Bacterial quorum sensing and metabolic slowing in a cooperative population

Abstract

Acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) controls the production of numerous intra- and extracellular products across many species of Proteobacteria. Although these cooperative activities are often costly at an individual level, they provide significant benefits to the group. Other potential roles for QS include the restriction of nutrient acquisition and maintenance of metabolic homeostasis of individual cells in a crowded but cooperative population. Under crowded conditions, QS may function to modulate and coordinate nutrient utilization and the homeostatic primary metabolism of individual cells. Here, we show that QS down-regulates glucose uptake, substrate level and oxidative phosphorylation, and de novo nucleotide biosynthesis via the activity of the QS-dependent transcriptional regulator QsmR (quorum sensing master regulator R) in the rice pathogen Burkholderia glumae. Systematic analysis of glucose uptake and core primary metabolite levels showed that QS deficiency perturbed nutrient acquisition, and energy and nucleotide metabolism, of individuals within the group. The QS mutants grew more rapidly than the wild type at the early exponential stage and outcompeted wild-type cells in coculture. Metabolic slowing of individuals in a QS-dependent manner indicates that QS acts as a metabolic brake on individuals when cells begin to mass, implying a mechanism by which AHL-mediated QS might have evolved to ensure homeostasis of the primary metabolism of individuals under crowded conditions.

Fig. 1.

Down-regulation of glucose uptake mediated by QS and outgrowth of QS mutants. (A) Expression of the phosphoenolpyruvate-protein phosphotransferase-encoding gene (ptsI) in B. glumae. The expression levels of ptsI were higher in the QS-null mutants than the wild type, as revealed by quantitative reverse transcription-PCR. (B) Total amounts of [¹³C] after feeding of d-glucose-1-[¹³C] in the wild type and QS mutants of B. glumae. The total cellular amounts of [¹³C] were higher in QS mutants than the wild type as assessed by integration of peaks generated by [¹³C]-NMR spectroscopy. Error bars represent the error ranges of triplicate experiments. (C) Difference of growth rate between the wild-type BGR1 and the QS mutants. The QS mutants grew more rapidly at the early exponential stage than did the wild type, and statistical analysis of the data are presented in SI Appendix, Fig. S4. (D) Competition assays between the tofR mutant BGS1 and the wild-type BGR1. The tofR mutant BGS1 outcompeted the wild-type strain in coculture.

Fig. 2.

QS-dependent changes in the levels of metabolites involved in central carbon metabolism. The levels of metabolites of the Entner–Doudoroff and pentose phosphate pathways were plotted on pathway maps. The x and y axes refer to two different incubation times and show metabolite concentrations (in pmol per 10⁹ cells), respectively. Red, blue, and black lines indicate wild-type BGR1, the tofI mutant BGS2, and the qsmR mutant BGS9, respectively. Blunt arrows in red indicate QsmR-mediated repression of target genes. Each value is an average of data from three independent experiments, and statistical analysis of the data are presented in SI Appendix, Fig. S5.

Fig. 3.

Repression of substrate-level and oxidative phosphorylation, and de novo nucleotide biosynthesis, by QsmR. (A) The expression levels of nine genes involved in substrate-level and oxidative phosphorylation, and de novo nucleotide biosynthesis, in the wild type and QS mutants of B. glumae. The expression levels were determined by quantitative reverse transcription-PCR. (B) The activities of pyruvate kinase and adenylate kinase were higher in the QS mutants than wild-type B. glumae. (C) Direct binding of QsmR to the promoter regions of the ptsI, atpE, and ndk genes. The electrophoretic mobility-shift assay confirmed direct control of ptsI, atpE, and ndk by QsmR. Error bars represent the error ranges of triplicate experiments.

Fig. 4.

Down-regulation of nucleotide metabolism by QsmR. The levels of intermediates of nucleotide metabolism were plotted on pathway maps. The x and y axes refer to two different incubation times and show metabolite concentrations (in pmol per 10⁹ cells), respectively. Red, blue, and black lines indicate wild-type BGR1, the tofI mutant BGS2, and the qsmR mutant BGS9, respectively. Blunt arrows in red indicate QsmR-mediated repression of target genes. Each value is an average of data from at least three independent experiments, and statistical analyses of the data are presented in SI Appendix, Fig. S5.