The fitness burden imposed by synthesising quorum sensing signals.

It is now well established that bacterial populations utilize cell-to-cell signaling (quorum-sensing, QS) to control the production of public goods and other co-operative behaviours. Evolutionary theory predicts that both the cost of signal production and the response to signals should incur fitness costs for producing cells. Although costs imposed by the downstream consequences of QS have been shown, the cost of QS signal molecule (QSSM) production and its impact on fitness has not been examined. We measured the fitness cost to cells of synthesising QSSMs by quantifying metabolite levels in the presence of QSSM synthases. We found that: (i) bacteria making certain QSSMs have a growth defect that exerts an evolutionary cost, (ii) production of QSSMs negatively correlates with intracellular concentrations of QSSM precursors, (iii) the production of heterologous QSSMs negatively impacts the production of a native QSSM that shares common substrates, and (iv) supplementation with exogenously added metabolites partially rescued growth defects imposed by QSSM synthesis. These data identify the sources of the fitness costs incurred by QSSM producer cells, and indicate that there may be metabolic trade-offs associated with QS signaling that could exert selection on how signaling evolves.

Quantitative liquid chromatography-tandem mass spectrometry profiling of activated methyl cycle metabolites involved in LuxS-dependent quorum sensing in Escherichia coli.

A rapid, selective, and sensitive liquid chromatography-tandem mass spectrometry assay has been developed and validated for the simultaneous quantification of the metabolites and precursors of the activated methyl cycle, reported in preliminary form by Heurlier et al. (2009) [43]. Analytes were extracted from Escherichia coli MG1655 and chemically derivatized as N(O,S)-iso-butyloxycarbonyl iso-butyl esters using iso-butyl chloroformate in an aqueous iso-butanol/pyridine environment. S-Adenosylmethionine, S-adenosylhomocysteine, S-ribosylhomocysteine, homocysteine, methionine, cystathionine, cysteine, and homoserine were quantified by liquid chromatography-positive ion tandem electrospray ionization mass spectrometry. Internal standards were isotopically labeled [(13)CD(3)]methionine and S-adenosylcysteine. Linearity of the assay was established up to a concentration of 700 microg/g cell dry weight for each analyte. The validated assay was used to quantitatively profile the intracellular activated methyl cycle metabolites as a function of growth in E. coli MG1655 and its derivative Deltapfs and DeltaluxS mutants to determine the metabolic consequences of a disruption to the activated methyl cycle and, hence, LuxS-dependent quorum sensing.