Quorum sensing in bacterial species that use degenerate autoinducers can be tuned by using structurally identical non-native ligands

Disrupting quorum sensing as a strategy to inhibit bacterial virulence in human, animal, and plant pathogens

The development of sustainable alternatives to conventional antimicrobials is needed to address bacterial virulence while avoiding selecting resistant strains in a variety of fields, including human, animal, and plant health. Quorum sensing (QS), a bacterial communication system involved in noxious bacterial phenotypes such as virulence, motility, and biofilm formation, is of utmost interest. In this study, we harnessed the potential of the lactonase SsoPox to disrupt QS of human, fish, and plant pathogens. Lactonase treatment significantly alters phenotypes including biofilm formation, motility, and infection capacity. In plant pathogens, SsoPox decreased the production of plant cell wall degrading enzymes in Pectobacterium carotovorum and reduced the maceration of onions infected by Burkholderia glumae. In human pathogens, lactonase treatment significantly reduced biofilm formation in Acinetobacter baumannii, Burkholderia cepacia, and Pseudomonas aeruginosa, with the cytotoxicity of the latter being reduced by SsoPox treatment. In fish pathogens, lactonase treatment inhibited biofilm formation and bioluminescence in Vibrio harveyi and affected QS regulation in Aeromonas salmonicida. QS inhibition can thus be used to largely impact the virulence of bacterial pathogens and would constitute a global and sustainable approach for public, crop, and livestock health in line with the expectations of the One Health initiative.

Quorum Sensing Regulation as a Target for Antimicrobial Therapy

Some bacterial species use a cell-to-cell communication mechanism called Quorum Sensing (QS). Bacteria release small diffusible molecules, usually termed signals which allow the activation of beneficial phenotypes that guarantee bacterial survival and the expression of a diversity of virulence genes in response to an increase in population density. The study of the molecular mechanisms that relate signal molecules with bacterial pathogenesis is an area of growing interest due to its use as a possible therapeutic alternative through the development of synthetic analogues of autoinducers as a strategy to regulate bacterial communication as well as the study of bacterial resistance phenomena, the study of these relationships is based on the structural diversity of natural or synthetic autoinducers and their ability to inhibit bacterial QS, which can be approached with a molecular perspective from the following topics: i) Molecular signals and their role in QS regulation; ii) Strategies in the modulation of Quorum Sensing; iii) Analysis of Bacterial QS circuit regulation strategies; iv) Structural evolution of natural and synthetic autoinducers as QS regulators. This mini-review allows a molecular view of the QS systems, showing a perspective on the importance of the molecular diversity of autoinducer analogs as a strategy for the design of new antimicrobial agents.

β-Cyclodextrin Encapsulation of Synthetic AHLs: Drug Delivery Implications and Quorum-Quenching Exploits

Many bacteria, such as Pseudomonas aeruginosa, regulate phenotypic switching in a population density-dependent manner through a phenomenon known as quorum sensing (QS). For Gram-negative bacteria, QS relies on the synthesis, transmission, and perception of low-molecular-weight signal molecules that are predominantly N-acyl-l-homoserine lactones (AHLs). Efforts to disrupt AHL-mediated QS have largely focused on the development of synthetic AHL analogues (SAHLAs) that are structurally similar to native AHLs. However, like AHLs, these molecules tend to be hydrophobic and are poorly soluble under aqueous conditions. Water-soluble macrocycles, such as cyclodextrins (CDs), that encapsulate hydrophobic guests have long been used by both the agricultural and pharmaceutical industries to overcome the solubility issues associated with hydrophobic compounds of interest. Conveniently, CDs have also demonstrated anti-AHL-mediated QS effects. Here, using fluorescence spectroscopy, NMR spectrometry, and mass spectrometry, we evaluate the affinity of SAHLAs, as well as their hydrolysis products, for β-CD inclusion. We also evaluated the ability of these complexes to inhibit wild-type P. aeruginosa virulence in a Caenorhabditis elegans host infection study, for the first time. Our efforts confirm the potential of β-CDs for the improved delivery of SAHLAs at the host/microbial interface, expanding the utility of this approach as a strategy for probing and controlling QS.

Chemical Control of Quorum Sensing in E. coli: Identification of Small Molecule Modulators of SdiA and Mechanistic Characterization of a Covalent Inhibitor

Enterohemorrhagic Escherichia coli (EHEC) is the causative agent of severe diarrheal disease in humans. Cattle are the natural reservoir of EHEC, and approximately 75% of EHEC infections in humans stem from bovine products. Many common bacterial pathogens, including EHEC, rely on chemical communication systems, such as quorum sensing (QS), to regulate virulence and facilitate host colonization. EHEC uses SdiA from E. coli (SdiA_EC), an orphan LuxR-type receptor, to sense N-acyl l-homoserine lactone (AHL) QS signals produced by other members of the bovine enteric microbiome. SdiA_EC regulates two phenotypes critical for colonizing cattle: acid resistance and the formation of attaching and effacing lesions. Despite the importance of SdiA_EC, there is very little known about its selectivity for different AHL signals, and no chemical inhibitors that act specifically on SdiA_EC have been reported. Such compounds would represent valuable tools to study the roles of QS in EHEC virulence. To identify chemical modulators of SdiA_EC and delineate the structure-activity relationships (SARs) for AHL activity in this receptor, we report herein the screening of a focused library composed largely of AHLs and AHL analogues in an SdiA_EC reporter assay. We describe the identity and SARs of potent modulators of SdiA_EC activity, examine the promiscuity of SdiA_EC, characterize the mechanism of a covalent inhibitor, and provide phenotypic assay data to support that these compounds can control SdiA_EC-dependent acid resistance in E. coli. These SdiA_EC modulators could be used to advance the study of LuxR-type receptor/ligand interactions, the biological roles of orphan LuxR-type receptors, and potential QS-based therapeutic approaches.

Quorum Sensing Behavior in the Model Unicellular Eukaryote Chlamydomonas reinhardtii

Microbial communities display behavioral changes in response to variable environmental conditions. In some bacteria, motility increases as a function of cell density, allowing for population dispersal before the onset of nutrient scarcity. Utilizing automated particle tracking, we now report on a population-dependent increase in the swimming speeds of the photosynthetic unicellular eukaryotes Chlamydomonas reinhardtii and C. moewussi. Our findings confirm that this acceleration in swimming speed arises as a function of culture density, rather than with age and/or nutrient availability. Furthermore, this phenomenon depends on the synthesis and detection of a low-molecular-weight compound which can be transferred between cultures and stimulates comparable effects across both species, supporting the existence of a conserved phenomenon, not unlike bacterial quorum sensing, among members of this genus. The potential expansion of density-dependent phenomena to a new group of unicellular eukaryotes provides important insight into how microbial populations evolve and regulate "social" behaviors.

Potent modulation of the CepR quorum sensing receptor and virulence in a Burkholderia cepacia complex member using non-native lactone ligands

The Burkholderia cepacia complex (Bcc) is a family of closely related bacterial pathogens that are the causative agent of deadly human infections. Virulence in Bcc species has been shown to be controlled by the CepI/CepR quorum sensing (QS) system, which is mediated by an N-acyl L-homoserine lactone (AHL) signal (C₈-AHL) and its cognate LuxR-type receptor (CepR). Chemical strategies to block QS in Bcc members would represent an approach to intercept this bacterial communication process and further delineate its role in infection. In the current study, we sought to identify non-native AHLs capable of agonizing or antagonizing CepR, and thereby QS, in a Bcc member. We screened a library of AHL analogs in cell-based reporters for CepR, and identified numerous highly potent CepR agonists and antagonists. These compounds remain active in a Bcc member, B. multivorans, with one agonist 250-fold more potent than the native ligand C₈-AHL, and can affect QS-controlled motility. Further, the CepR antagonists prolong C. elegans survival in an infection model. These AHL analogs are the first reported non-native molecules that both directly modulate CepR and impact QS-controlled phenotypes in a Bcc member, and represent valuable chemical tools to assess the role of QS in Bcc infections.

Quorum Sensing Signal Selectivity and the Potential for Interspecies Cross Talk

Many species of proteobacteria communicate with kin and coordinate group behaviors through a form of cell-cell signaling called acyl-homoserine lactone (AHL) quorum sensing (QS). Most AHL receptors are thought to be specific for their cognate signal, ensuring that bacteria cooperate and share resources only with closely related kin cells. Although specificity is considered fundamental to QS, there are reports of "promiscuous" receptors that respond broadly to nonself signals. These promiscuous responses expand the function of QS systems to include interspecies interactions and have been implicated in both interspecies competition and cooperation. Because bacteria are frequently members of polymicrobial communities, AHL cross talk between species could have profound impacts. To better understand the prevalence of QS promiscuity, we measured the activity of seven QS receptors in their native host organisms. To facilitate comparison of our results to previous studies, we also measured receptor activity using heterologous expression in Escherichia coli We found that the standard E. coli methods consistently overestimate receptor promiscuity and sensitivity and that overexpression of the receptors is sufficient to account for the discrepancy between native and E. coli reporters. Additionally, receptor overexpression resulted in AHL-independent activity in Pseudomonas aeruginosa Using our activation data, we developed a quantitative score of receptor selectivity. We find that the receptors display a wide range of selectivity and that most receptors respond sensitively and strongly to at least one nonself signal, suggesting a broad potential for cross talk between QS systems.IMPORTANCE Specific recognition of cognate signals is considered fundamental to cell signaling circuits as it creates fidelity in the communication system. In bacterial quorum sensing (QS), receptor specificity ensures that bacteria cooperate only with kin. There are examples, however, of QS receptors that respond promiscuously to multiple signals. "Eavesdropping" by these promiscuous receptors can be beneficial in both interspecies competition and cooperation. Despite their potential significance, we know little about the prevalence of promiscuous QS receptors. Further, many studies rely on methods requiring receptor overexpression, which is known to increase apparent promiscuity. By systematically studying QS receptors in their natural parent strains, we find that the receptors display a wide range of selectivity and that there is potential for significant cross talk between QS systems. Our results provide a basis for hypotheses about the evolution and function of promiscuous signal receptors and for predictions about interspecies interactions in complex microbial communities.

Selection of Appropriate Autoinducer Analogues for the Modulation of Quorum Sensing at the Host-Bacterium Interface

Bacteria regulate a variety of phenotypes in response to their population density using quorum sensing (QS). This phenomenon is regulated by small molecule or peptide signals, the best characterized of which are the N-acyl l-homoserine lactones (AHLs) utilized by Gram-negative bacteria. As many QS-controlled phenotypes, notably pathogenicity and symbiosis, can profoundly impact host eukaryotes, there is significant interest in developing methods for modulating QS signaling and either ameliorating or augmenting these phenotypes. One strategy has been the use of non-native AHL analogues to agonize or antagonize specific AHL receptors. This approach is complicated, however, by the potential for prospective hosts to respond to both native AHLs and synthetic analogues. Accordingly, identifying AHL analogues with little or no activity toward eukaryotes is important in developing QS modulation as a strategy for the regulation of prokaryotic behaviors. Herein, we utilize the model plant Arabidopsis thaliana to characterize eukaryotic responses to a variety of synthetic AHL analogues to identify structural elements of existing scaffolds that may elicit responses in prospective hosts. Our results indicate that, while many of these compounds have no discernible effect on A. thaliana, some elicit strong phenotypes similar to those produced by auxin, a hormone involved in almost all aspects of plant development. We outline concentrations and chemical scaffolds that are ideal for deployment on plant hosts for the regulation of QS. This approach should be exportable to other eukaryotes for the selection of optimal AHL tools for the study of QS at the host-microbe interface.

Non-native autoinducer analogs capable of modulating the SdiA quorum sensing receptor in Salmonella enterica serovar Typhimurium

Quorum sensing (QS) allows many common bacterial pathogens to coordinate group behaviors such as virulence factor production, host colonization, and biofilm formation at high population densities. This cell–cell signaling process is regulated by N-acyl L-homoserine lactone (AHL) signals, or autoinducers, and LuxR-type receptors in Gram-negative bacteria. SdiA is an orphan LuxR-type receptor found in Escherichia, Salmonella, Klebsiella, and Enterobacter genera that responds to AHL signals produced by other species and regulates genes involved in several aspects of host colonization. The inhibition of QS using non-native small molecules that target LuxR-type receptors offers a non-biocidal approach for studying, and potentially controlling, virulence in these bacteria. To date, few studies have characterized the features of AHLs and other small molecules capable of SdiA agonism, and no SdiA antagonists have been reported. Herein, we report the screening of a set of AHL analogs to both uncover agonists and antagonists of SdiA and to start to delineate structure–activity relationships (SARs) for SdiA:AHL interactions. Using a cell-based reporter of SdiA in Salmonella enterica serovar Typhimurium, several non-natural SdiA agonists and the first set of SdiA antagonists were identified and characterized. These compounds represent new chemical probes for exploring the mechanisms by which SdiA functions during infection and its role in interspecies interactions. Moreover, as SdiA is highly stable when produced in vitro, these compounds could advance fundamental studies of LuxR-type receptor:ligand interactions that engender both agonism and antagonism.

Chemical probes of quorum sensing: from compound development to biological discovery

Bacteria can utilize chemical signals to coordinate the expression of group-beneficial behaviors in a method of cell-cell communication called quorum sensing (QS). The discovery that QS controls the production of virulence factors and biofilm formation in many common pathogens has driven an explosion of research aimed at both deepening our fundamental understanding of these regulatory networks and developing chemical agents that can attenuate QS signaling. The inherently chemical nature of QS makes studying these pathways with small molecule tools a complementary approach to traditional microbiology techniques. Indeed, chemical tools are beginning to yield new insights into QS regulation and provide novel strategies to inhibit QS. Here, we review the most recent advances in the development of chemical probes of QS systems in Gram-negative bacteria, with an emphasis on the opportunistic pathogen Pseudomonas aeruginosa We first describe reports of novel small molecule modulators of QS receptors and QS signal synthases. Next, in several case studies, we showcase how chemical tools have been deployed to reveal new knowledge of QS biology and outline lessons for how researchers might best target QS to combat bacterial virulence. To close, we detail the outstanding challenges in the field and suggest strategies to overcome these issues.

Biocontrol of the Potato Blackleg and Soft Rot Diseases Caused by Dickeya dianthicola

Development of protection tools targeting Dickeya species is an important issue in the potato production. Here, we present the identification and the characterization of novel biocontrol agents. Successive screenings of 10,000 bacterial isolates led us to retain 58 strains that exhibited growth inhibition properties against several Dickeya sp. and/or Pectobacterium sp. pathogens. Most of them belonged to the Pseudomonas and Bacillus genera. In vitro assays revealed a fitness decrease of the tested Dickeya sp. and Pectobacterium sp. pathogens in the presence of the biocontrol agents. In addition, four independent greenhouse assays performed to evaluate the biocontrol bacteria effect on potato plants artificially contaminated with Dickeya dianthicola revealed that a mix of three biocontrol agents, namely, Pseudomonas putida PA14H7 and Pseudomonas fluorescens PA3G8 and PA4C2, repeatedly decreased the severity of blackleg symptoms as well as the transmission of D. dianthicola to the tuber progeny. This work highlights the use of a combination of biocontrol strains as a potential strategy to limit the soft rot and blackleg diseases caused by D. dianthicola on potato plants and tubers.

Potent and Selective Modulation of the RhlR Quorum Sensing Receptor by Using Non-native Ligands: An Emerging Target for Virulence Control in Pseudomonas aeruginosa

Pseudomonas aeruginosa uses N-acylated L-homoserine lactone signals and a triumvirate of LuxR-type receptor proteins--LasR, RhlR, and QscR--for quorum sensing (QS). Each of these receptors can contribute to QS activation or repression and, thereby, the control of myriad virulence phenotypes in this pathogen. LasR has traditionally been considered to be at the top of the QS receptor hierarchy in P. aeruginosa; however, recent reports suggest that RhlR plays a more prominent role in infection than originally predicted, in some circumstances superseding that of LasR. Herein, we report the characterization of a set of synthetic, small-molecule agonists and antagonists of RhlR. Using E. coli reporter strains, we demonstrated that many of these compounds can selectively activate or inhibit RhlR instead of LasR and QscR. Moreover, several molecules maintain their activities in P. aeruginosa at concentrations analogous to native RhlR signal levels. These compounds represent useful chemical probes to study the role of RhlR in the complex QS circuitry of P. aeruginosa, its direct (and indirect) effects on virulence, and its overall merit as a target for anti-infective therapy.

Plant responses to bacterial N-acyl L-homoserine lactones are dependent on enzymatic degradation to L-homoserine

Many bacteria use quorum sensing (QS) to regulate phenotypes that ultimately benefit the bacterial population at high cell densities. These QS-dependent phenotypes are diverse and can have significant impacts on the bacterial host, including virulence factor production, motility, biofilm formation, bioluminescence, and root nodulation. As bacteria and their eukaryotic hosts have coevolved over millions of years, it is not surprising that certain hosts appear to be able to sense QS signals, potentially allowing them to alter QS outcomes. Recent experiments have established that eukaryotes have marked responses to the N-acyl l-homoserine lactone (AHL) signals used by Gram-negative bacteria for QS, and the responses of plants to AHLs have received considerable scrutiny to date. However, the molecular mechanisms by which plants, and eukaryotes in general, sense bacterial AHLs remain unclear. Herein, we report a systematic analysis of the responses of the model plants Arabidopsis thaliana and Medicago truncatula to a series of native AHLs and byproducts thereof. Our results establish that AHLs can significantly alter seedling growth in an acyl-chain length dependent manner. Based upon A. thaliana knockout studies and in vitro biochemical assays, we conclude that the observed growth effects are dependent upon AHL amidolysis by a plant-derived fatty acid amide hydrolase (FAAH) to yield l-homoserine. The accumulation of l-homoserine appears to encourage plant growth at low concentrations by stimulating transpiration, while higher concentrations inhibit growth by stimulating ethylene production. These results offer new insights into the mechanisms by which plant hosts can respond to QS signals and the potential role of QS in interkingdom associations.

Modified N-acyl-homoserine lactones as chemical probes for the elucidation of plant-microbe interactions

Gram-negative bacteria often use N-acyl-homoserine lactones (AHLs) as signal molecules to monitor their local population densities and to regulate gene-expression in a process called "Quorum Sensing" (QS). This cell-to-cell communication allows bacteria to adapt to environmental changes and to behave as multicellular communities. QS plays a key role in both bacterial virulence towards the host and symbiotic interactions with other organisms. Plants also perceive AHLs and respond to them with changes in gene expression or modifications in development. Herein, we report the synthesis of new AHL-derivatives for the investigation and identification of AHL-interacting proteins. We show that our new compounds are still recognised by different bacteria and that a novel biotin-tagged-AHL derivative interacts with a bacterial AHL receptor.

Non-native acylated homoserine lactones reveal that LuxIR quorum sensing promotes symbiont stability

Quorum sensing, a group behaviour coordinated by a diffusible pheromone signal and a cognate receptor, is typical of bacteria that form symbioses with plants and animals. LuxIR-type N-acyl L-homoserine (AHL) quorum sensing is common in Gram-negative Proteobacteria, and many members of this group have additional quorum-sensing networks. The bioluminescent symbiont Vibrio fischeri encodes two AHL signal synthases: AinS and LuxI. AinS-dependent quorum sensing converges with LuxI-dependent quorum sensing at the LuxR regulatory element. Both AinS- and LuxI-mediated signalling are required for efficient and persistent colonization of the squid host, Euprymna scolopes. The basis of the mutualism is symbiont bioluminescence, which is regulated by both LuxI- and AinS-dependent quorum sensing, and is essential for maintaining a colonization of the host. Here, we used chemical and genetic approaches to probe the dynamics of LuxI- and AinS-mediated regulation of bioluminescence during symbiosis. We demonstrate that both native AHLs and non-native AHL analogues can be used to non-invasively and specifically modulate induction of symbiotic bioluminescence via LuxI-dependent quorum sensing. Our data suggest that the first day of colonization, during which symbiont bioluminescence is induced by LuxIR, is a critical period that determines the stability of the V. fischeri population once symbiosis is established.

Non-native N-aroyl L-homoserine lactones are potent modulators of the quorum sensing receptor RpaR in Rhodopseudomonas palustris

Quorum sensing (QS) is a process by which bacteria use low-molecular-weight signaling molecules (or autoinducers) to assess their local population densities and alter gene expression levels at high cell numbers. Many Gram-negative bacteria use N-acyl L-homoserine lactones (AHLs) with aliphatic acyl groups as signaling molecules for QS. However, bacteria that utilize AHLs with aroyl acyl groups have been recently discovered; they include the metabolically versatile soil bacterium Rhodopseudomonas palustris, which uses p-coumaroyl HL (p-cAHL) as its QS signal. This autoinducer is especially unusual because its acyl group is believed to originate from a monolignol (i.e., p-coumarate) produced exogenously by plants in the R. palustris environment, rather than through the endogenous fatty acid biosynthesis pathway like other native AHLs. As such, p-cAHL could signal not only bacterial density, but also the availability of an exogenous plant-derived substrate and might even constitute an interkingdom signal. Like other Gram-negative bacteria, QS in R. palustris is controlled by the p-cAHL signal binding its cognate LuxR-type receptor, RpaR. We sought to determine if non-native aroyl HLs (ArHLs) could potentially activate or inhibit RpaR in R. palustris, and thereby modulate QS in this bacterium. Herein, we report the testing of a set of synthetic ArHLs for RpaR agonism and antagonism by using a R. palustris reporter strain. Several potent non-native RpaR agonists and antagonists were identified. Additionally, the screening data revealed that lower concentrations of ArHL are required to strongly agonize RpaR than to antagonize it. Structure-activity relationship analyses of the active ArHLs indicated that potent RpaR agonists tend to have sterically small substituents on their aryl groups, most notably in the ortho position. In turn, the most potent RpaR antagonists were based on either the phenylpropionyl HL (PPHL) or the phenoxyacetyl HL (POHL) scaffold, and many contained an electron-withdrawing group at either the meta or para positions of the aryl ring. To our knowledge, the compounds reported herein represent the first abiotic chemical modulators of RpaR, and more generally, the first abiotic ligands capable of intercepting QS in bacteria that utilize native ArHL signals. In view of the origins of the p-cAHL signal in R. palustris, the largely unknown role of QS in this bacterium, and R. palustris' unique environmental lifestyles, we anticipate that these compounds could be valuable as chemical probes to study QS in R. palustris in a range of fundamental and applied contexts.

Exploiting quorum sensing to confuse bacterial pathogens

Cell-cell communication, or quorum sensing, is a widespread phenomenon in bacteria that is used to coordinate gene expression among local populations. Its use by bacterial pathogens to regulate genes that promote invasion, defense, and spread has been particularly well documented. With the ongoing emergence of antibiotic-resistant pathogens, there is a current need for development of alternative therapeutic strategies. An antivirulence approach by which quorum sensing is impeded has caught on as a viable means to manipulate bacterial processes, especially pathogenic traits that are harmful to human and animal health and agricultural productivity. The identification and development of chemical compounds and enzymes that facilitate quorum-sensing inhibition (QSI) by targeting signaling molecules, signal biogenesis, or signal detection are reviewed here. Overall, the evidence suggests that QSI therapy may be efficacious against some, but not necessarily all, bacterial pathogens, and several failures and ongoing concerns that may steer future studies in productive directions are discussed. Nevertheless, various QSI successes have rightfully perpetuated excitement surrounding new potential therapies, and this review highlights promising QSI leads in disrupting pathogenesis in both plants and animals.

N,N'-alkylated Imidazolium-derivatives act as quorum-sensing inhibitors targeting the Pectobacterium atrosepticum-induced symptoms on potato tubers

Bacteria belonging to the Pectobacterium genus are the causative agents of the blackleg and soft-rot diseases that affect potato plants and tubers worldwide. In Pectobacterium, the expression of the virulence genes is controlled by quorum-sensing (QS) and N-acylhomoserine lactones (AHLs). In this work, we screened a chemical library of QS-inhibitors (QSIs) and AHL-analogs to find novel QSIs targeting the virulence of Pectobacterium. Four N,N′-bisalkylated imidazolium salts were identified as QSIs; they were active at the μM range. In potato tuber assays, two of them were able to decrease the severity of the symptoms provoked by P. atrosepticum. This work extends the range of the QSIs acting on the Pectobacterium-induced soft-rot disease.

Synthesis and biological evaluation of triazole-containing N-acyl homoserine lactones as quorum sensing modulators

Many bacterial species are capable of assessing their local population densities through a cell-cell signaling mechanism termed quorum sensing (QS). This intercellular communication process is mediated by small molecule or peptide ligands and their cognate protein receptors. Numerous pathogens use QS to initiate virulence once they achieve a threshold cell number on a host. Consequently, approaches to intercept QS have attracted considerable attention as potential anti-infective therapies. Our interest in the development of small molecule tools to modulate QS pathways motivated us to evaluate triazole-containing analogs of natural N-acyl L-homoserine lactone (AHL) signals as non-native QS agonists and antagonists in Gram-negative bacteria. We synthesized 72 triazole derivatives of five broad structure types in high yields and purities using efficient Cu(I)-catalyzed azide-alkyne couplings. These compounds were evaluated for their ability to activate or inhibit two QS receptors from two prevalent pathogens - LasR from Pseudomonas aeruginosa and AbaR from Acinetobacter baumannii- using bacterial reporter strains. Several triazole derivatives were identified that were capable of strongly modulating the activity of LasR and AbaR. These compounds represent a new and synthetically accessible class of AHL analogs, and could find utility as chemical tools to study QS and its role in bacterial virulence.

Attenuation of quorum sensing in the pathogen Acinetobacter baumannii using non-native N-Acyl homoserine lactones

Many bacterial pathogens use quorum sensing (QS) to control virulence. As a result, the development of methods to intercept QS has attracted significant interest as a potential anti-infective therapy. Acinetobacter baumannii has emerged as a pan-drug-resistant pathogen and displays a remarkable ability to persist in hospital settings despite desiccation and antimicrobial treatment. Recent studies have shown that A. baumannii QS mutants have limited motility and fail to form mature biofilms; these phenotypes are linked to its ability to persist on biotic and abiotic surfaces and increase its pathogenicity. A. baumannii uses N-(3-hydroxydodecanoyl)-l-homoserine lactone (OH-dDHL) and its putative cognate receptor, AbaR, for QS. We sought to identify non-native ligands capable of blocking or promoting AbaR activity in A. baumannii for use as chemical probes to modulate QS phenotypes in this pathogen. We screened a focused library of synthetic, non-native N-acyl homoserine lactones (AHLs) to identify such compounds, and several highly potent antagonists and agonists were uncovered, with IC(50) and EC(50) values in the low micromolar range, respectively. The strongest AbaR antagonists largely contained aromatic acyl groups, whereas the AbaR agonists closely resembled OH-dDHL. Notably, the 10 most potent AbaR antagonists also strongly inhibited A. baumannii motility, and five antagonists reduced biofilm formation in A. baumannii by up to 40%. The discovery of these compounds is significant, as they represent, to our knowledge, the first non-native modulators of QS in A. baumannii to be reported and could find utility as new tools to study the role and timing of QS phenotypes in A. baumannii infections.

Chemical methods to interrogate bacterial quorum sensing pathways

Bacteria frequently manifest distinct phenotypes as a function of cell density in a phenomenon known as quorum sensing (QS). This intercellular signalling process is mediated by "chemical languages" comprised of low-molecular weight signals, known as autoinducers, and their cognate receptor proteins. As many of the phenotypes regulated by QS can have a significant impact on the success of pathogenic or mutualistic prokaryotic-eukaryotic interactions, there is considerable interest in methods to probe and modulate QS pathways with temporal and spatial control. Such methods would be valuable for both basic research in bacterial ecology and in practical medicinal, agricultural, and industrial applications. Toward this goal, considerable recent research has been focused on the development of chemical approaches to study bacterial QS pathways. In this Perspective, we provide an overview of the use of chemical probes and techniques in QS research. Specifically, we focus on: (1) combinatorial approaches for the discovery of small molecule QS modulators, (2) affinity chromatography for the isolation of QS receptors, (3) reactive and fluorescent probes for QS receptors, (4) antibodies as quorum "quenchers," (5) abiotic polymeric "sinks" and "pools" for QS signals, and (6) the electrochemical sensing of QS signals. The application of such chemical methods can offer unique advantages for both elucidating and manipulating QS pathways in culture and under native conditions.

Attenuation of virulence in pathogenic bacteria using synthetic quorum-sensing modulators under native conditions on plant hosts

Quorum sensing (QS) is often critical in both pathogenic and mutualistic relationships between bacteria and their eukaryotic hosts. Gram-negative bacteria typically use N-acylated l-homoserine lactone (AHL) signals for QS. We have identified a number of synthetic AHL analogues that are able to strongly modulate QS in culture-based, reporter gene assays. While informative, these assays represent idealized systems, and their relevance to QS under native conditions is often unclear. As one of our goals is to utilize synthetic QS modulators to study bacterial communication under native conditions, identifying robust host-bacteria model systems for their evaluation is crucial. We reasoned that the host-pathogen interaction between Solanum tuberosum (potato) and the Gram-negative pathogen Pectobacterium carotovora would be ideal for such studies as we have identified several potent, synthetic QS modulators for this pathogen, and infection assays in potato are facile. Herein, we report on our development of this host-pathogen system, and another in Phaseolus vulgaris (green bean), as a means for monitoring the ability of abiotic AHLs to modulate QS-regulated virulence in host infection assays. Our assays confirmed that QS modulators previously identified through culture-based assays largely retained their activity profiles when introduced into the plant host. However, inhibition of virulence in wild-type infections was highly dependent on the timing of compound dosing. This study is the first to demonstrate that our AHL analogues are active in wild-type bacteria in their native eukaryotic hosts and provides compelling evidence for the application of these molecules as probes to study QS in a range of organisms and environments.

Thiolactone modulators of quorum sensing revealed through library design and screening

Quorum sensing (QS) is a process by which bacteria use small molecules or peptidic signals to assess their local population densities. At sufficiently high density, bacteria can alter gene expression levels to regulate group behaviors involved in a range of important and diverse phenotypes, including virulence factor production, biofilm formation, root nodulation, and bioluminescence. Gram-negative bacteria most commonly use N-acylated l-homoserine lactones (AHLs) as their QS signals. The AHL lactone ring is hydrolyzed relatively rapidly at biological pH, and the ring-opened product is QS inactive. We seek to identify AHL analogues with heightened hydrolytic stability, and thereby potentially heightened activity, for use as non-native modulators of bacterial QS. As part of this effort, we probed the utility of thiolactone analogues in the current study as QS agonists and antagonists in Gram-negative bacteria. A focused library of thiolactone analogs was designed and rapidly synthesized in solution. We examined the activity of the library as agonists and antagonists of LuxR-type QS receptors in Pseudomonas aeruginosa (LasR), Vibrio fischeri (LuxR), and Agrobacterium tumefaciens (TraR) using bacterial reporter strains. The thiolactone library contained several highly active compounds, including some of the most active LuxR inhibitors and the most active synthetic TraR agonist reported to date. Analysis of a representative thiolactone analog revealed that its hydrolysis half-life was almost double that of its parent AHL in bacterial growth medium.

Design, synthesis, and biological evaluation of abiotic, non-lactone modulators of LuxR-type quorum sensing

Quorum sensing (QS) is a cell-cell signaling mechanism that allows bacteria to monitor their population size and alter their behavior at high cell densities. Gram-negative bacteria use N-acylated L-homoserine lactones (AHLs) as their primary signals for QS. These signals are susceptible to lactone hydrolysis in biologically relevant media, and the ring-opened products are inactive QS signals. We have previously identified a range of non-native AHLs capable of strongly agonizing and antagonizing QS in Gram-negative bacteria. However, these abiotic AHLs are also prone to hydrolysis and inactivation and thereby have a relatively short time window for use (∼12-48 h). Non-native QS modulators with reduced or no hydrolytic instability could have enhanced potencies and would be valuable as tools to study the mechanisms of QS in a range of environments (for example, on eukaryotic hosts). This study reports the design and synthesis of two libraries of new, non-hydrolyzable AHL mimics. The libraries were screened for QS modulatory activity using LasR, LuxR, and TraR bacterial reporter strains, and several new, abiotic agonists and antagonists of these receptors were identified.