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

Abstract

AbstractBackgroundOur recent ‘-omics’ comparisons ofStreptococcus mutanswild-type andlrgAB-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 aS. mutanspyruvate uptake system.ResultsS. mutansexcretes 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 phaselrgABinduction. The degree oflrgABinduction was reduced by high extracellular levels of pyruvate, suggesting thatlrgABinduction is subject to negative feedback regulation, likely through the LytST TCS, which is required for expression oflrgAB. Stationary phaselrgABinduction 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 triggeringlrgABexpression.S. mutansgrowth was inhibited by high concentrations of 3FP, implying that pyruvate uptake is necessary forS. mutansexponential phase growth and occurs in a Lrg-independent manner. Finally, we found that stationary phaselrgABinduction is modulated by hydrogen peroxide (H2O2) and by co-cultivation with H2O2-producingS. gordonii.ConclusionsPyruvate may provideS. mutanswith 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.