Abstract
Metabolic homeostasis in cooperative bacteria is achieved by modulating primary metabolism in a quorum sensing (QS)-dependent manner. A perturbed metabolism in QS mutants causes physiological stress in the rice bacterial pathogen Burkholderia glumae. Here, we show that increased bacterial osmolality in B. glumae is caused by unusually high cellular concentrations of glutamate and betaine generated by QS deficiencies. QS negatively controls glutamate uptake and the expression of genes involved in the glutamine synthetase and glutamine oxoglutarate aminotransferase cycles. Thus, cellular glutamate levels were significantly higher in the QS mutants than in the wild type, and they caused hyperosmotic cellular conditions. Under the hypotonic conditions of the periplasm in the QS mutants, outer membrane bulging and vesiculation were observed, although these changes were rescued by knocking out the gltI gene, which encodes a glutamate transporter. Outer membrane modifications were not detected in the wild type. These results suggest that QS-dependent glutamate metabolism is critical for homeostatic osmolality. We suggest that outer membrane bulging and vesiculation might be the outcome of a physiological adaptation to relieve hypotonic osmotic stress in QS mutants. Our findings reveal how QS functions to maintain bacterial osmolality in a cooperative population.
Highlights
Is bifunctional because it represses sets of genes involved in primary metabolism, including glucose uptake, substrate and oxidative phosphorylation, and de novo nucleotide biosynthesis[13,14]
Based on previous results showing that glutamate levels in the tofI mutant BGS2 and the qsmR mutant BGS9 were higher than that in the wild-type BGR1, we assessed the glutamate uptake ability by feeding L-glutamate-3-[13C] in LB broth to each of these strains. 13C-labeled glutamate was transported at significantly higher levels in the quorum sensing (QS) mutants than in the wild type as determined by [13C]-nuclear magnetic resonance (NMR) spectra (Fig. 1a; Supplementary Fig. S1)
In addition to differential uptake rates of glutamate between the wild type and the QS mutants, we determined whether the expression of genes involved in nitrogen metabolism is controlled by QS because glutamate levels can be influenced by the glutamine synthetase (GS) and glutamine oxoglutarate aminotransferase (GOGAT) cycles
Summary
Is bifunctional because it represses sets of genes involved in primary metabolism, including glucose uptake, substrate and oxidative phosphorylation, and de novo nucleotide biosynthesis[13,14]. Our previous metabolome analyses showed that glutamate, glutamine, and betaine were accumulated at significantly higher levels in QS mutants than in the wild type[13] We found outer membrane bulging and vesiculation in the QS mutants but not in the wild type of B. glumae. We examined the molecular basis underlying imbalances in glutamate and glutamine levels in QS mutants and determined whether QS-mediated metabolic homeostasis is important for the maintenance of bacterial osmolality. We investigated whether such osmotic stress conditions could represent the cause of outer membrane bulging and vesiculation observed in QS mutants. We found that bacterial osmolality strongly corresponds to metabolic disorders present in B. glumae QS mutants and showed that outer membrane vesiculation occurs under high osmolality resulting from QS deficiency
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