) after meal consumption, and from endogenous bacterial ROS-producers that colonize the dentition as constituents of dental plaque. We hypothesize that the PerR (SMU.593) and SloR (SMU.186) metalloregulatory proteins in S. mutans contribute to oxidative stress tolerance by regulating the expression of genes responsive to H2O2 challenge. The results of qRT-PCR experiments with S. mutans cultures exposed to 0.5mM H2O2 reveal perR transcription that is responsive to the peroxide stressor, and sloR transcription that is subject to PerR repression. The results of gel shift assays support direct binding of a PerR homolog to the S. mutans sloR promoter at Fur and PerR consensus sequences on the UA159 chromosome. In addition, transcription of the S. mutans tpx and dpr antioxidant genes is upregulated in a perR/sloR double knockout mutant, consistent with heightened resistance of the S. mutans GMS802 perR-deficient strain when challenged with H2O2. Cumulatively, these results reveal a relationship of reciprocity between the PerR and SloR metalloregulators during the S. mutans response to oxidative stress and begin to elucidate the fitness strategies that evolved to foster S. mutans survival and persistence in the transient environments of the human oral cavity.IMPORTANCEIn 2020, untreated dental caries, especially in the permanent dentition, ranked among the most prevalent infectious diseases worldwide. Moreover, caries disproportionately affects children and individuals of low socioeconomic status. Untreated caries can lead to systemic health problems and has been associated with extended school and work absences, inappropriate use of emergency departments, and an inability for military forces to deploy. In combination with public health policy, research aimed at alleviating S. mutans-induced tooth decay is important because it can improve oral health, as well as overall health, especially for underserved populations. This research is focused on the S. mutans SloR and PerR metalloregulatory proteins that can help inform the development of therapeutics aimed at alleviating and potentially preventing dental caries.