Characterization of LrgAB as a stationary phase-specific pyruvate uptake system in Streptococcus mutans

Our recent '-omics' comparisons of Streptococcus mutans wild-type and lrgAB-mutant revealed that this organism undergoes dynamic cellular changes in the face of multiple exogenous stresses, consequently affecting its comprehensive virulence traits. In this current study, we further demonstrate that LrgAB functions as a S. mutans pyruvate uptake system.

S. mutans excretes pyruvate during growth as an overflow metabolite, and appears to uptake this excreted pyruvate via LrgAB once the primary carbon source is exhausted. This utilization of excreted pyruvate was tightly regulated by glucose levels and stationary growth phase lrgAB induction. The degree of lrgAB induction was reduced by high extracellular levels of pyruvate, suggesting that lrgAB induction is subject to negative feedback regulation, likely through the LytST TCS, which is required for expression of lrgAB. Stationary phase lrgAB induction was efficiently inhibited by low concentrations of 3FP, a toxic pyruvate analogue, without affecting cell growth, suggesting that accumulated pyruvate is sensed either directly or indirectly by LytS, subsequently triggering lrgAB expression. S. mutans growth was inhibited by high concentrations of 3FP, implying that pyruvate uptake is necessary for S. mutans exponential phase growth and occurs in a Lrg-independent manner. Finally, we found that stationary phase lrgAB induction is modulated by hydrogen peroxide (H₂O₂) and by co-cultivation with H₂O₂-producing S. gordonii.

Pyruvate may provide S. mutans with an alternative carbon source under limited growth conditions, as well as serving as a buffer against exogenous oxidative stress_. Given the hypothesized role of LrgAB in cell death and lysis, these data also provide an important basis for how these processes are functionally and mechanically connected to key metabolic pathways such as pyruvate metabolism.