Structure of the autoinducer required for expression of Pseudomonas aeruginosa virulence genes

Genetic control of bacterial biofilms

Nearly all bacterial species, including pathogens, have the ability to form biofilms. Biofilms are defined as structured ecosystems in which microbes are attached to surfaces and embedded in a matrix composed of polysaccharides, eDNA, and proteins, and their development is a multistep process. Bacterial biofilms constitute a large medical problem due to their extremely high resistance to various types of therapeutics, including conventional antibiotics. Several environmental and genetic signals control every step of biofilm development and dispersal. From among the latter, quorum sensing, cyclic diguanosine-5'-monophosphate, and small RNAs are considered as the main regulators. The present review describes the control role of these three regulators in the life cycles of biofilms built by Pseudomonas aeruginosa, Staphylococcus aureus, Salmonella enterica serovar Typhimurium, and Vibrio cholerae. The interconnections between their activities are shown. Compounds and strategies which target the activity of these regulators, mainly quorum sensing inhibitors, and their potential role in therapy are also assessed.

A novel and widespread class of ketosynthase is responsible for the head-to-head condensation of two acyl moieties in bacterial pyrone biosynthesis

The biosynthesis of photopyrones, novel quorum sensing signals in Photorhabdus, has been studied by heterologous expression of the photopyrone synthase PpyS catalyzing the head-to-head condensation of two acyl moieties. The biochemical mechanism of pyrone formation has been investigated by amino acid exchange and bioinformatic analysis. Additionally, the evolutionary origin of PpyS has been studied by phylogenetic analyses also revealing homologous enzymes in Pseudomonas sp. GM30 responsible for the biosynthesis of pseudopyronines including a novel derivative. Moreover this novel class of ketosynthases is only distantly related to other pyrone-forming enzymes identified in the biosynthesis of the potent antibiotics myxopyronin and corallopyronin.

Quorum Sensing Protects Pseudomonas aeruginosa against Cheating by Other Species in a Laboratory Coculture Model

Many species of bacteria use a cell-cell communication system called quorum sensing (QS) to coordinate group activities. QS systems frequently regulate the production of exoproducts. Some of these products, such as proteases, are "public goods" that are shared among the population and vulnerable to cheating by nonproducing members of the population. Because the QS system of the opportunistic pathogen Pseudomonas aeruginosa regulates several public goods, it can serve as a model for studying cooperation. Bacteria also commonly regulate antimicrobial production through QS. In this study, we focused on the hypothesis that QS-regulated antimicrobials may be important for P. aeruginosa to protect against cheating by another bacterial species, Burkholderia multivorans. We assessed laboratory cocultures of P. aeruginosa and B. multivorans and investigated the importance of three P. aeruginosa QS-regulated antimicrobials, hydrogen cyanide, rhamnolipids, and phenazines, for competition. We found that P. aeruginosa dominates cocultures with B. multivorans and that the three antimicrobials together promote P. aeruginosa competitiveness, with hydrogen cyanide contributing the greatest effect. We show that these QS-regulated antimicrobials are also critical for P. aeruginosa to prevent B. multivorans from cheating under nutrient conditions where both species require a P. aeruginosa quorum-regulated protease for growth. Together our results highlight the importance of antimicrobials in protecting cooperating populations from exploitation by other species that can act as cheaters.

IMPORTANCE

Cooperative behaviors are threatened by social cheating, wherein individuals do not produce but nonetheless benefit from shared public goods. Bacteria have been shown to use several genetic mechanisms to restrain the emergence of cheaters from within the population, but public goods might also be used by other bacterial species in the vicinity. We demonstrate that a public good produced by Pseudomonas aeruginosa can be used by another species, Burkholderia multivorans, to obtain carbon and energy. We also show that P. aeruginosa antimicrobials that are coregulated with the public good prevent invasion by the cheating species. Our results demonstrate that cross-species cheating can occur and that coregulation of public goods with antimicrobials may stabilize cooperative behavior in mixed microbial communities.

Nitrite-Oxidizing Bacterium Nitrobacter winogradskyi Produces N-Acyl-Homoserine Lactone Autoinducers

Nitrobacter winogradskyi is a chemolithotrophic bacterium that plays a role in the nitrogen cycle by oxidizing nitrite to nitrate. Here, we demonstrate a functional N-acyl-homoserine lactone (acyl-HSL) synthase in this bacterium. The N. winogradskyi genome contains genes encoding a putative acyl-HSL autoinducer synthase (nwi0626, nwiI) and a putative acyl-HSL autoinducer receptor (nwi0627, nwiR) with amino acid sequences 38 to 78% identical to those in Rhodopseudomonas palustris and other Rhizobiales. Expression of nwiI and nwiR correlated with acyl-HSL production during culture. N. winogradskyi produces two distinct acyl-HSLs, N-decanoyl-l-homoserine lactone (C10-HSL) and a monounsaturated acyl-HSL (C10:1-HSL), in a cell-density- and growth phase-dependent manner, during batch and chemostat culture. The acyl-HSLs were detected by bioassay and identified by ultraperformance liquid chromatography with information-dependent acquisition mass spectrometry (UPLC-IDA-MS). The C=C bond in C10:1-HSL was confirmed by conversion into bromohydrin and detection by UPLC-IDA-MS.

Identification of sdiA-regulated genes in a mouse commensal strain of Enterobacter cloacae

Many bacteria determine their population density using quorum sensing. The most intensively studied mechanism of quorum sensing utilizes proteins of the LuxI family to synthesize a signaling molecule of the acylhomoserine lactone (AHL) type, and a protein of the LuxR family to bind AHL and regulate transcription. Genes regulated by quorum sensing often encode functions that are most effective when a group of bacteria are working cooperatively (e.g., luminescence, biofilm formation, host interactions). Bacteria in the Escherichia, Salmonella, Klebsiella, and Enterobacter genera do not encode an AHL synthase but they do encode an AHL receptor of the LuxR family, SdiA. Instead of detecting their own AHL synthesis, these organisms use SdiA to detect the AHLs synthesized by other bacterial species. In this study, we used a genetic screen to identify AHL-responsive genes in a commensal Enterobacter cloacae strain that was isolated from a laboratory mouse. The genes include a putative type VI secretion system, copA (a copper transporter), and fepE (extends O-antigen chain length). A new transposon mutagenesis strategy and suicide vectors were used to construct an sdiA mutant of E. cloacae. The AHL-responsiveness of all fusions was entirely sdiA-dependent, although some genes were regulated by sdiA in the absence of AHL.

Using surface enhanced Raman scattering to analyze the interactions of protein receptors with bacterial quorum sensing modulators

Many members of the LuxR family of quorum sensing (QS) transcriptional activators, including LasR of Pseudomonas aeruginosa, are believed to require appropriate acyl-homoserine lactone (acyl-HSL) ligands to fold into an active conformation. The failure to purify ligand-free LuxR homologues in nonaggregated form at the high concentrations required for their structural characterization has limited the understanding of the mechanisms by which QS receptors are activated. Surface-enhanced Raman scattering (SERS) is a vibrational spectroscopy technique that can be applied to study proteins at extremely low concentrations in their active state. The high sensitivity of SERS has allowed us to detect molecular interactions between the ligand-binding domain of LasR (LasRLBD) as a soluble apoprotein and modulators of P. aeruginosa QS. We found that QS activators and inhibitors produce differential SERS fingerprints in LasRLBD, and in combination with molecular docking analysis provide insight into the relevant interaction mechanism. This study reveals signal-specific structural changes in LasR upon ligand binding, thereby confirming the applicability of SERS to analyze ligand-induced conformational changes in proteins.

Immunization with 3-oxododecanoyl-L-homoserine lactone-r-PcrV conjugate enhances survival of mice against lethal burn infections caused by Pseudomonas aeruginosa

Quorum Sensing and type III secretion system play an important role in the virulence of Pseudomonas (P.) aeruginosa in burn wound infections. We aimed to explore the feasibility of using 3-oxo-C₁₂-HSL-r-PcrV conjugate as a candidate vaccine against P. aeruginosa caused infections. 3-oxo-C₁₂-HSL-r-PcrV conjugate was prepared and used for immunization of mice (10 μg, subcutaneous, three times, at 2-week intervals). Mice were divided into five groups: I: PcrV; II: 3-oxo-C₁₂-HSL-r-PcrV (10 μg); III: 3-oxo-C₁₂-HSL-r-PcrV (20 μg); IV: 3-oxo-C₁₂-HSL; and V: PBS receiving groups. After each shot of immunization, total and isotype antibody responses against corresponding antigen were measured to determine the immunization efficacy. One month after the last immunization, all groups were burned and challenged subeschar with P. aeruginosa PAO1. Survival rate and bacterial quantity in the skin and internal organs (liver and spleen) were evaluated 25-hr after burn infection. Immunization with 3-oxo-C₁₂-HSL-r-PcrV significantly increased total IgG and specific subclass antibodies (IgG₁, IgG₂a, IgG₂b, and IgM) in the serum of the groups II and III compared to the control group (p<0.001). While all the control mice (PBS injected group) died within 2 days after bacterial challenge, 64% of the group I, 78% of group II, and 86% of group III, survived within 14 days after challenge. Interestingly, bacterial burden in the liver and spleen of 3-oxo-C₁₂-HSL-r-PcrV injected group (III) was significantly lower than the control group (p<0.001). The present study proposed two-component vaccine to inhibit Pseudomonas infections in burned mouse.

Sub-MICs of Mentha piperita essential oil and menthol inhibits AHL mediated quorum sensing and biofilm of Gram-negative bacteria

Bacterial quorum sensing (QS) is a density dependent communication system that regulates the expression of certain genes including production of virulence factors in many pathogens. Bioactive plant extract/compounds inhibiting QS regulated gene expression may be a potential candidate as antipathogenic drug. In this study anti-QS activity of peppermint (Mentha piperita) oil was first tested using the Chromobacterium violaceum CVO26 biosensor. Further, the findings of the present investigation revealed that peppermint oil (PMO) at sub-Minimum Inhibitory Concentrations (sub-MICs) strongly interfered with acyl homoserine lactone (AHL) regulated virulence factors and biofilm formation in Pseudomonas aeruginosa and Aeromonas hydrophila. The result of molecular docking analysis attributed the QS inhibitory activity exhibited by PMO to menthol. Assessment of ability of menthol to interfere with QS systems of various Gram-negative pathogens comprising diverse AHL molecules revealed that it reduced the AHL dependent production of violacein, virulence factors, and biofilm formation indicating broad-spectrum anti-QS activity. Using two Escherichia coli biosensors, MG4/pKDT17 and pEAL08-2, we also confirmed that menthol inhibited both the las and pqs QS systems. Further, findings of the in vivo studies with menthol on nematode model Caenorhabditis elegans showed significantly enhanced survival of the nematode. Our data identified menthol as a novel broad spectrum QS inhibitor.

Emergence of polymyxin B resistance influences pathogenicity in Pseudomonas aeruginosa mutators

The interplay between polymyxin B pharmacodynamics and pathogenicity was examined in Pseudomonas aeruginosa PAO1 and isogenic DNA repair-deficient mutators (mutM and mutS strains). Against mutS mutators, polymyxin B initial killing was concentration dependent, with >99.9% bacterial reduction at 2 h followed by regrowth and resistance. The pre- versus postexposed strains were inoculated real time into Galleria mellonella waxworms, resulting in increased median survival times from 20 h to 23 h (P < 0.001). Emergence of resistance in mutS P. aeruginosa resulted in attenuation of virulence.

N-3-(oxododecanoyl)-L-homoserine lactone promotes the induction of regulatory T-cells by preventing human dendritic cell maturation

N-3-(Oxododecanoyl)-L-homoserine lactone (C12) is a small bacterial signaling molecule secreted by Pseudomonas aeruginosa (PA), which activates mammalian cells through TLR4-independent mechanisms. C12 acts as an immunosuppressant and it has been shown to modulate murine bone marrow-derived dendritic cell-mediated T-helper 2 (Th2) cell polarizations in vitro. In the present study, we initially examined the impact of C12 on the maturation of human monocyte-derived dendritic cells (Mo-DCs) and the induction of regulatory T-cells (iTregs) in culture. Our findings demonstrate that C12-treated Mo-DCs failed to undergo lipopolysaccharide (LPS)-induced maturation. At the molecular level, C12 blocked the upregulation of surface molecules, including CD11c, HLA-DR, CD40, and CD80, and it switched to an interleukin (IL)-10(high), IL-12p70(low) phenotype. Moreover, C12 selectively inhibited the capacity of Mo-DCs to stimulate the proliferation of allogeneic CD4(+) T-cells. Otherwise, the C12-treated Mo-DCs promoted the generation of CD4(+)CD25(+)Foxp3(+)-induced regulatory T-cells (iTregs) and enhanced their IL-10 and transforming growth factor (TGF)-β production associated with reduced interferon (IFN)-γ and IL-12p70 production. These findings provide new insights towards understanding the persistence of chronic inflammation in PA infection.

Post-transcriptional regulation of gene PA5507 controls Pseudomonas quinolone signal concentration in P. aeruginosa

Pseudomonas aeruginosa can sense and respond to a myriad of environmental signals and utilizes a system of small molecules to communicate through intercellular signaling. The small molecule 2-heptyl-3-hydroxy-4-quinolone (Pseudomonas Quinolone Signal [PQS]) is one of these signals and its synthesis is important for virulence. Previously, we identified an RpiR-type transcriptional regulator, QapR, that positively affects PQS production by repressing the qapR operon. An in-frame deletion of this regulator caused P. aeruginosa to produce a greatly reduced concentration of PQS. Here, we report that QapR translation is linked to the downstream gene PA5507. We found that introduction of a premature stop codon within qapR eliminates transcriptional autorepression of the qapR operon as expected but has no effect on PQS concentration. This was investigated with a series of lacZ reporter fusions which showed that translation of QapR must terminate at, or close to, the native qapR stop codon in order for translation of PA5507 to occur. Also, it was shown that truncation of the 5' end of the qapR transcript permitted PA5507 translation without translation of QapR. Our findings led us to conclude that PA5507 transcription and translation are both tightly controlled by QapR and this control is important for PQS homeostasis.

Investigational therapies targeting quorum-sensing for the treatment of Staphylococcus aureus infections

INTRODUCTION

Antibiotic resistance is a serious global health concern for developed and developing nations. MRSA represents a particularly severe public health threat that is associated with high morbidity and mortality. The lack of novel antibiotics has led scientists to explore therapies targeting bacterial virulence mechanisms and virulence regulators, including those controlling cell-cell communication.

AREAS COVERED

The authors discuss the role of quorum-sensing in Staphylococcus aureus infections and components of the system that are being targeted using novel investigational drugs. In particular, the authors examine the role of the accessory gene regulator (Agr) system in virulence regulation of S. aureus pathogenesis. Finally, the authors present and compare natural and synthetic compounds that have been found to interfere with Agr functionality.

EXPERT OPINION

There is a great need to develop new therapeutic methods to combat S. aureus infections. These include anti-virulence therapies that target key global regulators involved with the establishment and propagation of infection. Several molecules have been found to interfere with S. aureus virulence regulation, especially those targeting the Agr quorum-sensing signaling molecule. These preliminary findings warrant further investigation and validation, with the goal of refining a compound that has broad-spectrum inhibitory effects on most S. aureus strains and Agr subtypes.

Potent Irreversible Inhibitors of LasR Quorum Sensing in Pseudomonas aeruginosa

Antagonism of quorum sensing represents a promising new antivirulence approach for the treatment of bacterial infection. The development of a novel series of non-natural irreversible antagonists of P. aeruginosa LasR is described. The lead compounds identified (25 and 28) display potent LasR antagonist activity and inhibit expression of the P. aeruginosa virulence factors pyocyanin and biofilm formation in PAO1 and PA14.

Development of potent inhibitors of pyocyanin production in Pseudomonas aeruginosa

The development of new approaches for the treatment of antimicrobial-resistant infections is an urgent public health priority. The Pseudomonas aeruginosa pathogen, in particular, is a leading source of infection in hospital settings, with few available treatment options. In the context of an effort to develop antivirulence strategies to combat bacterial infection, we identified a series of highly effective small molecules that inhibit the production of pyocyanin, a redox-active virulence factor produced by P. aeruginosa. Interestingly, these new antagonists appear to suppress P. aeruginosa virulence factor production through a pathway that is independent of LasR and RhlR.

Quorum sensing and policing of Pseudomonas aeruginosa social cheaters

The bacterium Pseudomonas aeruginosa is an opportunistic human pathogen that uses a quorum sensing signal cascade to activate expression of dozens of genes when sufficient population densities have been reached. Quorum sensing controls production of several key virulence factors, including secreted proteases such as elastase. Cooperating groups of bacteria growing on protein are susceptible to social cheating by quorum-sensing defective mutants. A possible way to restrict cheater emergence is by policing where cooperators produce costly goods to sanction or punish cheats. The P. aeruginosa LasR-LasI quorum sensing system controls genes including those encoding proteases and also those encoding a second quorum-sensing system, the RhlR-RhlI system, which controls numerous genes including those for cyanide production. By using RhlR quorum sensing mutants and cyanide synthesis mutants, we show that cyanide production is costly and cyanide-producing cooperators use cyanide to punish LasR-null social cheaters. Cooperators are less susceptible to cyanide than are LasR mutants. These experiments demonstrate policing in P. aeruginosa, provide a mechanistic understanding of policing, and show policing involves the cascade organization of the two quorum sensing systems in this bacterium.

Functional amyloids keep quorum-sensing molecules in check

The mechanism by which extracellular metabolites, including redox mediators and quorum-sensing signaling molecules, traffic through the extracellular matrix of biofilms is poorly explored. We hypothesize that functional amyloids, abundant in natural biofilms and possessing hydrophobic domains, retain these metabolites. Using surface plasmon resonance, we demonstrate that the quorum-sensing (QS) molecules, 2-heptyl-3-hydroxy-4(1H)-quinolone and N-(3-oxododecanoyl)-l-homoserine lactone, and the redox mediator pyocyanin bind with transient affinity to functional amyloids from Pseudomonas (Fap). Their high hydrophobicity predisposes them to signal-amyloid interactions, but specific interactions also play a role. Transient interactions allow for rapid association and dissociation kinetics, which make the QS molecules bioavailable and at the same time secure within the extracellular matrix as a consequence of serial bindings. Retention of the QS molecules was confirmed using Pseudomonas aeruginosa PAO1-based 2-heptyl-3-hydroxy-4(1H)-quinolone and N-(3-oxododecanoyl)-l-homoserine lactone reporter assays, showing that Fap fibrils pretreated with the QS molecules activate the reporters even after sequential washes. Pyocyanin retention was validated by electrochemical analysis of pyocyanin-pretreated Fap fibrils subjected to the same washing process. Results suggest that QS molecule-amyloid interactions are probably important in the turbulent environments commonly encountered in natural habitats.

Sodium houttuyfonate affects production of N-acyl homoserine lactone and quorum sensing-regulated genes expression in Pseudomonas aeruginosa

Quorum sensing (QS) is a means of cell-to-cell communication that uses diffusible signaling molecules that are sensed by the population to determine population density, thus allowing co-ordinate gene regulation in response to population density. In Pseudomonas aeruginosa, production of the QS signaling molecule, N-acyl homoserine lactone (AHL), co-ordinates expression of key factors of pathogenesis, including biofilm formation and toxin secretion. It is predicted that the inhibition of AHL sensing would provide an effective clinical treatment to reduce the expression of virulence factors and increase the effectiveness of antimicrobial agents. We previously demonstrated that sodium houttuyfonate (SH), commonly used in traditional Chinese medicine to treat infectious diseases, can effectively inhibit QS-regulated processes, including biofilm formation. Here, using a model system, we demonstrate that SH causes the dose-dependent inhibition of AHL production, through down-regulation of the AHL biosynthesis gene, lasI. Addition of SH also resulted in down-regulation of expression of the AHL sensor and transcriptional regulator, LasR, and inhibited the production of the QS-regulated virulence factors, pyocyanin and LasA. These results suggest that the antimicrobial activity of SH may be due to its ability to disrupt QS in P. aeruginosa.

Surface attachment induces Pseudomonas aeruginosa virulence

Pseudomonas aeruginosa infects every type of host that has been examined by deploying multiple virulence factors. Previous studies of virulence regulation have largely focused on chemical cues, but P. aeruginosa may also respond to mechanical cues. Using a rapid imaging-based virulence assay, we demonstrate that P. aeruginosa activates virulence in response to attachment to a range of chemically distinct surfaces, suggesting that this bacterial species responds to mechanical properties of its substrates. Surface-activated virulence requires quorum sensing, but activating quorum sensing does not induce virulence without surface attachment. The activation of virulence by surfaces also requires the surface-exposed protein PilY1, which has a domain homologous to a eukaryotic mechanosensor. Specific mutation of the putative PilY1 mechanosensory domain is sufficient to induce virulence in non-surface-attached cells, suggesting that PilY1 mediates surface mechanotransduction. Triggering virulence only when cells are both at high density and attached to a surface—two host-nonspecific cues—explains how P. aeruginosa precisely regulates virulence while maintaining broad host specificity.

Sodium ascorbate as a quorum sensing inhibitor of Pseudomonas aeruginosa

AIMS

Quorum sensing circuits regulate virulence factors in Pseudomonas aeruginosa and coordinate bacterial pathogenicity. We are interested in exploring available medications for their antiquorum sensing activity.

METHODS AND RESULTS

First, we determined the MIC of ascorbate against Ps. aeruginosa strain PAO1, and all further experiments used concentrations below the MIC so that results could not be caused by reduced viability. Tests of subinhibitory concentrations of sodium ascorbate on cell signals were performed using a reporter strain assay. Sub-MICs of sodium ascorbate resulted in significant reduction of the signalling molecules C4-HSL and 3-oxo-C12-HSL (P < 0·01). The influence of sub-MIC of sodium ascorbate on virulence factors was also determined and ascorbate treatment led to significant depression of elastase, protease and haemolysin activities. In addition, inhibition of pyocyanin production, attenuation of biofilm formation and alteration of Pseudomonas motility was observed. Analysis by RT-PCR tested the effect of ascorbate on the expression of QS regulatory genes. Expression of QS regulatory genes, lasI, lasR, rhlI, rhlR, pqsR and pqsA, was repressed compared to untreated Ps. aeruginosa PAO1, confirming that ascorbate QS inhibition works on gene expression at the molecular level.

CONCLUSION

Sodium ascorbate, even at low concentrations, inhibited QS and related virulence factors of Ps. aeruginosa PAO1.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrated that sodium ascorbate could function as signal modulator and virulence inhibitor in Ps. aeruginosa.

An evolving perspective on the Pseudomonas aeruginosa orphan quorum sensing regulator QscR

Many Proteobacteria govern responses to changes in cell density by using acyl-homoserine lactone (AHL) quorum-sensing (QS) signaling. Similar to the LuxI-LuxR system described in Vibrio fischeri, a minimal AHL QS circuit comprises a pair of genes, a luxI-type synthase gene encoding an enzyme that synthesizes an AHL and a luxR-type AHL-responsive transcription regulator gene. In most bacteria that utilize AHL QS, cognate luxI and luxR homologs are found in proximity to each other on the chromosome. However, a number of recent reports have identified luxR homologs that are not linked to luxI homologs; in some cases luxR homologs have been identified in bacteria that have no luxI homologs. A luxR homolog without a linked luxI homologs is termed an orphan or solo. One of the first reports of an orphan was on QscR in Pseudomonas aeruginosa. The qscR gene was revealed by whole genome sequencing and has been studied in some detail. P. aeruginosa encodes two AHL synthases and three AHL responsive receptors, LasI-LasR form a cognate synthase-receptor pair as do RhlI-RhlR. QscR lacks a linked synthase and responds to the LasI-generated AHL. QS regulation of gene expression in P. aeruginosa employs multiple signals and occurs in the context of other interconnected regulatory circuits that control diverse physiological functions. QscR affects virulence of P. aeruginosa, and although it shows sensitivity to the LasI-generated AHL, 3-oxo-dodecanoylhomoserine lactone, it's specificity is relaxed compared to LasR and can respond equally well to several AHLs. QscR controls a set of genes that overlaps the set regulated by LasR. QscR is comparatively easy to purify and study in vitro, and has become a model for understanding the biochemistry of LuxR homologs. In fact there is a crystal structure of QscR bound to the LasI-generated AHL. Here, we review the current state of research concerning QscR and highlight recent advances in our understanding of its structure and biochemistry.

Serum influences the expression of Pseudomonas aeruginosa quorum-sensing genes and QS-controlled virulence genes during early and late stages of growth

In response to diverse environmental stimuli at different infection sites, Pseudomonas aeruginosa, a serious nosocomial pathogen, coordinates the production of different virulence factors through a complicated network of the hierarchical quorum-sensing (QS) systems including the las, rhl, and the 2-alkyl-4-quinolone-related QS systems. We recently showed that at early stages of growth serum alters the expression of numerous P. aeruginosa genes. In this study, we utilized transcriptional analysis and enzyme assays to examine the effect of serum on the QS and QS-controlled virulence factors during early and late phases of growth of the P. aeruginosa strain PAO1. At early phase, serum repressed the transcription of lasI, rhlI, and pqsA but not lasR or rhlR. However, at late phase, serum enhanced the expression of all QS genes. Serum produced a similar effect on the synthesis of the autoinducers 3OC₁₂-HSL, C₄-HSL, and HHQ/PQS. Additionally, serum repressed the expression of several QS-controlled genes in the early phase, but enhanced them in the late phase. Furthermore, serum influenced the expression of different QS-positive (vqsR, gacA, and vfr) as well as QS-negative (rpoN, qscR, mvaT, and rsmA) regulatory genes at either early or late phases of growth. However, with the exception of PAOΔvfr, we detected comparable levels of lasI/lasR expression in PAO1 and PAO1 mutants defective in these regulatory genes. At late stationary phase, serum failed to enhance lasI/lasR expression in PAOΔvfr. These results suggest that depending on the phase of growth, serum differentially influenced the expression of P. aeruginosa QS and QS-controlled virulence genes. In late phase, serum enhanced the expression of las genes through vfr.

A Sinorhizobium meliloti-specific N-acyl homoserine lactone quorum-sensing signal increases nodule numbers in Medicago truncatula independent of autoregulation

N-acyl homoserine lactones (AHLs) act as quorum sensing signals that regulate cell-density dependent behaviors in many gram-negative bacteria, in particular those important for plant-microbe interactions. AHLs can also be recognized by plants, and this may influence their interactions with bacteria. Here we tested whether the exposure to AHLs affects the nodule-forming symbiosis between legume hosts and rhizobia. We treated roots of the model legume, Medicago truncatula, with a range of AHLs either from its specific symbiont, Sinorhizobium meliloti, or from the potential pathogens, Pseudomonas aeruginosa and Agrobacterium vitis. We found increased numbers of nodules formed on root systems treated with the S. meliloti-specific AHL, 3-oxo-C14-homoserine lactone, at a concentration of 1 μM, while the other AHLs did not result in significant changes to nodule numbers. We did not find any evidence for altered nodule invasion by the rhizobia. Quantification of flavonoids that could act as nod gene inducers in S. meliloti did not show any correlation with increased nodule numbers. The effects of AHLs were specific for an increase in nodule numbers, but not lateral root numbers or root length. Increased nodule numbers following 3-oxo-C14-homoserine lactone treatment were under control of autoregulation of nodulation and were still observed in the autoregulation mutant, sunn4 (super numeric nodules4). However, increases in nodule numbers by 3-oxo-C14-homoserine lactone were not found in the ethylene-insensitive sickle mutant. A comparison between M. truncatula with M. sativa (alfalfa) and Trifolium repens (white clover) showed that the observed effects of AHLs on nodule numbers were specific to M. truncatula, despite M. sativa nodulating with the same symbiont. We conclude that plant perception of the S. meliloti-specific 3-oxo-C14-homoserine lactone influences nodule numbers in M. truncatula via an ethylene-dependent, but autoregulation-independent mechanism.

The hierarchy quorum sensing network in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes severe and persistent infections in immune compromised individuals and cystic fibrosis sufferers. The infection is hard to eradicate as P. aeruginosa has developed strong resistance to most conventional antibiotics. The problem is further compounded by the ability of the pathogen to form biofilm matrix, which provides bacterial cells a protected environment withstanding various stresses including antibiotics. Quorum sensing (QS), a cell density-based intercellular communication system, which plays a key role in regulation of the bacterial virulence and biofilm formation, could be a promising target for developing new strategies against P. aeruginosa infection. The QS network of P. aeruginosa is organized in a multi-layered hierarchy consisting of at least four interconnected signaling mechanisms. Evidence is accumulating that the QS regulatory network not only responds to bacterial population changes but also could react to environmental stress cues. This plasticity should be taken into consideration during exploration and development of anti-QS therapeutics.

DJ-1 interacts with and regulates paraoxonase-2, an enzyme critical for neuronal survival in response to oxidative stress

Loss-of-function mutations in DJ-1 (PARK7) gene account for about 1% of all familial Parkinson's disease (PD). While its physiological function(s) are not completely clear, DJ-1 protects neurons against oxidative stress in both in vitro and in vivo models of PD. The molecular mechanism(s) through which DJ-1 alleviates oxidative stress-mediated damage remains elusive. In this study, we identified Paraoxonase-2 (PON2) as an interacting target of DJ-1. PON2 activity is elevated in response to oxidative stress and DJ-1 is crucial for this response. Importantly, we showed that PON2 deficiency hypersensitizes neurons to oxidative stress induced by MPP⁺ (1-methyl-4-phenylpyridinium). Conversely, over-expression of PON2 protects neurons in this death paradigm. Interestingly, PON2 effectively rescues DJ-1 deficiency-mediated hypersensitivity to oxidative stress. Taken together, our data suggest a model by which DJ-1 exerts its antioxidant activities, at least partly through regulation of PON2.

Aspirin is an efficient inhibitor of quorum sensing, virulence and toxins in Pseudomonas aeruginosa

Quorum sensing (QS) plays a vital role in regulation of virulence factors and toxins in Pseudomonas aeruginosa, which can cause serious human infections. Therefore, the QS system in P. aeruginosa may be an important target for pharmacological intervention. Activity of aspirin on the QS system was assessed using a reporter strain assay and confirmed using RT-PCR to test expression of virulence factors and toxins. In addition, molecular modeling techniques including docking, flexible alignment and surface mapping were also applied to further understand aspirin's potential QS inhibition activity. Aspirin (6 mg/ml) showed significant reduction (p < 0.01) of quorum sensing signals in P. aeruginosa, including expression of elastase, total proteases, and pyocyanin (p < 0.01) without affecting bacterial viability. Aspirin also significantly reduced organism motility and biofilm production (p < 0.01) and decreased expression of lasI, lasR, rhlI, rhlR, pqsA and pqsR genes by 38, 72, 69, 72, 74 and 43% respectively. Moreover, the expression of Pseudomonas toxins exoS and exoY was reduced by 47 and 55% respectively. The molecular modeling analysis suggests the QS inhibitory action of aspirin occurs through interaction of aspirin's aryl group and Tyr-88 of the LasR receptor, by strong π-π stacking interactions, which associated with a conformational change of the receptor-aspirin complex. The inhibitory effect of aspirin on virulence factors was specific to P. aeruginosa as aspirin at sub-MIC did not affect the biofilm or motility of Escherichia coli. To summarize, the collective data demonstrate that low concentrations of aspirin inhibit quorum sensing of P. aeruginosa.

N-acyl homoserine lactone-mediated quorum sensing with special reference to use of quorum quenching bacteria in membrane biofouling control

Membrane biofouling remains a severe problem to be addressed in wastewater treatment systems affecting reactor performance and economy. The finding that many wastewater bacteria rely on N-acyl homoserine lactone-mediated quorum sensing to synchronize their activities essential for biofilm formations; the quenching bacterial quorum sensing suggests a promising approach for control of membrane biofouling. A variety of quorum quenching compounds of both synthetic and natural origin have been identified and found effective in inhibition of membrane biofouling with much less environmental impact than traditional antimicrobials. Work over the past few years has demonstrated that enzymatic quorum quenching mechanisms are widely conserved in several prokaryotic organisms and can be utilized as a potent tool for inhibition of membrane biofouling. Such naturally occurring bacterial quorum quenching mechanisms also play important roles in microbe-microbe interactions and have been used to develop sustainable nonantibiotic antifouling strategies. Advances in membrane fabrication and bacteria entrapment techniques have allowed the implication of such quorum quenching bacteria for better design of membrane bioreactor with improved antibiofouling efficacies. In view of this, the present paper is designed to review and discuss the recent developments in control of membrane biofouling with special emphasis on quorum quenching bacteria that are applied in membrane bioreactors.

Biomolecular Mechanisms of Pseudomonas aeruginosa and Escherichia coli Biofilm Formation

Pseudomonas aeruginosa and Escherichia coli are the most prevalent Gram-negative biofilm forming medical device associated pathogens, particularly with respect to catheter associated urinary tract infections. In a similar manner to Gram-positive bacteria, Gram-negative biofilm formation is fundamentally determined by a series of steps outlined more fully in this review, namely adhesion, cellular aggregation, and the production of an extracellular polymeric matrix. More specifically this review will explore the biosynthesis and role of pili and flagella in Gram-negative adhesion and accumulation on surfaces in Pseudomonas aeruginosa and Escherichia coli. The process of biofilm maturation is compared and contrasted in both species, namely the production of the exopolysaccharides via the polysaccharide synthesis locus (Psl), pellicle Formation (Pel) and alginic acid synthesis in Pseudomonas aeruginosa, and UDP-4-amino-4-deoxy-l-arabinose and colonic acid synthesis in Escherichia coli. An emphasis is placed on the importance of the LuxR homologue sdiA; the luxS/autoinducer-II; an autoinducer-III/epinephrine/norepinephrine and indole mediated Quorum sensing systems in enabling Gram-negative bacteria to adapt to their environments. The majority of Gram-negative biofilms consist of polysaccharides of a simple sugar structure (either homo- or heteropolysaccharides) that provide an optimum environment for the survival and maturation of bacteria, allowing them to display increased resistance to antibiotics and predation.

Molecular mechanisms of master regulator VqsM mediating quorum-sensing and antibiotic resistance in Pseudomonas aeruginosa

The Pseudomonas aeruginosa quorum-sensing (QS) systems contribute to bacterial homeostasis and pathogenicity. Although the AraC-family transcription factor VqsM has been characterized to control the production of virulence factors and QS signaling molecules, its detailed regulatory mechanisms still remain elusive. Here, we report that VqsM directly binds to the lasI promoter region, and thus regulates its expression. To identify additional targets of VqsM in P. aeruginosa PAO1, we performed chromatin immunoprecipitation (ChIP) followed by high-throughput DNA sequencing (ChIP-seq) and detected 48 enriched loci harboring VqsM-binding peaks in the P. aeruginosa genome. The direct regulation of these genes by VqsM has been confirmed by electrophoretic mobility shift assays and quantitative real-time polymerase chain reactions. A VqsM-binding motif was identified by using the MEME suite and verified by footprint assays in vitro. In addition, VqsM directly bound to the promoter regions of the antibiotic resistance regulator NfxB and the master type III secretion system (T3SS) regulator ExsA. Notably, the vqsM mutant displayed more resistance to two types of antibiotics and promoted bacterial survival in a mouse model, compared to wild-type PAO1. Collectively, this work provides new cues to better understand the detailed regulatory networks of QS systems, T3SS, and antibiotic resistance.

Pseudomonas aeruginosa biofilm growth inhibition on medical plastic materials by immobilized esterases and acylase

Biofilms are matrix-encapsulated cell aggregates that cause problems in technical and health-related areas; for example, 65 % of all human infections are biofilm associated. This is mainly due to their ameliorated resistance against antimicrobials and immune systems. Pseudomonas aeruginosa, a biofilm-forming organism, is commonly responsible for nosocomial infections. Biofilm development is partly mediated by signal molecules, such as acyl-homoserine lactones (AHLs) in Gram-negative bacteria. We applied horse liver esterase, porcine kidney acylase, and porcine liver esterase; these can hydrolyze AHLs, thereby inhibiting biofilm formation. As biofilm infections are often related to foreign material introduced into the human body, we immobilized the enzymes on medical plastic materials. Biofilm formation was quantified by Crystal Violet staining and confocal laser scanning microscopy, revealing up to 97 % (on silicone), 54 % (on polyvinyl chloride), and 77 % (on polyurethane) reduced biomass after 68 h growth.

Detection of quorum sensing activity in the multidrug-resistant clinical isolate Pseudomonas aeruginosa strain GB11

A multidrug-resistant clinical bacteria strain GB11 was isolated from a wound swab on the leg of a patient. Identity of stain GB11 as Pseudomonas aeruginosa was validated by using matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS). Detection of the production of signaling molecules, N-acylhomoserine lactones (AHLs), was conducted using three different bacterial biosensors. A total of four different AHLs were found to be produced by strain GB11, namely N-butyryl homoserine lactone (C4-HSL), N-hexanoylhomoserine lactone (C6-HSL), N-octanoyl homoserine lactone (C8-HSL) and N-3-oxo-dodecanoylhomoserine lactone (3-oxo-C12-HSL) using high resolution liquid chromatography tandem mass spectrometry (LC-MS/MS). Of these detected AHLs, 3-oxo-C12-HSL was found to be the most abundant AHL produced by P. aeruginosa GB11.

Correlation between group behavior and quorum sensing in Pseudomonas aeruginosa isolated from patients with hospital-acquired pneumonia

BACKGROUND

This study investigated the correlation between the expression of the Las and Rhl quorum-sensing (QS) systems and the communal behavior (motility, biofilm formation, and pyocyanin production) of Pseudomonas aeruginosa (P. aeruginosa) isolated from patients with hospital-acquired pneumonia.

METHODS

We analyzed 138 P. aeruginosa isolates from 48 patients (30 men and 18 women; age 68.18±15.08 years). P. aeruginosa clinical isolates were assessed for Las and Rhl gene expression and bacterial motility, biofilm formation, and pyocyanin production.

RESULTS

P. aeruginosa swimming, twitching, and swarming motility positively correlated with the expression of LasI, LasR, and RhlI (P<0.05) but not with that of RhlR (P>0.05). At all analyzed time points, a significant positive correlation was found between biofilm formation and the expression of LasI, LasR (P<0.01), and RhlI (P<0.05 for day 1, P<0.01 for days 7 and 14), whereas RhlR expression positively correlated with biofilm formation only on day 14 (P<0.05). On days 1 and 7, positive correlation was observed between pyocyanin production and the levels of LasI and RhlI (P<0.05). In bacterial clearance cases, the expression of QS-related genes and the group behavior of the pathogen did not correlate (P>0.05). However, in cases of persistent P. aeruginosa infection, the changes in LasI and LasR gene expression were positively correlated with those in bacterial motility (P<0.05), and the changes in LasI, LasR, RhlI, and RhlR expression showed a significant positive association with those in biofilm formation (P<0.01).

CONCLUSIONS

In patients with hospital-acquired pneumonia, the expression of the Las and Rhl QS genes was associated with bacterial motility, biofilm formation, and pyocyanin production, suggesting an involvement of the QS genes in the clearance of pathogenic P. aeruginosa in patients.

Pseudomonas aeruginosa biofilms: mechanisms of immune evasion

The opportunistic gram-negative bacterium Pseudomonas aeruginosa is implicated in many chronic infections and is readily isolated from chronic wounds, medical devices, and the lungs of cystic fibrosis patients. P. aeruginosa is believed to persist in the host organism due to its capacity to form biofilms, which protect the aggregated, biopolymer-embedded bacteria from the detrimental actions of antibiotic treatments and host immunity. A key component in the protection against innate immunity is rhamnolipid, which is a quorum sensing (QS)-regulated virulence factor. QS is a cell-to-cell signaling mechanism used to coordinate expression of virulence and protection of aggregated biofilm cells. Rhamnolipids are known for their ability to cause hemolysis and have been shown to cause lysis of several cellular components of the human immune system, for example, macrophages and polymorphonuclear leukocytes (PMNs). In this chapter, the interplay between P. aeruginosa and the PMNs in chronic infections is discussed with focus on the role of rhamnolipids and extracellular DNA.

A conserved suppressor mutation in a tryptophan auxotroph results in dysregulation of Pseudomonas quinolone signal synthesis

Pseudomonas aeruginosa is a common nosocomial pathogen that relies on three cell-to-cell signals to regulate multiple virulence factors. The Pseudomonas quinolone signal (PQS; 2-heptyl-3-hydroxy-4-quinolone) is one of these signals, and it is known to be important for P. aeruginosa pathogenesis. PQS is synthesized in a multistep reaction that condenses anthranilate and a fatty acid. In P. aeruginosa, anthranilate is produced via the kynurenine pathway and two separate anthranilate synthases, TrpEG and PhnAB, the latter of which is important for PQS synthesis. Others have previously shown that a P. aeruginosa tryptophan auxotroph could grow on tryptophan-depleted medium with a frequency of 10(-5) to 10(-6). These revertants produced more pyocyanin and had increased levels of phnA transcript. In this study, we constructed similar tryptophan auxotroph revertants and found that the reversion resulted from a synonymous G-to-A nucleotide mutation within pqsC. This change resulted in increased pyocyanin and decreased PQS, along with an increase in the level of the pqsD, pqsE, and phnAB transcripts. Reporter fusion and reverse transcriptase PCR studies indicated that a novel transcript containing pqsD, pqsE, and phnAB occurs in these revertants, and quantitative real-time PCR experiments suggested that the same transcript appears in the wild-type strain under nutrient-limiting conditions. These results imply that the PQS biosynthetic operon can produce an internal transcript that increases anthranilate production and greatly elevates the expression of the PQS signal response protein PqsE. This suggests a novel mechanism to ensure the production of both anthranilate and PQS-controlled virulence factors.

Quorum sensing activity of Hafnia alvei isolated from packed food

Quorum sensing (QS) is a mechanism adopted by bacteria to regulate expression of genes according to population density. N-acylhomoserine lactones (AHLs) are a type of QS signalling molecules commonly found in Gram-negative bacteria which have been reported to play a role in microbial spoilage of foods and pathogenesis. In this study, we isolated an AHL-producing Hafnia alvei strain (FB1) from spherical fish pastes. Analysis via high resolution triple quadrupole liquid chromatography/mass spectrometry (LC/MS) on extracts from the spent supernatant of H. alvei FB1 revealed the existence of two short chain AHLs: N-(3-oxohexanoyl) homoserine lactone (3-oxo-C6-HSL) and N-(3-oxo- octanoyl) homoserine lactone (3-oxo-C8-HSL). To our knowledge, this is the first report of the production of AHLs, especially 3-oxo-C8-HSL, by H. alvei.

Pseudomonas aeruginosa quorum-sensing molecule N-(3-oxododecanoyl) homoserine lactone attenuates lipopolysaccharide-induced inflammation by activating the unfolded protein response

N-3-oxododecanoyl homoserine lactone (3-oxo-C12-HSL), a quorum-sensing signal molecule produced by Pseudomonas aeruginosa (P. aeruginosa), is involved in the expression of bacterial virulence factors and in the modulation of host immune responses by directly disrupting nuclear factor-κB (NF-κB) signaling and inducing cell apoptosis. The unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress may suppress inflammatory responses in the later phase by blocking NF-κB activation. It was recently demonstrated that 3-oxo-C12-HSL may induce UPR in human aortic endothelial cells (HAECs). Therefore, 3-oxo-C12-HSL may also inhibit NF-κB activation and suppress inflammatory responses by activating UPR. However, the possible underlying mechanism has not been fully elucidated. Accordingly, we investigated the effects of 3-oxo-C12-HSL on cellular viability, UPR activation, lipopolysaccharide (LPS)-induced NF-κB activation and inflammatory response in the RAW264.7 mouse macrophage cell line. Treatment with 6.25 μM 3-oxo-C12-HSL was not found to affect the viability of RAW264.7 cells. However, pretreating RAW264.7 cells with 6.25 μM 3-oxo-C12-HSL effectively triggered UPR and increased the expression of UPR target genes, such as CCAAT/enhancer-binding protein β (C/EBP β) and CCAAT/enhancer-binding protein-homologous protein (CHOP). The expression of C/EBP β and CHOP was found to be inversely correlated with LPS-induced NF-κB activation. 3-Oxo-C12-HSL pretreatment was also shown to inhibit LPS-stimulated proinflammatory cytokine production. Hence, 3-oxo-C12-HSL may attenuate LPS-induced inflammation via UPR-mediated NF-κB inhibition without affecting cell viability. This may be another mechanism through which P. aeruginosa evades the host immune system and maintains a persistent infection.

Bicyclic brominated furanones: a new class of quorum sensing modulators that inhibit bacterial biofilm formation

Both natural and synthetic brominated furanones are known to inhibit biofilm formation by bacteria, but their toxicity to mammalian cells is often not reported. Here, we designed and synthesized a new class of brominated furanones (BBFs) that contained a bicyclic structure having one bromide group with well-defined regiochemistry. This class of molecules exhibited reduction in the toxicity to mammalian cells (human neuroblastoma SK-N-SH) and did not inhibit bacteria (Pseudomonas aeruginosa and Escherichia coli) growth, but retained the inhibitory activity towards biofilm formation of bacteria. In addition, all the BBFs inhibited the production of virulence factor elastase B in P. aeruginosa. To explore the effect of BBFs on quorum sensing, we used a reporter gene assay and found that 6-BBF and 7-BBF exhibited antagonistic activities for LasR protein in the lasI quorum sensing circuit, while 5-BBF showed agonistic activity for the rhlI quorum sensing circuit. This study suggests that structural variation of brominated furanones can be designed for targeted functions to control biofilm formation.

Metabolite transfer with the fermentation product 2,3-butanediol enhances virulence by Pseudomonas aeruginosa

The respiratory tract of cystic fibrosis (CF) patients harbor persistent microbial communities (CF airway microbiome) with Pseudomonas aeruginosa emerging as a dominant pathogen. Within a polymicrobial infection, interactions between co-habitant microbes can be important for pathogenesis, but even when considered, these interactions are not well understood. Here, we show with in vitro experiments that, compared with glucose, common fermentation products from co-habitant bacteria significantly increase virulence factor production, antimicrobial activity and biofilm formation of P. aeruginosa. The maximum stimulating effect was produced with the fermentation product 2,3-butanediol, which is a substrate for P. aeruginosa, resulting in a metabolic relationship between fermenters and this pathogen. The global transcription regulator LasI LasR, which controls quorum sensing, was upregulated threefold with 2,3-butanediol, resulting in higher phenazine and exotoxin concentrations and improved biofilm formation. This indicates that the success of P. aeruginosa in CF airway microbiomes could be governed by the location within the food web with fermenting bacteria. Our findings suggest that interbacterial metabolite transfer in polymicrobial infections stimulates virulence of P. aeruginosa and could have a considerable impact on disease progression.

Physiological framework for the regulation of quorum sensing-dependent public goods in Pseudomonas aeruginosa

Many bacteria possess cell density-dependent quorum-sensing (QS) systems that often regulate cooperative secretions involved in host-microbe or microbe-microbe interactions. These secretions, or "public goods," are frequently coregulated by stress and starvation responses. Here we provide a physiological rationale for such regulatory complexity in the opportunistic pathogen Pseudomonas aeruginosa. Using minimal-medium batch and chemostat cultures, we comprehensively characterized specific growth rate-limiting macronutrients as key triggers for the expression of extracellular enzymes and metabolites directly controlled by the las and rhl QS systems. Expression was unrelated to cell density, depended on the secreted product's elemental composition, and was induced only when the limiting nutrient was not also a building block of the product; rhl-dependent products showed the strongest response, caused by the largely las-independent induction of the regulator RhlR and its cognate signal. In agreement with the prominent role of the rhl system, slow growth inverted the las-to-rhl signal ratio, previously considered a characteristic distinguishing between planktonic and biofilm lifestyles. Our results highlight a supply-driven, metabolically prudent regulation of public goods that minimizes production costs and thereby helps stabilize cooperative behavior. Such regulation would be beneficial for QS-dependent public goods that act broadly and nonspecifically, and whose need cannot always be accurately assessed by the producing cell. Clear differences in the capacities of the las and rhl systems to integrate starvation signals help explain the existence of multiple QS systems in one cell.

Bacteria clustering by polymers induces the expression of quorum-sensing-controlled phenotypes

Bacteria deploy a range of chemistries to regulate their behaviour and respond to their environment. Quorum sensing is one method by which bacteria use chemical reactions to modulate pre-infection behaviour such as surface attachment. Polymers that can interfere with bacterial adhesion or the chemical reactions used for quorum sensing are therefore a potential means to control bacterial population responses. Here, we report how polymeric 'bacteria sequestrants', designed to bind to bacteria through electrostatic interactions and therefore inhibit bacterial adhesion to surfaces, induce the expression of quorum-sensing-controlled phenotypes as a consequence of cell clustering. A combination of polymer and analytical chemistry, biological assays and computational modelling has been used to characterize the feedback between bacteria clustering and quorum sensing signalling. We have also derived design principles and chemical strategies for controlling bacterial behaviour at the population level.

A quorum-sensing inhibitor blocks Pseudomonas aeruginosa virulence and biofilm formation

Quorum sensing is a chemical communication process that bacteria use to regulate collective behaviors. Disabling quorum-sensing circuits with small molecules has been proposed as a potential strategy to prevent bacterial pathogenicity. The human pathogen Pseudomonas aeruginosa uses quorum sensing to control virulence and biofilm formation. Here, we analyze synthetic molecules for inhibition of the two P. aeruginosa quorum-sensing receptors, LasR and RhlR. Our most effective compound, meta-bromo-thiolactone (mBTL), inhibits both the production of the virulence factor pyocyanin and biofilm formation. mBTL also protects Caenorhabditis elegans and human lung epithelial cells from killing by P. aeruginosa. Both LasR and RhlR are partially inhibited by mBTL in vivo and in vitro; however, RhlR, not LasR, is the relevant in vivo target. More potent antagonists do not exhibit superior function in impeding virulence. Because LasR and RhlR reciprocally control crucial virulence factors, appropriately tuning rather than completely inhibiting their activities appears to hold the key to blocking pathogenesis in vivo.

Biochar and microbial signaling: production conditions determine effects on microbial communication

Charcoal has a long soil residence time, which has resulted in its production and use as a carbon sequestration technique (biochar). A range of biological effects can be triggered by soil biochar that can positively and negatively influence carbon storage, such as changing the decomposition rate of organic matter and altering plant biomass production. Sorption of cellular signals has been hypothesized to underlie some of these effects, but it remains unknown whether the binding of biochemical signals occurs, and if so, on time scales relevant to microbial growth and communication. We examined biochar sorption of N-3-oxo-dodecanoyl-L-homoserine lactone, an acyl-homoserine lactone (AHL) intercellular signaling molecule used by many gram-negative soil microbes to regulate gene expression. We show that wood biochars disrupt communication within a growing multicellular system that is made up of sender cells that synthesize AHL and receiver cells that express green fluorescent protein in response to an AHL signal. However, biochar inhibition of AHL-mediated cell-cell communication varied, with the biochar prepared at 700 °C (surface area of 301 m(2)/g) inhibiting cellular communication 10-fold more than an equivalent mass of biochar prepared at 300 °C (surface area of 3 m(2)/g). These findings provide the first direct evidence that biochars elicit a range of effects on gene expression dependent on intercellular signaling, implicating the method of biochar preparation as a parameter that could be tuned to regulate microbial-dependent soil processes, like nitrogen fixation and pest attack of root crops.

A new class of quorum quenching molecules from Staphylococcus species affects communication and growth of gram-negative bacteria

The knowledge that many pathogens rely on cell-to-cell communication mechanisms known as quorum sensing, opens a new disease control strategy: quorum quenching. Here we report on one of the rare examples where Gram-positive bacteria, the ‘Staphylococcus intermedius group’ of zoonotic pathogens, excrete two compounds in millimolar concentrations that suppress the quorum sensing signaling and inhibit the growth of a broad spectrum of Gram-negative beta- and gamma-proteobacteria. These compounds were isolated from Staphylococcus delphini. They represent a new class of quorum quenchers with the chemical formula N-[2-(1H-indol-3-yl)ethyl]-urea and N-(2-phenethyl)-urea, which we named yayurea A and B, respectively. In vitro studies with the N-acyl homoserine lactone (AHL) responding receptor LuxN of V. harveyi indicated that both compounds caused opposite effects on phosphorylation to those caused by AHL. This explains the quorum quenching activity. Staphylococcal strains producing yayurea A and B clearly benefit from an increased competitiveness in a mixed community.

While studying the potential interaction of staphylococci with Gram-negative bacteria, we came across another communication system in a Staphylococcus species group, which consists of closely related coagulase-positive bacterial species that play a role as zoonotic pathogens. We found that these species excrete two small compounds that inhibit both the expression of QS-controlled toxins and other QS-regulated compounds as well as growth in Gram-negative bacteria. The excreted compounds, which we named yayurea A and B, were isolated from S. delphini and structurally characterized. They represent new bacterial products, which quench the QS regulation in a wide spectrum of Gram-negative bacteria by stimulating the LuxN-mediated phosphorylation of LuxU. Furthermore, growth of yayurea A and B producing S. delphini is not suppressed by respiratory toxins when co-cultured with P. aeruginosa. This suggests that the quorum quenchers have a function in self-protection and competitiveness in natural environments shared with Gram-negatives. Here we show one of the rare cases of inter-phylum interference between firmicutes (Gram-positive) and beta-/gammaproteobacteria (Gram-negative).