The bacterial quorum sensing molecule, 2-heptyl-3-hydroxy-4-quinolone (PQS), inhibits signal transduction mechanisms in brain tissue and is behaviorally active in mice.

Interkingdom communication between bacteria and host organisms is one of the most interesting research topics in biology. Quorum sensing molecules produced by Gram-negative bacteria, such as acylated homoserine lactones and quinolones, have been shown to interact with host cell receptors, stimulating innate immunity and bacterial clearance. To our knowledge, there is no evidence that these molecules influence CNS function. Here, we have found that low micromolar concentrations of the Pseudomonas aeruginosa quorum sensing autoinducer, 2-heptyl-3-hydroxy-4-quinolone (PQS), inhibited polyphosphoinositide hydrolysis in mouse brain slices, whereas four selected acylated homoserine lactones were inactive. PQS also inhibited forskolin-stimulated cAMP formation in brain slices. We therefore focused on PQS in our study. Biochemical effects of PQS were not mediated by the bitter taste receptors, T2R4 and T2R16. Interestingly, submicromolar concentrations of PQS could be detected in the serum and brain tissue of adult mice under normal conditions. Levels increased in five selected brain regions after single i.p. injection of PQS (10 mg/kg), peaked after 15 min, and returned back to normal between 1 and 4 h. Systemically administered PQS reduced spontaneous locomotor activity, increased the immobility time in the forced swim test, and largely attenuated motor response to the psychostimulant, methamphetamine. These findings offer the first demonstration that a quorum sensing molecule specifically produced by Pseudomonas aeruginosa is centrally active and influences cell signaling and behavior. Quorum sensing autoinducers might represent new interkingdom signaling molecules between ecological communities of commensal, symbiotic, and pathogenic microorganisms and the host CNS.

A novel bacterial l-arginine sensor controlling c-di-GMP levels in Pseudomonas aeruginosa.

Nutrients such as amino acids play key roles in shaping the metabolism of microorganisms in natural environments and in host-pathogen interactions. Beyond taking part to cellular metabolism and to protein synthesis, amino acids are also signaling molecules able to influence group behavior in microorganisms, such as biofilm formation. This lifestyle switch involves complex metabolic reprogramming controlled by local variation of the second messenger 3', 5'-cyclic diguanylic acid (c-di-GMP). The intracellular levels of this dinucleotide are finely tuned by the opposite activity of dedicated diguanylate cyclases (GGDEF signature) and phosphodiesterases (EAL and HD-GYP signatures), which are usually allosterically controlled by a plethora of environmental and metabolic clues. Among the genes putatively involved in controlling c-di-GMP levels in P. aeruginosa, we found that the multidomain transmembrane protein PA0575, bearing the tandem signature GGDEF-EAL, is an l-arginine sensor able to hydrolyse c-di-GMP. Here, we investigate the basis of arginine recognition by integrating bioinformatics, molecular biophysics and microbiology. Although the role of nutrients such as l-arginine in controlling the cellular fate in P. aeruginosa (including biofilm, pathogenicity and virulence) is already well established, we identified the first l-arginine sensor able to link environment sensing, c-di-GMP signaling and biofilm formation in this bacterium.

Novel genetic tools to tackle c-di-GMP-dependent signalling in Pseudomonas aeruginosa.

AIMS: To develop new genetic tools for studying 3',5'-cyclic diguanylic acid (c-di-GMP) signalling in Pseudomonas aeruginosa. METHODS AND RESULTS: Plasmid pPcdrA::lux, carrying a transcriptional fusion between the c-di-GMP responsive promoter PcdrA and the luxCDABE reporter genes, has been generated and validated in purpose-built P. aeruginosa strains in which c-di-GMP levels can be increased or reduced upon arabinose-dependent induction of c-di-GMP synthetizing or degrading enzymes. CONCLUSIONS: The reporter systems described so far were able to detect a decrease in the c-di-GMP levels only in engineered strains overproducing c-di-GMP. Conversely, pPcdrA::lux could be used for studying any process or chemical compound expected to cause both an increase or a decrease with respect to the c-di-GMP levels produced by wild type P. aeruginosa. Another relevant aspect of this study has been the development of novel and improved genetic devices for the fine arabinose-dependent control of c-di-GMP levels in P. aeruginosa. SIGNIFICANCE AND IMPACT OF THE STUDY: The genetic tools developed and validated in this study could facilitate investigations tackling the c-di-GMP signalling process on different fields, from cellular physiology to drug-discovery research.