Welcome to the Krell laboratory.

The Krell laboratory is part of the research group “Environmental Microbiology and Biodegradation”. The laboratory is located at the Estación Experimental del Zaidín in Granada (Spain) which is part of the Spanish National Research Council (CSIC).

Two systems, based on direct and indirect ligand recognition at chemoreceptors, mediate chemotaxis to inorganic phosphate in Pseudomonas aeruginosa

Rico-Jiménez et al. (2016) Scientific Reports 6:28967.

Inorganic phosphate (Pi) is a central signaling molecule that modulates virulence in various pathogens. In Pseudomonas aeruginosa, low Pi concentrations induce transcriptional alterations that increase virulence. P. aeruginosa can move chemotactically in Pi gradients, a process which forcibly alters virulence related gene expression.

 Chemotaxis is mediated by the two non-paralogous chemoreceptors CtpH and CtpL. Whereas CtpH mediates taxis to high Pi concentrations, CtpL is responsible for the taxis to low Pi concentrations. Interestingly, both receptors differ in the type of ligand binding domain (LBD) since CtpH has a 4-helix bundle LBD and CtpL a helical bimodular (HBM) LBD.

 In Rico Jiménez et al. we show that CtpH and CtpL function is based on different mechanisms. Isothermal titration calorimetry experiments demonstrate that CtpH recognizes Pi directly (below).

In contrast, CtpL does not recognize Pi directly. Pull-down experiments have led to the identification of the periplasmic binding protein (PBP) PstS as CtpL ligand. ITC studies showed that PstS binds Pi with the very high affinity of 7 nM and Analytical Ultracentrifugation showed a 1 : 1 PstS/CtpL-LBD stoichiometry (below). PstS is a soluble periplasmic protein that as part of the Pi transporter binds to the membrane bound PstA and PstC transporter subunits.

Quantitative capillary chemotaxis assays revealed that the deletion of the pstS and ctpH genes abolished Pi chemotaxis indicating that PstS is the only Pi shuttle for CtpL. Taken together, the model shown below is proposed for Pi chemotaxis and transport. PstS has thus a double function since it delivers Pi to the transporter and the chemotaxis receptor. The expression of pstS is tightly controlled by Pi, which permits a coordination of Pi transport and chemotaxis.

Chemoreceptors that are stimulated by PBPs have been so far reported in enterobacteria for sugars and dipeptides. The identification of a PBP dependent chemoreceptor in a different bacterial order and for a different type of ligand indicates that such systems are widespread in nature.

Identification of a plant compound that modulates Pseudomonas aeruginosa quorum sensing

Corral-Lugo et al. Science Signaling (2016) 9(409):ra1

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Bacteria communicate with each other via quorum sensing (QS) mechanisms. For many pathogens it has been shown that QS regulates the expression of virulence related genes. It is also known that plants produce and secrete compounds that interfere with pathogen QS signaling, which is considered to be a defense mechanism. However, little is known about the identity of these plant compounds. In Corral-Lugo et al. Science Signaling (2016) 9(409):ra1 (direct link) we identify rosmarinic acid (RA) as a plant compound that interferes with P. aeruginosa PAO1 QS. We show that RA binds tightly to the QS regulator RhlR and in silico docking experiments indicate that RA competes with the cognate C-4-homoserine lactone (C4-HSL) for binding.

Homology model of the RhlR effector binding domain containing RA (red) and C4-HSL (green) in their best-fit positions as determined by in silico docking experiments.

In vitro transcription assays demonstrate that RA has a higher capacity to stimulate RhlR-mediated transcription than the cognate C4-HSL.

In vitro transcription from the hcnABC promoter in the presence of RhlR as well as C4-HSL, RA and chlorogenic acid (compound structurally similar to RA that did not bind to RhlR).

Gene expression studies show that RA enhances the expression from different promoters that were previously shown to be controlled by RhlR. The addition of RA to P. aeruginosa cultures caused a number of typical QS-mediated and virulence-related phenotypes such as increases in pyocyanin and elastase production as well as a stimulation of biofilm formation.

Left: Biofilm formation in the presence of RA and chlorogenic acid. Right: Pyocyanin production (green colour) in the absence and presence of different RA concentrations.

It has been shown previously that P. aeruginosa infection causes RA secretion from plant roots and we hypothesize that the secretion of the QS agonist RA causes premature gene expression. Further work will show to what extent such premature expression modulates the efficiency of plant infection.

Selected publications

Matilla, M.A., Krell, T. (2017) The effect of bacterial chemotaxis on host infection and pathogenicity. FEMS Microbiol Rev. doi: 10.1093/femsre/fux052.

Ortega, Á., Zhulin, I.B., Krell, T. (2017) Sensory Repertoire of Bacterial Chemoreceptors. Microbiol Mol Biol Rev. doi: 10.1128/MMBR.00033-17.

Rico-Jiménez M, Reyes-Darias JA, Ortega Á, Díez Peña AI, Morel B, Krell T. (2016) Two different mechanisms mediate chemotaxis to inorganic phosphate in Pseudomonas aeruginosa. Scientific Reports 6:28967.

Corral-Lugo A, Daddaoua A, Ortega A, Espinosa-Urgel M, Krell T. (2016) Rosmarinic acid is a homoserine lactone mimic produced by plants that activates a bacterial quorum-sensing regulator. Science Signaling 9(409):ra1.

Martín-Mora D, Reyes-Darias JA, Ortega A, Corral-Lugo A, Matilla MA, Krell T. (2016) McpQ is a specific citrate chemoreceptor that responds preferentially to citrate/metal ion complexes. Environ Microbiol.  18:3284-3295.

Fernández M, Morel B, Corral-Lugo A, Krell T. (2016) Identification of a chemoreceptor that specifically mediates chemotaxis toward metabolizable purine derivatives. Mol Microbiol. 99:34-42.

Krell T. (2015) Tackling the bottleneck in bacterial signal transduction research: high-throughput identification of signal molecules. Mol. Microbiol. 96: 685-8.

Reyes-Darias JA, García V, Rico-Jiménez M, Corral-Lugo A, Lesouhaitier O, Juárez-Hernández D, Yang Y, Bi S, Feuilloley M, Muñoz-Rojas J, Sourjik V, Krell T. (2015). Specific gamma-aminobutyrate (GABA) chemotaxis in Pseudomonads with different lifestyle. Mol. Microbiol. 97:488-501.

Reyes-Darias JA, Yang Y, Sourjik V, Krell T. (2015) Correlation between signal input and output in PctA and PctB amino acid chemoreceptor of Pseudomonas aeruginosa. Mol. Microbiol. 2015. 96, 513-25.

García Fontana, C., Corral-Lugo, A., Krell, T. (2014) Specificity of the CheR2 Methyltransferase in Pseudomonas aeruginosa is Directed by C-Terminal Pentapeptides in Chemoreceptors. Science Signaling 7 (320) ra34.

Rico-Jiménez M., Muñoz-Martínez F., García-Fontana C., Fernandez M., Morel B., Ortega A., Ramos J.L., Krell T. (2013) Paralogous chemoreceptors mediate chemotaxis towards protein amino acids and the non-protein amino acid gamma-aminobutyrate (GABA). Mol. Microbiol. 88 (6): 1230-1243.

Garcia-Fontana C., Reyes-Darias J.A., Munoz-Martinez F., Alfonso C., Morel B., Ramos J.L., Krell T. (2013) High specificity in CheR methyltransferase function: CheR2 of Pseudomonas putida is essential for chemotaxis whereas CheR1 is involved in biofilm formation. J. Biol. Chem. 288 (26):18987-99.

Pineda-Molina, E., Reyes-Darias, J.A., Lacal, J., Ramos, J.L., García-Ruiz, J.M., Gavira, J.A., Krell, T. (2012) Evidence for chemoreceptors with bimodular ligand binding regions harboring two signal-binding sites. Proc. Acad. Natl. Sci. USA. 109, 18926-18931.