Genetic Characterization of Glyoxalase Pathway in Oral Streptococci and its Contribution to Interbacterial Competition

Abstract

Substantial quantities of Reactive Electrophile Species (RES), including methylglyoxal and glyoxal, are generated by microbes and humans. To understand the impact of RES on oral microbial homeostasis, genetic analyses were performed on the glyoxalase pathway in Streptococcus mutans (SMU) and Streptococcus sanguinis (SSA). Loss of glyoxalase I (LguL), which catalyzes the rate-limiting reaction in RES degradation, reduced methylglyoxal and glyoxal tolerance to a far greater extent in SMU than in SSA, decreasing the competitiveness of SMU over SSA in planktonic cultures. MICs showed an overall greater RES tolerance by SMU than SSA; a finding consistent with the ability of methylglyoxal to induce the expression of lguL in SMU, but not in SSA. Computational analysis identified a novel paralogue of LguL in most streptococci represented by SMU.1112c in SMU. {Delta}SMU.1112c showed a minor decrease in methylglyoxal tolerance under certain conditions, but a significant growth defect on fructose; a phenotype reversed by the deletion of a fructose-1-phosphate-generating sugar: phosphotransferase system or addition of glutathione (GSH) to the medium. Further, deletion of the glucose-PTS in SMU increased RES tolerance partly through enhanced expression of the pyruvate-dehydrogenase complex. Consistent with the requirement of GSH for methylglyoxal detoxification, deletion of glutathione synthetase (gshAB) in SMU significantly reduced RES resistance. This study reveals the critical roles of RES in fitness and interbacterial competition and the effects of PTS in modulating RES metabolism. The fact that RES may impact the pathogenic potential of the oral microbiome via differential effects on beneficial and pathogenic species warrants further investigation. ImportanceAs highly reactive byproducts of sugar metabolism, very little is known regarding the contribution of methylglyoxal or related aldehyde compounds to oral health. The need to better understand the influence of these reactive electrophile species (RES) to microbial physiology and ecology is made more urgent by the widespread condition of hyperglycemia in humans, which is associated with elevated RES levels. Our study showed a significantly greater ability of a major caries pathobiont, Streptococcus mutans, to tolerate methylglyoxal and glyoxal than many commensal oral streptococci. Genetic analysis of methylglyoxal degradation in the pathobiont and commensals identified significant differences in genetic structure and gene regulation patterns that could contribute to differential fitness by constituents of the dental microbiota and ecologic shift in the presence of RES.