Quinolone signaling in the cell-to-cell communication system of Pseudomonas aeruginosa

Biological and clinical significance of quorum sensing alkylquinolones: current analytical and bioanalytical methods for their quantification

Quorum sensing (QS) is a sophisticated bacterial communication system which plays a key role in the virulence and biofilm formation of many pathogens. The Pseudomonas aeruginosa QS network consists of four sets of connected systems (las, rlh, pqs and iqs) hierarchically organized. The pqs system involves characteristic autoinducers (AI), most of them sharing an alkylquinolone (AQ) structure, and is able to carry out several relevant biological functions besides its main signalling activity. Their role in bacterial physiology and pathogenicity has been widely studied. Indeed, the presence of these metabolites in several body fluids and infected tissues has pointed to their potential value as biomarkers of infection. In this review, we summarize the most recent findings about the biological implications and the clinical significance of the main P. aeruginosa AQs. These findings have encouraged the development of analytical and bioanalytical techniques addressed to assess the role of these metabolites in bacterial growth and survival, during pathogenesis or as biomarkers of infections. The availability of highly sensitive reliable analytical methods suitable for clinical analysis would allow getting knowledge about pathogenesis and disease prognosis or progression, supporting clinicians on the decision-making process for the management of these infections and guiding them on the application of more effective and appropriate treatments. The benefits from the implementation of the point-of-care (PoC)-type testing in infectious disease diagnostics, which are seen to improve patient outcomes by promoting earlier therapeutic interventions, are also discussed.

Malonate utilization by Pseudomonas aeruginosa affects quorum-sensing and virulence and leads to formation of mineralized biofilm-like structures

Pseudomonas aeruginosa is an opportunistic pathogen that uses malonate among its many carbon sources. We recently reported that, when grown in blood from trauma patients, P. aeruginosa expression of malonate utilization genes was upregulated. In this study, we explored the role of malonate utilization and its contribution to P. aeruginosa virulence. We grew P. aeruginosa strain PA14 in M9 minimal medium containing malonate (MM9) or glycerol (GM9) as a sole carbon source and assessed the effect of the growth on quorum sensing, virulence factors, and antibiotic resistance. Growth of PA14 in MM9, compared to GM9, reduced the production of elastases, rhamnolipids, and pyoverdine; enhanced the production of pyocyanin and catalase; and increased its sensitivity to norfloxacin. Growth in MM9 decreased extracellular levels of N-acylhomoserine lactone autoinducers, an effect likely associated with increased pH of the culture medium; but had little effect on extracellular levels of PQS. At 18 hr of growth in MM9, PA14 formed biofilm-like structures or aggregates that were associated with biomineralization, which was related to increased pH of the culture medium. These results suggest that malonate significantly impacts P. aeruginosa pathogenesis by influencing the quorum sensing systems, the production of virulence factors, biofilm formation, and antibiotic resistance.

An Immunochemical Approach to Quantify and Assess the Potential Value of the Pseudomonas Quinolone Signal as a Biomarker of Infection

Quorum sensing (QS) is a bacterial cell density-based communication system using low molecular weight signals called autoinducers (AIs). Identification and quantification of these molecules could provide valuable information related to the stage of colonization or infection as well as the stage of the disease. With this scenario, we report here for the first time the development of antibodies against the PQS (pseudomonas quinolone signal), the main signaling molecule from the pqs QS system of Pseudomonas aeruginosa, and the development of a microplate-based enzyme-linked immunosorbent assay (ELISA) able of quantifying this molecule in complex biological media in the low nanometer range (LOD, 0.36 ± 0.14 nM in culture broth media). Moreover, the PQS ELISA here reported has been found to be robust and reliable, providing accurate results in culture media. The technique allowed us to follow up the PQS profile of the release of bacterial clinical isolates obtained from patients of different disease status. A clear correlation was found between the PQS immunoreactivity equivalents and the chronic or acute infection conditions, which supports the reported differences on virulence and behavior of these bacterial strains due to their adaptation capability to the host environment. The results obtained point to the potential of the PQS as a biomarker of infection and to the value of the antibodies and the technology developed for improving diagnosis and management of P. aeruginosa infections based on the precise identification of the pathogen, appropriate stratification of the patients according to their disease status, and knowledge of the disease progression.

Baicalin Represses Type Three Secretion System of Pseudomonas aeruginosa through PQS System

Therapeutics that target the virulence of pathogens rather than their viability offer a promising alternative for treating infectious diseases and circumventing antibiotic resistance. In this study, we searched for anti-virulence compounds against Pseudomonas aeruginosa from Chinese herbs and investigated baicalin from Scutellariae radix as such an active anti-virulence compound. The effect of baicalin on a range of important virulence factors in P. aeruginosa was assessed using luxCDABE-based reporters and by phenotypical assays. The molecular mechanism of the virulence inhibition by baicalin was investigated using genetic approaches. The impact of baicalin on P. aeruginosa pathogenicity was evaluated by both in vitro assays and in vivo animal models. The results show that baicalin diminished a plenty of important virulence factors in P. aeruginosa, including the Type III secretion system (T3SS). Baicalin treatment reduced the cellular toxicity of P. aeruginosa on the mammalian cells and attenuated in vivo pathogenicity in a Drosophila melanogaster infection model. In a rat pulmonary infection model, baicalin significantly reduced the severity of lung pathology and accelerated lung bacterial clearance. The PqsR of the Pseudomonas quinolone signal (PQS) system was found to be required for baicalin's impact on T3SS. These findings indicate that baicalin is a promising therapeutic candidate for treating P. aeruginosa infections.

Signal Synthase-Type versus Catabolic Monooxygenases: Retracing 3-Hydroxylation of 2-Alkylquinolones and Their N-Oxides by Pseudomonas aeruginosa and Other Pulmonary Pathogens

The multiple biological activities of 2-alkylquinolones (AQs) are crucial for virulence of Pseudomonas aeruginosa, conferring advantages during infection and in polymicrobial communities. Whereas 2-heptyl-3-hydroxyquinolin-4(1H)-one (the "Pseudomonas quinolone signal" [PQS]) is an important quorum sensing signal molecule, 2-alkyl-1-hydroxyquinolin-4(1H)-ones (also known as 2-alkyl-4-hydroxyquinoline N-oxides [AQNOs]) are antibiotics inhibiting respiration. Hydroxylation of the PQS precursor 2-heptylquinolin-4(1H)-one (HHQ) by the signal synthase PqsH boosts AQ quorum sensing. Remarkably, the same reaction, catalyzed by the ortholog AqdB, is used by Mycobacteroides abscessus to initiate degradation of AQs. The antibiotic 2-heptyl-1-hydroxyquinolin-4(1H)-one (HQNO) is hydroxylated by Staphylococcus aureus to the less toxic derivative PQS-N-oxide (PQS-NO), a reaction probably also catalyzed by a PqsH/AqdB ortholog. In this study, we provide a comparative analysis of four AQ 3-monooxygenases of different organisms. Due to the major impact of AQ/AQNO 3-hydroxylation on the biological activities of the compounds, we surmised adaptations on the enzymatic and/or physiological level to serve either the producer or target organisms. Our results indicate that all enzymes share similar features and are incapable of discriminating between AQs and AQNOs. PQS-NO, hence, occurs as a native metabolite of P. aeruginosa although the unfavorable AQNO 3-hydroxylation is minimized by export as shown for HQNO, involving at least one multidrug efflux pump. Moreover, M. abscessus is capable of degrading the AQNO heterocycle by concerted action of AqdB and dioxygenase AqdC. However, S. aureus and M. abscessus orthologs disfavor AQNOs despite their higher toxicity, suggesting that catalytic constraints restrict evolutionary adaptation and lead to the preference of non-N-oxide substrates by AQ 3-monooxygenases.IMPORTANCE Pseudomonas aeruginosa, Staphylococcus aureus, and Mycobacteroides abscessus are major players in bacterial chronic infections and particularly common colonizers of cystic fibrosis (CF) lung tissue. Whereas S. aureus is an early onset pathogen in CF, P. aeruginosa establishes at later stages. M. abscessus occurs at all stages but has a lower epidemiological incidence. The dynamics of how these pathogens interact can affect survival and therapeutic success. 2-Alkylquinolone (AQ) and 2-alkylhydroxyquinoline N-oxide (AQNO) production is a major factor of P. aeruginosa virulence. The 3-position of the AQ scaffold is critical, both for attenuation of AQ toxicity or degradation by competitors, as well as for full unfolding of quorum sensing. Despite lacking signaling functionality, AQNOs have the strongest impact on suppression of Gram-positives. Because evidence for 3-hydroxylation of AQNOs has been reported, it is desirable to understand the extent by which AQ 3-monooxygenases contribute to manipulation of AQ/AQNO equilibrium, resistance, and degradation.

Disproof of the Proposed Structures of Bradyoxetin, a Putative Bradyrhizobium japonicum Signaling Molecule, and HMCP, a Putative Ralstonia solanacearum Quorum-Sensing Molecule

First, we revisited the reported NMR data of bradyoxetin, a putative cell density factor of Bradyrhizobium japonicum, and found some inconsistencies in the proposed structure. To elucidate the correct structure, we synthesized model oxetane compounds and confirmed that the NMR data of the synthetic compounds did not match those of the reported bradyoxetin. After reinterpreting the reported NMR data, we concluded that bradyoxetin must be chloramphenicol. Next, some derivatives of 2-hydroxy-4-((methylamino)(phenyl)methyl)cyclopentanone (HMCP), which is a putative quorum-sensing molecule of Ralstonia solanacearum, were synthesized. The NMR spectra of the synthesized compounds were completely different from those of the reported natural products. Based on theoretical studies, including the estimation of ¹H and ¹³C NMR chemical shifts using density functional theory calculations, we confirmed the correctness of the structure of the synthesized compound. These results strongly suggest that the proposed structure of HMCP could be incorrect.

Microbiologically-Synthesized Nanoparticles and Their Role in Silencing the Biofilm Signaling Cascade

The emergence of bacterial resistance to antibiotics has led to the search for alternate antimicrobial treatment strategies. Engineered nanoparticles (NPs) for efficient penetration into a living system have become more common in the world of health and hygiene. The use of microbial enzymes/proteins as a potential reducing agent for synthesizing NPs has increased rapidly in comparison to physical and chemical methods. It is a fast, environmentally safe, and cost-effective approach. Among the biogenic sources, fungi and bacteria are preferred not only for their ability to produce a higher titer of reductase enzyme to convert the ionic forms into their nano forms, but also for their convenience in cultivating and regulating the size and morphology of the synthesized NPs, which can effectively reduce the cost for large-scale manufacturing. Effective penetration through exopolysaccharides of a biofilm matrix enables the NPs to inhibit the bacterial growth. Biofilm is the consortia of sessile groups of microbial cells that are able to adhere to biotic and abiotic surfaces with the help extracellular polymeric substances and glycocalyx. These biofilms cause various chronic diseases and lead to biofouling on medical devices and implants. The NPs penetrate the biofilm and affect the quorum-sensing gene cascades and thereby hamper the cell-to-cell communication mechanism, which inhibits biofilm synthesis. This review focuses on the microbial nano-techniques that were used to produce various metallic and non-metallic nanoparticles and their "signal jamming effects" to inhibit biofilm formation. Detailed analysis and discussion is given to their interactions with various types of signal molecules and the genes responsible for the development of biofilm.

A phage-encoded anti-activator inhibits quorum sensing in Pseudomonas aeruginosa

The arms race between bacteria and phages has led to the evolution of diverse anti-phage defenses, several of which are controlled by quorum-sensing pathways. In this work, we characterize a quorum-sensing anti-activator protein, Aqs1, found in Pseudomonas phage DMS3. We show that Aqs1 inhibits LasR, the master regulator of quorum sensing, and present the crystal structure of the Aqs1-LasR complex. The 69-residue Aqs1 protein also inhibits PilB, the type IV pilus assembly ATPase protein, which blocks superinfection by phages that require the pilus for infection. This study highlights the remarkable ability of small phage proteins to bind multiple host proteins and disrupt key biological pathways. As quorum sensing influences various anti-phage defenses, Aqs1 provides a mechanism by which infecting phages might simultaneously dampen multiple defenses. Because quorum-sensing systems are broadly distributed across bacteria, this mechanism of phage counter-defense may play an important role in phage-host evolutionary dynamics.

Rhamnolipids produced by Pseudomonas: from molecular genetics to the market

Rhamnolipids are biosurfactants with a wide range of industrial applications that entered into the market a decade ago. They are naturally produced by Pseudomonas aeruginosa and some Burkholderia species. Occasionally, some strains of different bacterial species, like Pseudomonas chlororaphis NRRL B-30761, which have acquired RL-producing ability by horizontal gene transfer, have been described. P. aeruginosa, the ubiquitous opportunistic pathogenic bacterium, is the best rhamnolipids producer, but Pseudomonas putida has been used as heterologous host for the production of this biosurfactant with relatively good yields. The molecular genetics of rhamnolipids production by P. aeruginosa has been widely studied not only due to the interest in developing overproducing strains, but because it is coordinately regulated with the expression of different virulence-related traits by the quorum-sensing response. Here, we highlight how the research of the molecular mechanisms involved in rhamnolipid production have impacted the development of strains that are suitable for industrial production of this biosurfactant, as well as some perspectives to improve these industrial useful strains.

Tackling Antimicrobial Resistance with Small Molecules Targeting LsrK: Challenges and Opportunities

Antimicrobial resistance (AMR) is a growing threat with severe health and economic consequences. The available antibiotics are losing efficacy, and the hunt for alternative strategies is a priority. Quorum sensing (QS) controls biofilm and virulence factors production. Thus, the quenching of QS to prevent pathogenicity and to increase bacterial susceptibility to antibiotics is an appealing therapeutic strategy. The phosphorylation of autoinducer-2 (a mediator in QS) by LsrK is a crucial step in triggering the QS cascade. Thus, LsrK represents a valuable target in fighting AMR. Few LsrK inhibitors have been reported so far, allowing ample room for further exploration. This perspective aims to provide a comprehensive analysis of the current knowledge about the structural and biological properties of LsrK and the state-of-the-art technology for LsrK inhibitor design. We elaborate on the challenges in developing novel LsrK inhibitors and point out promising avenues for further research.

Anti-quorum Sensing and Anti-biofilm Activity of Zinc Oxide Nanospikes

This study evaluates the impact of two separate incubation periods (4 and 6 weeks) on the morphology of sol-gel-fabricated ZnO nanospikes (ZNs), that is, ZN1 and ZN2, respectively. We further analyzed the inhibitory effects of ZN1 and ZN2 on quorum sensing (QS) and biofilm formation in Pseudomonas aeruginosa (PAO1) and Chromobacterium violaceum (strains 12472 and CVO26). The size of the synthesized ZNs was in the range of 40-130 nm, and finer nanoparticles were synthesized after an incubation period of 6 weeks. Treatment with ZNs decreased the production of violacein in the pathogen without affecting the bacterial growth, which indicated that ZNs inhibited the QS signaling regulated by N-acyl homoserine lactone. ZN2 had a higher inhibitory action on the virulence factor productivity than ZN1. Furthermore, ZN2-treated cells displayed a substantial decrease in azocasein-degrading protease activity (80%), elastase activity (83%), and pyocyanin production (85%) relative to untreated P. aeruginosa PAO1 cells. Treatment with ZN2 decreased swarming motility and exopolysaccharide production by 89 and 85%, respectively. ZN2 was effective against both the las & pqs systems of P. aeruginosa and exhibited broad-spectrum activity. Additionally, ZN2 was more efficient in inhibiting the biofilm formation at the attachment stage than ZN1. These findings revealed that in P. aeruginosa, ZN2 demonstrated inhibitory effects on QS as well as on the development of biofilms. Thus, ZN2 can be potentially used to treat drug-resistant P. aeruginosa infections.

Bacterial Alkyl-4-quinolones: Discovery, Structural Diversity and Biological Properties

The alkyl-4-quinolones (AQs) are a class of metabolites produced primarily by members of the Pseudomonas and Burkholderia genera, consisting of a 4-quinolone core substituted by a range of pendant groups, most commonly at the C-2 position. The history of this class of compounds dates back to the 1940s, when a range of alkylquinolones with notable antibiotic properties were first isolated from Pseudomonas aeruginosa. More recently, it was discovered that an alkylquinolone derivative, the Pseudomonas Quinolone Signal (PQS) plays a key role in bacterial communication and quorum sensing in Pseudomonas aeruginosa. Many of the best-studied examples contain simple hydrocarbon side-chains, but more recent studies have revealed a wide range of structurally diverse examples from multiple bacterial genera, including those with aromatic, isoprenoid, or sulfur-containing side-chains. In addition to their well-known antimicrobial properties, alkylquinolones have been reported with antimalarial, antifungal, antialgal, and antioxidant properties. Here we review the structural diversity and biological activity of these intriguing metabolites.

Pseudomonas Quinolone Signal-Induced Outer Membrane Vesicles Enhance Biofilm Dispersion in Pseudomonas aeruginosa

Bacterial biofilms are major contributors to chronic infections in humans. Because they are recalcitrant to conventional therapy, they present a particularly difficult treatment challenge. Identifying factors involved in biofilm development can help uncover novel targets and guide the development of antibiofilm strategies. Pseudomonas aeruginosa causes surgical site, burn wound, and hospital-acquired infections and is also associated with aggressive biofilm formation in the lungs of cystic fibrosis patients. A potent but poorly understood contributor to P. aeruginosa virulence is the ability to produce outer membrane vesicles (OMVs). OMV trafficking has been associated with cell-cell communication, virulence factor delivery, and transfer of antibiotic resistance genes. Because OMVs have almost exclusively been studied using planktonic cultures, little is known about their biogenesis and function in biofilms. Several groups have shown that Pseudomonas quinolone signal (PQS) induces OMV formation in P. aeruginosa Our group described a biophysical mechanism for this and recently showed it is operative in biofilms. Here, we demonstrate that PQS-induced OMV production is highly dynamic during biofilm development. Interestingly, PQS and OMV synthesis are significantly elevated during dispersion compared to attachment and maturation stages. PQS biosynthetic and receptor mutant biofilms were significantly impaired in their ability to disperse, but this phenotype was rescued by genetic complementation or exogenous addition of PQS. Finally, we show that purified OMVs can actively degrade extracellular protein, lipid, and DNA. We therefore propose that enhanced production of PQS-induced OMVs during biofilm dispersion facilitates cell escape by coordinating the controlled degradation of biofilm matrix components.IMPORTANCE Treatments that manipulate biofilm dispersion hold the potential to convert chronic drug-tolerant biofilm infections from protected sessile communities into released populations that are orders-of-magnitude more susceptible to antimicrobial treatment. However, dispersed cells often exhibit increased acute virulence and dissemination phenotypes. A thorough understanding of the dispersion process is therefore critical before this promising strategy can be effectively employed. Pseudomonas quinolone signal (PQS) has been implicated in early biofilm development, but we hypothesized that its function as an outer membrane vesicle (OMV) inducer may contribute at multiple stages. Here, we demonstrate that PQS and OMVs are differentially produced during Pseudomonas aeruginosa biofilm development and provide evidence that effective biofilm dispersion is dependent on the production of PQS-induced OMVs, which likely act as delivery vehicles for matrix-degrading enzymes. These findings lay the groundwork for understanding OMV contributions to biofilm development and suggest a model to explain the controlled matrix degradation that accompanies biofilm dispersion in many species.

Burkholderia thailandensis Methylated Hydroxyalkylquinolines: Biosynthesis and Antimicrobial Activity in Cocultures

The bacterium Burkholderia thailandensis produces an arsenal of secondary metabolites that have diverse structures and roles in the ecology of this soil-dwelling bacterium. In coculture experiments, B. thailandensis strain E264 secretes an antimicrobial that nearly eliminates another soil bacterium, Bacillus subtilis strain 168. To identify the antimicrobial, we used a transposon mutagenesis approach. This screen identified antimicrobial-defective mutants with insertions in the hmqA, hmqC, and hmqF genes involved in biosynthesis of a family of 2-alkyl-4(1H)-quinolones called 4-hydroxy-3-methyl-2-alkenylquinolines (HMAQs), which are closely related to the Pseudomonas aeruginosa 4-hydroxy-2-alkylquinolines (HAQs). Insertions also occurred in the previously uncharacterized gene BTH_II1576 ("hmqL"). The results confirm that BTH_II1576 is involved in generating N-oxide derivatives of HMAQs (HMAQ-NOs). Synthetic HMAQ-NO is active against B. subtilis 168, showing ∼50-fold more activity than HMAQ. Both the methyl group and the length of the carbon side chain account for the high activity of HMAQ-NO. The results provide new information on the biosynthesis and activities of HMAQs and reveal new insight into how these molecules might be important for the ecology of B. thailandensis IMPORTANCE The soil bacterium Burkholderia thailandensis produces 2-alkyl-4(1H)-quinolones that are mostly methylated 4-hydroxyalkenylquinolines, a family of relatively unstudied metabolites similar to molecules also synthesized by Pseudomonas aeruginosa Several of the methylated 4-hydroxyalkenylquinolines have antimicrobial activity against other species. We show that Bacillus subtilis strain 168 is particularly susceptible to N-oxidated methylalkenylquinolines (HMAQ-NOs). We confirmed that HMAQ-NO biosynthesis requires the previously unstudied protein HmqL. These results provide new information about the biology of 2-alkyl-4(1H)-quinolones, particularly the methylated 4-hydroxyalkenylquinolines, which are unique to B. thailandensis This study also has importance for understanding B. thailandensis secondary metabolites and has implications for potential therapeutic development.

Static Growth Promotes PrrF and 2-Alkyl-4(1H)-Quinolone Regulation of Type VI Secretion Protein Expression in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that is frequently associated with both acute and chronic infections. P. aeruginosa possesses a complex regulatory network that modulates nutrient acquisition and virulence, but our knowledge of these networks is largely based on studies with shaking cultures, which are not likely representative of conditions during infection. Here, we provide proteomic, metabolic, and genetic evidence that regulation by iron, a critical metallonutrient, is altered in static P. aeruginosa cultures. Specifically, we observed a loss of iron-induced expression of proteins for oxidative phosphorylation, tricarboxylic acid (TCA) cycle metabolism under static conditions. Moreover, we identified type VI secretion as a target of iron regulation in P. aeruginosa cells under static but not shaking conditions, and we present evidence that this regulation occurs via PrrF small regulatory RNA (sRNA)-dependent production of 2-alkyl-4(1H)-quinolone metabolites. These results yield new iron regulation paradigms in an important opportunistic pathogen and highlight the need to redefine iron homeostasis in static microbial communities.IMPORTANCE Host-mediated iron starvation is a broadly conserved signal for microbial pathogens to upregulate expression of virulence traits required for successful infection. Historically, global iron regulatory studies in microorganisms have been conducted in shaking cultures to ensure culture homogeneity, yet these conditions are likely not reflective of growth during infection. Pseudomonas aeruginosa is a well-studied opportunistic pathogen and model organism for iron regulatory studies. Iron homeostasis is maintained through the Fur protein and PrrF small regulatory sRNAs, the functions of which are highly conserved in many other bacterial species. In the current study, we examined how static growth affects the known iron and PrrF regulons of P. aeruginosa, leading to the discovery of novel PrrF-regulated virulence processes. This study demonstrates how the utilization of distinct growth models can enhance our understanding of basic physiological processes that may also affect pathogenesis.

The Roles of Microbial Cell-Cell Chemical Communication Systems in the Modulation of Antimicrobial Resistance

Rapid emergence of antimicrobial resistance (AMR) has become a critical challenge worldwide. It is of great importance to understand how AMR is modulated genetically in order to explore new antimicrobial strategies. Recent studies have unveiled that microbial communication systems, which are known to play key roles in regulation of bacterial virulence, are also associated with the formation and regulation of AMR. These microbial cell-to-cell chemical communication systems, including quorum sensing (QS) and pathogen-host communication mechanisms, rely on detection and response of various chemical signal molecules, which are generated either by the microbe itself or host cells, to activate the expression of virulence and AMR genes. This article summarizes the generic signaling mechanisms of representative QS and pathogen-host communications systems, reviews the current knowledge regarding the roles of these chemical communication systems in regulation of AMR, and describes the strategies developed over the years for blocking bacterial chemical communication systems in disease control. The research progress in this field suggests that the bacterial cell-cell communication systems are a promising target not only for disease control but also for curbing the problem of microbial drug resistance.

Heterologous Rhamnolipid Biosynthesis: Advantages, Challenges, and the Opportunity to Produce Tailor-Made Rhamnolipids

The first heterologous expression of genes responsible for the production of rhamnolipids was already implemented in the mid-1990s during the functional identification of the rhlAB operon. This was the starting shot for multiple approaches to establish the rhamnolipid biosynthesis in different host organisms. Since most of the native rhamnolipid producing organisms are human or plant pathogens, the intention for these ventures was the establishment of non-pathogenic organisms as heterologous host for the production of rhamnolipids. The pathogenicity of producing organisms is one of the bottlenecks for applications of rhamnolipids in many industrial products especially foods and cosmetics. The further advantage of heterologous rhamnolipid production is the circumvention of the complex regulatory network, which regulates the rhamnolipid biosynthesis in wild type production strains. Furthermore, a suitable host with an optimal genetic background to provide sufficient amounts of educts allows the production of tailor-made rhamnolipids each with its specific physico-chemical properties depending on the contained numbers of rhamnose sugar residues and the numbers, chain length and saturation degree of 3-hydroxyfatty acids. The heterologous expression of rhl genes can also enable the utilization of unusual carbon sources for the production of rhamnolipids depending on the host organism.

Functional characterization of quorum sensing LuxR-type transcriptional regulator, EasR in Enterobacter asburiae strain L1

Over the past decades, Enterobacter spp. have been identified as challenging and important pathogens. The emergence of multidrug-resistant Enterobacteria especially those that produce Klebsiella pneumoniae carbapenemase has been a very worrying health crisis. Although efforts have been made to unravel the complex mechanisms that contribute to the pathogenicity of different Enterobacter spp., there is very little information associated with AHL-type QS mechanism in Enterobacter spp. Signaling via N-acyl homoserine lactone (AHL) is the most common quorum sensing (QS) mechanism utilized by Proteobacteria. A typical AHL-based QS system involves two key players: a luxI gene homolog to synthesize AHLs and a luxR gene homolog, an AHL-dependent transcriptional regulator. These signaling molecules enable inter-species and intra-species interaction in response to external stimuli according to population density. In our recent study, we reported the genome of AHL-producing bacterium, Enterobacter asburiae strain L1. Whole genome sequencing and in silico analysis revealed the presence of a pair of luxI/R genes responsible for AHL-type QS, designated as easI/R, in strain L1. In a QS system, a LuxR transcriptional protein detects and responds to the concentration of a specific AHL controlling gene expression. In E. asburiae strain L1, EasR protein binds to its cognate AHLs, N-butanoyl homoserine lactone (C4-HSL) and N–hexanoyl homoserine lactone (C6-HSL), modulating the expression of targeted genes. In this current work, we have cloned the 693 bp luxR homolog of strain L1 for further characterization. The functionality and specificity of EasR protein in response to different AHL signaling molecules to activate gene transcription were tested and validated with β-galactosidase assays. Higher β-galactosidase activities were detected for cells harboring EasR, indicating EasR is a functional transcriptional regulator. This is the first report documenting the cloning and characterization of transcriptional regulator, luxR homolog of E. asburiae.

During bacteremia, Pseudomonas aeruginosa PAO1 adapts by altering the expression of numerous virulence genes including those involved in quorum sensing

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that produces numerous virulence factors and causes serious infections in trauma patients and patients with severe burns. We previously showed that the growth of P. aeruginosa in blood from severely burned or trauma patients altered the expression of numerous genes. However, the specific influence of whole blood from healthy volunteers on P. aeruginosa gene expression is not known. Transcriptome analysis of P. aeruginosa grown for 4 h in blood from healthy volunteers compared to that when grown in laboratory medium revealed that the expression of 1085 genes was significantly altered. Quorum sensing (QS), QS-related, and pyochelin synthesis genes were downregulated, while genes of the type III secretion system and those for pyoverdine synthesis were upregulated. The observed effect on the QS and QS-related genes was shown to reside within serum fraction: growth of PAO1 in the presence of 10% human serum from healthy volunteers significantly reduced the expression of QS and QS-regulated genes at 2 and 4 h of growth but significantly enhanced their expression at 8 h. Additionally, the production of QS-regulated virulence factors, including LasA and pyocyanin, was also influenced by the presence of human serum. Serum fractionation experiments revealed that part of the observed effect resides within the serum fraction containing <10-kDa proteins. Growth in serum reduced the production of many PAO1 outer membrane proteins but enhanced the production of others including OprF, a protein previously shown to play a role in the regulation of QS gene expression. These results suggest that factor(s) within human serum: 1) impact P. aeruginosa pathogenesis by influencing the expression of different genes; 2) differentially regulate the expression of QS and QS-related genes in a growth phase- or time-dependent mechanism; and 3) manipulate the production of P. aeruginosa outer membrane proteins.

Rational Drug Design for Pseudomonas aeruginosa PqsA Enzyme: An in silico Guided Study to Block Biofilm Formation

Pseudomonas aeruginosa is an opportunistic gram-negative bacterium implicated in acute and chronic nosocomial infections and a leading cause of patient mortality. Such infections occur owing to biofilm formation that confers multidrug resistance and enhanced pathogenesis to the bacterium. In this study, we used a rational drug design strategy to inhibit the quorum signaling system of P. aeruginosa by designing potent inhibitory lead molecules against anthranilate-CoA ligase enzyme encoded by the pqsA gene. This enzyme produces autoinducers for cell-to-cell communication, which result in biofilm formation, and thus plays a pivotal role in the virulence of P. aeruginosa. A library of potential drug molecules was prepared by performing ligand-based screening using an available set of enzyme inhibitors. Subsequently, structure-based virtual screening was performed to identify compounds showing the best binding conformation with the target enzyme and forming a stable complex. The two hit compounds interact with the binding site of the enzyme through multiple short-range hydrophilic and hydrophobic interactions. Molecular dynamic simulation and MM-PBSA/GBSA results to calculate the affinity and stability of the hit compounds with the PqsA enzyme further confirmed their strong interactions. The hit compounds might be useful in tackling the resistant phenotypes of this pathogen.

Bacterial chatter in chronic wound infections

One of the hallmark characteristics of chronic diabetic wounds is the presence of biofilm-forming bacteria. Bacteria encapsulated in a biofilm may coexist as a polymicrobial community and communicate with each other through a phenomenon termed quorum sensing (QS). Here, we describe the QS circuits of bacterial species commonly found in chronic diabetic wounds. QS relies on diffusion of signaling molecules and the local concentration changes of these molecules that bacteria experience in wounds. These biochemical signaling pathways play a role not only in biofilm formation and virulence but also in wound healing. They are, therefore, key to understanding the distinctive nature of these infections. While several in vivo and in vitro models exist to study QS in wounds, there has been limited progress in understanding the interplay between QS molecules and host factors that contribute to wound healing. Lastly, we examine the potential of targeting QS for both diagnosis and therapeutic intervention purposes.

Screening of Bacterial Quorum Sensing Inhibitors in a Vibrio fischeri LuxR-Based Synthetic Fluorescent E. coli Biosensor

A library of 23 pure compounds of varying structural and chemical characteristics was screened for their quorum sensing (QS) inhibition activity using a synthetic fluorescent Escherichia coli biosensor that incorporates a modified version of lux regulon of Vibrio fischeri. Four such compounds exhibited QS inhibition activity without compromising bacterial growth, namely, phenazine carboxylic acid (PCA), 2-heptyl-3-hydroxy-4-quinolone (PQS), 1H-2-methyl-4-quinolone (MOQ) and genipin. When applied at 50 µM, these compounds reduced the QS response of the biosensor to 33.7% ± 2.6%, 43.1% ± 2.7%, 62.2% ± 6.3% and 43.3% ± 1.2%, respectively. A series of compounds only showed activity when tested at higher concentrations. This was the case of caffeine, which, when applied at 1 mM, reduced the QS to 47% ± 4.2%. In turn, capsaicin, caffeic acid phenethyl ester (CAPE), furanone and polygodial exhibited antibacterial activity when applied at 1mM, and reduced the bacterial growth by 12.8% ± 10.1%, 24.4% ± 7.0%, 91.4% ± 7.4% and 97.5% ± 3.8%, respectively. Similarly, we confirmed that trans-cinnamaldehyde and vanillin, when tested at 1 mM, reduced the QS response to 68.3% ± 4.9% and 27.1% ± 7.4%, respectively, though at the expense of concomitantly reducing cell growth by 18.6% ± 2.5% and 16% ± 2.2%, respectively. Two QS natural compounds of Pseudomonas aeruginosa, namely PQS and PCA, and the related, synthetic compounds MOQ, 1H-3-hydroxyl-4-quinolone (HOQ) and 1H-2-methyl-3-hydroxyl-4-quinolone (MHOQ) were used in molecular docking studies with the binding domain of the QS receptor TraR as a target. We offer here a general interpretation of structure-function relationships in this class of compounds that underpins their potential application as alternatives to antibiotics in controlling bacterial virulence.

Bacterial Quorum Sensing During Infection

Bacteria are highly interactive and possess an extraordinary repertoire of intercellular communication and social behaviors, including quorum sensing (QS). QS has been studied in detail at the molecular level, so mechanistic details are well understood in many species and are often involved in virulence. The use of different animal host models has demonstrated QS-dependent control of virulence determinants and virulence in several human pathogenic bacteria. QS also controls virulence in several plant pathogenic species. Despite the role QS plays in virulence during animal and plant laboratory-engineered infections, QS mutants are frequently isolated from natural infections, demonstrating that the function of QS during infection and its role in pathogenesis remain poorly understood and are fruitful areas for future research. We discuss the role of QS during infection in various organisms and highlight approaches to better understand QS during human infection. This is an important consideration in an era of growing antimicrobial resistance, when we are looking for new ways to target bacterial infections.

Isolation of 2-Alkyl-4-quinolones with Unusual Side Chains from a Chinese Pseudomonas aeruginosa Isolate

Chemical investigation of a Pseudomonas aeruginosa strain isolated from Hebei, China, led to the isolation of a suite of quinolones, quinolone-N-oxides, and phenazines, the structures of which were elucidated by detailed spectroscopic analysis. Most notable among the secondary metabolites isolated was an unprecedented 4-quinolone containing an S-methyl group in the side chain and a new derivative including a phenyl ring in the side chain, which expand significantly the variety of structural motifs found in the quinolones and raise interesting questions about their biosynthesis.

Synthesis and Antimicrobial Activity of Burkholderia-Related 4-Hydroxy-3-methyl-2-alkenylquinolines (HMAQs) and Their N-Oxide Counterparts

The Burkholderia genus offers a promising potential in medicine because of the diversity of biologically active natural products encoded in its genome. Some pathogenic Burkholderia spp. biosynthesize a specific class of antimicrobial 2-alkyl-4(1H)-quinolones, i.e., 4-hydroxy-3-methyl-2-alkenylquinolines (HMAQs) and their N-oxide derivatives (HMAQNOs). Herein, we report the synthesis of a series of six HMAQs and HMAQNOs featuring a trans-Δ² double bond at the C2-alkyl chain. The quinolone scaffold was obtained via the Conrad-Limpach approach, while the (E)-2-alkenyl chain was inserted through Suzuki-Miyaura cross-coupling under microwave radiation without noticeable isomerization according to the optimized conditions. Subsequent oxidation of enolate-protected HMAQs cleanly led to the formation of HMAQNOs following cleavage of the ethyl carbonate group. Synthetic HMAQs and HMAQNOs were evaluated in vitro for their antimicrobial activity against different Gram-negative and Gram-positive bacteria as well as against molds and yeasts. The biological results support and extend the potential of HMAQs and HMAQNOs as antimicrobials, especially against Gram-positive bacteria. We also confirm the involvement of HMAQs in the autoregulation of the Hmq system in Burkholderia ambifaria.

Quorum Quenching in a Novel Acinetobacter sp. XN-10 Bacterial Strain against Pectobacterium carotovorum subsp. carotovorum

Quorum sensing (QS) is a cell density-dependent mechanism that regulates the expression of specific genes in microbial cells. Quorum quenching (QQ) is a promising strategy for attenuating pathogenicity by interfering with the QS system of pathogens. N-Acyl-homoserine lactones (AHLs) act as signaling molecules in many Gram-negative bacterial pathogens and have received wide attention. In this study, a novel, efficient AHL-degrading bacterium, Acinetobacter sp. strain XN-10, was isolated from agricultural contaminated soil and evaluated for its degradation efficiency and potential use against QS-mediated pathogens. Strain XN-10 could effectively degrade N-(3-oxohexanoyl)-L-homoserine lactone (OHHL), N-hexanoyl-L-homoserine lactone (C6HSL), N-(3-oxododecanoyl)-L-homoserine lactone (3OC12HSL), and N-(3-oxooctanoyl)-L-homoserine lactone (3OC8HSL), which all belong to the AHL family. Analysis of AHL metabolic products by gas chromatography-mass spectrometry (GC-MS) led to the identification of N-cyclohexyl-propanamide, and pentanoic acid, 4-methyl, methyl ester as the main intermediate metabolites, revealing that AHL could be degraded by hydrolysis and dehydroxylation. All intermediates were transitory and faded away without any non-cleavable metabolites at the end of the experiment. Furthermore, strain XN-10 significantly attenuated the pathogenicity of Pectobacterium carotovorum subsp. carotovorum (Pcc) to suppress tissue maceration in carrots, potatoes, and Chinese cabbage. Taken together, our results shed light on the QQ mechanism of a novel AHL-degrading bacterial isolate, and they provide useful information which show potential for biocontrol of infectious diseases caused by AHL-dependent bacterial pathogens.

Functional Diversity of Quorum Sensing Receptors in Pathogenic Bacteria: Interspecies, Intraspecies and Interkingdom Level

The formation of biofilm by pathogenic bacteria is considered as one of the most powerful mechanisms/modes of resistance against the action of several antibiotics. Biofilm is formed as a structural adherent over the surfaces of host, food and equipments etc. and is further functionally coordinated by certain chemicals produced itself. These chemicals are known as quorum sensing (QS) signaling molecules and are involved in the cross talk at interspecies, intraspecies and interkingdom levels thus resulting in the production of virulence factors leading to pathogenesis. Bacteria possess receptors to sense these chemicals, which interact with the incoming QS molecules. It is followed by the secretion of virulence molecules, regulation of bioluminescence, biofilm formation, antibiotic resistance development and motility behavioral responses. In the natural environment, different bacterial species (Gram-positive and Gram-negative) produce QS signaling molecules that are structurally and functionally different. Recent and past research shows that various antagonistic molecules (naturally and chemically synthesized) are characterized to inhibit the formation of biofilm and attenuation of bacterial virulence by blocking the QS receptors. This review article describes about the diverse QS receptors at their structural, functional and production levels. Thus, by blocking these receptors with inhibitory molecules can be a potential therapeutic approach to control pathogenesis. Furthermore, these receptors can also be used as a structural platform to screen the most potent inhibitors with the help of bioinformatics approaches.

Mechanism of pyocyanin abolishment caused by mvaT mvaU double knockout in Pseudomonas aeruginosa PAO1

MvaT and MvaU are global transcriptional regulators belonging to the H-NS family, and pyocyanin is an important virulence factor produced by Pseudomonas aeruginosa. mvaT mvaU double knockout mutant of P. aeruginosa PAO1 demonstrated pyocyanin abolishment in the previous study. Here, we further explored the mechanism. Two main directions were studied: pyocyanin biosynthesis pathway and QS system. The effect on the expression of the pyocyanin biosynthesis genes was evaluated by promoter strength determination and Real-Time PCR assay, and significant changes leading to low pyocyanin production were found. The effect on the QS system was studied by signal molecule quantification using LC-MS/MS and related gene expression measurements using Real-Time PCR. In mvaT mvaU double knockout, the production of 3-oxo-C12-HSL obviously increased, while those of C4-HSL and PQS obviously decreased, and the changes can be recovered by mvaT or mvaU complementation. The expressions of transcriptional activator genes binding with QS system signal molecules were all decreased, resulting in decreased formation of signal-transcriptional activator complexes. And the decreased expression of rhlR and pqsE also led to the lower expression of phzA1 and phzA2. Further exploration found that QS system downregulation may be related to QsrO, a QS system repressor, which was highly upregulated with mvaT mvaU double knockout. Hence, the synthesis of pyocyanin was suffocated and the biofilm formation ability was decreased. These results were also confirmed by transcriptome analysis, which demonstrated similar gene expression changes of the aforementioned genes together with decreased expression of other virulence factor genes regulated by QS system.

Review of Potential Pseudomonas Weaponry, Relevant to the Pseudomonas-Aspergillus Interplay, for the Mycology Community

Pseudomonas aeruginosa is one of the most prominent opportunistic bacteria in airways of cystic fibrosis patients and in immunocompromised patients. These bacteria share the same polymicrobial niche with other microbes, such as the opportunistic fungus Aspergillus fumigatus. Their inter-kingdom interactions and diverse exchange of secreted metabolites are responsible for how they both fare in competition for ecological niches. The outcomes of their contests likely determine persistent damage and degeneration of lung function. With a myriad of virulence factors and metabolites of promising antifungal activity, P. aeruginosa products or their derivatives may prove useful in prophylaxis and therapy against A. fumigatus. Quorum sensing underlies the primary virulence strategy of P. aeruginosa, which serves as cell–cell communication and ultimately leads to the production of multiple virulence factors. Understanding the quorum-sensing-related pathogenic mechanisms of P. aeruginosa is a first step for understanding intermicrobial competition. In this review, we provide a basic overview of some of the central virulence factors of P. aeruginosa that are regulated by quorum-sensing response pathways and briefly discuss the hitherto known antifungal properties of these virulence factors. This review also addresses the role of the bacterial secretion machinery regarding virulence factor secretion and maintenance of cell–cell communication.

Role of quorum sensing in UVA-induced biofilm formation in Pseudomonas aeruginosa

Pseudomonas aeruginosa, a versatile bacterium present in terrestrial and aquatic environments and a relevant opportunistic human pathogen, is largely known for the production of robust biofilms. The unique properties of these structures complicate biofilm eradication, because they make the biofilms very resistant to diverse antibacterial agents. Biofilm development and establishment is a complex process regulated by multiple regulatory genetic systems, among them is quorum sensing (QS), a mechanism employed by bacteria to regulate gene transcription in response to population density. In addition, environmental factors such as UVA radiation (400-315 nm) have been linked to biofilm formation. In this work, we further investigate the mechanism underlying the induction of biofilm formation by UVA, analysing the role of QS in this phenomenon. We demonstrate that UVA induces key genes of the Las and Rhl QS systems at the transcriptional level. We also report that pelA and pslA genes, which are essential for biofilm formation and whose transcription depends in part on QS, are significantly induced under UVA exposure. Finally, the results demonstrate that in a relA strain (impaired for ppGpp production), the UVA treatment does not induce biofilm formation or QS genes, suggesting that the increase of biofilm formation due to exposure to UVA in P. aeruginosa could rely on a ppGpp-dependent QS induction.

A Quorum Quenching Bacterial Isolate Contains Multiple Substrate-Inducible Genes Conferring Degradation of Diffusible Signal Factor

Quorum quenching, which disrupts quorum sensing (QS) by either degradation of QS signals or interference of signal generation or perception, is a promising strategy for the prevention and control of QS-mediated bacterial infections. Diffusible signal factor (DSF) is widely conserved in many Gram-negative bacterial pathogens. In this study, we developed an efficient method for screening of highly active DSF degradation microorganisms. Among them, Pseudomonas sp. strain HS-18 showed a superior DSF degradation activity. Bioinformatics and genetic analyses showed that at least 4 genes, designated digA to digD, encoding fatty acyl coenzyme A ligase homologues, are responsible for DSF signal degradation. Interestingly, all 4 dig genes were induced by exogenous DSF, with digA being the most significantly induced. Expression of the dig genes in Xanthomonas campestris pv. campestris markedly reduced the accumulation of endogenous DSF, decreased production of virulence factors, and attenuated bacterial virulence on host plants. Similarly, application of strain HS-18 as a biocontrol agent could substantially reduce the disease severity caused by X. campestris pv. campestris These results unveil the molecular basis of a highly efficient DSF degradation bacterial isolate and present useful genes and biocontrol agents for control of the infectious diseases caused by DSF-dependent bacterial pathogens.IMPORTANCE Diffusible signal factor (DSF) represents a family of widely conserved quorum sensing signals involved in the regulation of virulence factor production in many Gram-negative bacterial pathogens. In this study, we developed a novel and efficient method for screening highly active DSF degradation microorganisms. With this method, we identified a bacterial isolate, Pseudomonas sp. strain HS-18, with a superb DSF degradation activity. We further found that strain HS-18 contains 4 genes responsible for DSF signal degradation, and significantly, these were induced by exogenous DSF molecules. These findings unveil the molecular basis of a highly efficient DSF degradation bacterial isolate and present useful methods, genes, and agents for control of the infectious diseases caused by DSF-dependent bacterial pathogens.

Structure-based Druggability Assessment of Anti-virulence Targets from Pseudomonas aeruginosa

Antimicrobial Resistance (AMR) represents a serious threat to health and the global economy. However, interest in antibacterial drug development has decreased substantially in recent decades. Meanwhile, anti-virulence drug development has emerged as an attractive alternative to fight AMR. Although several macromolecular targets have been explored for this goal, their druggability is a vital piece of information that has been overlooked. This review explores this subject by showing how structure- based freely available in silico tools, such as PockDrug and FTMap, might be useful for designing novel inhibitors of the pyocyanin biosynthesis pathway and improving the potency/selectivity of compounds that target the Pseudomonas aeruginosa quorum sensing mechanism. The information provided by hotspot analysis, along with binding site features, reveals novel druggable targets (PhzA and PhzS) that remain largely unexplored. However, it also highlights that in silico druggability prediction tools have several limitations that might be overcome in the near future. Meanwhile, anti-virulence drug targets should be assessed by complementary methods, such as the combined use of FTMap/PockDrug, once the consensus druggability classification reduces the risk of wasting resources on undruggable proteins.

Discovery of PqsE Thioesterase Inhibitors for Pseudomonas aeruginosa Using DNA-Encoded Small Molecule Library Screening

Pseudomonas aeruginosa is a leading cause of hospital-acquired infections in the United States. PqsE, a thioesterase enzyme, is vital for virulence of P. aeruginosa, making PqsE an attractive target for inhibition. Neither the substrate nor the product of PqsE catalysis has been identified. A library of 550 million DNA-encoded drug-like small molecules was screened for those that bind to the purified PqsE protein. The structures of the bound molecules were identified by high throughput sequencing of the attached DNA barcodes. Putative PqsE binders with the strongest affinity features were examined for inhibition of PqsE thioesterase activity in vitro. The most potent inhibitors were resynthesized off DNA and examined for the ability to alter PqsE thermal melting and for PqsE thioesterase inhibition. Here, we report the synthesis, biological activity, mechanism of action, and early structure-activity relationships of a series of 2-(phenylcarbamoyl)benzoic acids that noncompetitively inhibit PqsE. A small set of analogs designed to probe initial structure-activity relationships showed increases in potency relative to the original hits, the best of which has an IC₅₀ = 5 μM. Compound refinement is required to assess their in vivo activities as the current compounds do not accumulate in the P. aeruginosa cytosol. Our strategy validates DNA-encoded compound library screening as a rapid and effective method to identify catalytic inhibitors of the PqsE protein, and more generally, for discovering binders to bacterial proteins revealed by genetic screening to have crucial in vivo activities but whose biological functions have not been well-defined.

Pseudomonas Quinolone Signal molecule PQS behaves like a B Class inhibitor at the IQ site of mitochondrial complex I

Pseudomonas aeruginosa is a Gram‐negative bacterium of the proteobacteria class, and one of the most common causes of nosocomial infections. For example, it causes chronic pneumonia in cystic fibrosis patients. Patient sputum contains 2‐heptyl‐4‐hydroxyquinoline N‐oxide [HQNO] and Pseudomonas quorum sensing molecules such as the Pseudomonas quinolone signal [PQS]. It is known that HQNO inhibits the enzyme activity of mitochondrial and bacterial complex III at the Q_i (quinone reduction) site, but the target of PQS is not known. In this work we have shown that PQS has a negative effect on mitochondrial respiration in HeLa and A549 cells. It specifically inhibits the complex I of the respiratory chain. In vitro analyses showed a partially competitive inhibition with respect to ubiquinone at the I_Q site. In competing studies with Rotenone, PQS suppressed the ROS‐promoting effect of Rotenone, which is typical for a B‐type inhibitor. Prolonged incubation with PQS also had an effect on the activity of complex III.

The third quorum-sensing system of Pseudomonas aeruginosa: Pseudomonas quinolone signal and the enigmatic PqsE protein

Pseudomonas aeruginosa is an opportunistic pathogen that produces several virulence factors such as lectin A, pyocyanin, elastase and rhamnolipids. These compounds are controlled transcriptionally by three quorum-sensing circuits, two based on the synthesis and detection of N-acyl-homoserine-lactone termed the Las and Rhl system and a third system named the Pseudomonas quinolone signal (PQS) system, which is responsible for generating 2-alkyl-4(1 h)-quinolones (AQs). The transcriptional regulator called PqsR binds to the promoter of pqsABCDE in the presence of PQS or HHQ creating a positive feedback-loop. PqsE, encoded in the operon for AQ synthesis, is a crucial protein for pyocyanin production, activating the Rhl system by a still not fully understood mechanism. In turn, the regulation of the PQS system is modulated by Las and Rhl systems, which act positively and negatively, respectively. This review focuses on the PQS system, from its discovery to its role in Pseudomonas pathogenesis, such as iron depletion and pyocyanin synthesis that involves the PqsE protein - an intriguing player of this system.

The β-Lactamase Inhibitor Boronic Acid Derivative SM23 as a New Anti-Pseudomonas aeruginosa Biofilm

Pseudomonas aeruginosa is a Gram-negative nosocomial pathogen, often causative agent of severe device-related infections, given its great capacity to form biofilm. P. aeruginosa finely regulates the expression of numerous virulence factors, including biofilm production, by Quorum Sensing (QS), a cell-to-cell communication mechanism used by many bacteria. Selective inhibition of QS-controlled pathogenicity without affecting bacterial growth may represent a novel promising strategy to overcome the well-known and widespread drug resistance of P. aeruginosa. In this study, we investigated the effects of SM23, a boronic acid derivate specifically designed as β-lactamase inhibitor, on biofilm formation and virulence factors production by P. aeruginosa. Our results indicated that SM23: (1) inhibited biofilm development and production of several virulence factors, such as pyoverdine, elastase, and pyocyanin, without affecting bacterial growth; (2) decreased the levels of 3-oxo-C₁₂-HSL and C₄-HSL, two QS-related autoinducer molecules, in line with a dampened lasR/lasI system; (3) failed to bind to bacterial cells that had been preincubated with P. aeruginosa-conditioned medium; and (4) reduced both biofilm formation and pyoverdine production by P. aeruginosa onto endotracheal tubes, as assessed by a new in vitro model closely mimicking clinical settings. Taken together, our results indicate that, besides inhibiting β-lactamase, SM23 can also act as powerful inhibitor of P. aeruginosa biofilm, suggesting that it may have a potential application in the prevention and treatment of biofilm-associated P. aeruginosa infections.

Development and bioanalytical method validation of an LC-MS/MS assay for simultaneous quantitation of 2-alkyl-4(1H)-quinolones for application in bacterial cell culture and lung tissue

Pseudomonas aeruginosa is an opportunistic pathogen that produces numerous exoproducts during infection that help it evade the host immune system and procure nutrients from the host environment. Among these products are a family of secreted 2-alkyl-4(1H)-quinolone metabolites (AQs), which exhibit a range of biological activities. Here, we describe the validation of a liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based method for quantifying multiple AQ congeners in complex biological matrices. The assay was validated for selectivity, sensitivity, linearity, accuracy, precision, carryover, dilution integrity, recovery, matrix effects, and various aspects of stability (freeze-thaw, bench-top, long-term storage, and autosampler/post-preparative). Using authentic standards for 6 distinct AQ congeners, we report accurate quantitation within a linear range between 25 and 1000 nmol/L for all of the validated AQ standards. This method was successfully applied to quantify AQ concentrations in P. aeruginosa cell culture and in the lungs of mice infected with P. aeruginosa. Further, we confirmed the presence of unsaturated forms of several AQ congeners in cell culture.

Clinical significance of Pseudomonas aeruginosa 2-alkyl-4-quinolone quorum-sensing signal molecules for long-term outcomes in adults with cystic fibrosis

Introduction. Pseudomonas aeruginosa is an important respiratory pathogen in cystic fibrosis (CF), which is associated with an accelerated decline in lung function, frequent pulmonary exacerbations and increased mortality. P. aeruginosa produces intercellular signalling molecules including 2-alkyl-4-quinolones (AQs), which regulate virulence-factor production and biofilm formation in the CF airways. Studies have shown that AQs are detectable in the sputum and plasma of adults with CF and chronic pulmonary P. aeruginosa.Aim. We tested the hypothesis that the presence of six AQs in plasma or sputum obtained from adults with CF was associated with long-term adverse clinical outcomes.Methodology. We analysed clinical data over an 8 year follow period for 90 people with CF who had previously provided samples for AQ analysis at clinical stability. The primary outcome was all cause mortality or lung transplantation. Secondary outcomes were the rate of lung-function decline and the number of intravenous (IV) antibiotic days for pulmonary exacerbations.Results. There was no statistical association between the presence of any of the six measured AQs and the primary outcomes or the secondary outcome of decline in lung function. One of the six AQs was associated with IV antibiotic usage. The presence of 2-nonyl-3-hydroxy-4(1 h)-quinolone (C9-PQS) in sputum was associated with an increase in the number of IV antibiotic days in the follow-up period (Mann-Whitney; P=0.011).Conclusion. Further investigation to confirm the hypothesis that C9-PQS may be associated with increased antibiotic usage for pulmonary exacerbations is warranted as AQ-dependent signalling is a potential future target for anti-virulence therapies.

Flexible Fragment Growing Boosts Potency of Quorum-Sensing Inhibitors against Pseudomonas aeruginosa Virulence

Hit-to-lead optimization is a critical phase in drug discovery. Herein, we report on the fragment-based discovery and optimization of 2-aminopyridine derivatives as a novel lead-like structure for the treatment of the dangerous opportunistic pathogen Pseudomonas aeruginosa. We pursue an innovative treatment strategy by interfering with the Pseudomonas quinolone signal (PQS) quorum sensing (QS) system leading to an abolishment of bacterial pathogenicity. Our compounds act on the PQS receptor (PqsR), a key transcription factor controlling the expression of various pathogenicity determinants. In this target-driven approach, we made use of biophysical screening via surface plasmon resonance (SPR) followed by isothermal titration calorimetry (ITC)-enabled enthalpic efficiency (EE) evaluation. Hit optimization then involved growth vector identification and exploitation. Astonishingly, the latter was successfully achieved by introducing flexible linkers rather than rigid motifs leading to a boost in activity on the target receptor and anti-virulence potency.

Synthesis and Biological Evaluation of Novel L-Homoserine Lactone Analogs as Quorum Sensing Inhibitors of Pseudomonas aeruginosa

In this study, we synthesized four series of novel L-homoserine lactone analogs and evaluated their in vitro quorum sensing (QS) inhibitory activity against two biomonitor strains, Chromobacterium violaceum CV026 and Pseudomonas aeruginosa PAO1. Studies of the structure-activity relationships of the set of L-homoserine lactone analogs indicated that phenylurea-containing N-dithiocarbamated homoserine lactones are more potent than (Z)-4-bromo-5-(bromomethylene)-2(5H)-furanone (C30), a positive control for biofilm formation. In particular, compared with C30, QS inhibitor 11f significantly reduced the production of virulence factors (pyocyanin, elastase and rhamnolipid), swarming motility, the formation of biofilm and the mRNA level of QS-related genes regulated by the QS system of PAO1. These results reveal 11f as a potential lead compound for developing novel antibacterial quorum sensing inhibitors.

Differential Modulation of Quorum Sensing Signaling through QslA in Pseudomonas aeruginosa Strains PAO1 and PA14

Two clinical isolates of the opportunist pathogen Pseudomonas aeruginosa named PAO1 and PA14 are commonly studied in research laboratories. Despite the isolates being closely related, PA14 exhibits increased virulence compared to that of PAO1 in various models. To determine which players are responsible for the hypervirulence phenotype of the PA14 strain, we elected a transcriptomic approach through RNA sequencing. We found 2,029 genes that are differentially expressed between the two strains, including several genes that are involved with or regulated by quorum sensing (QS), known to control most of the virulence factors in P. aeruginosa Among them, we chose to focus our study on QslA, an antiactivator of QS whose expression was barely detectable in the PA14 strain according our data. We hypothesized that lack of expression of qslA in PA14 could be responsible for higher QS expression in the PA14 strain, possibly explaining its hypervirulence phenotype. After confirming that QslA protein was highly produced in PAO1 but not in the PA14 strain, we obtained evidence showing that a PAO1 deletion strain of qslA has faster QS gene expression kinetics than PA14. Moreover, known virulence factors activated by QS, such as (i) pyocyanin production, (ii) H2-T6SS (type VI secretion system) gene expression, and (iii) Xcp-T2SS (type II secretion system) machinery production and secretion, were all lower in PAO1 than in PA14, due to higher qslA expression. However, biofilm formation and cytotoxicity toward macrophages, although increased in PA14 compared to PAO1, were independent of QslA control. Together, our findings implicated differential qslA expression as a major determinant of virulence factor expression in P. aeruginosa strains PAO1 and PA14.IMPORTANCEPseudomonas aeruginosa is an opportunistic pathogen responsible for acute nosocomial infections and chronic pulmonary infections. P. aeruginosa strain PA14 is known to be hypervirulent in different hosts. Despite several studies in the field, the underlining molecular mechanisms sustaining this phenotype remain enigmatic. Here we provide evidence that the PA14 strain has faster quorum sensing (QS) kinetics than the PAO1 strain, due to the lack of QslA expression, an antiactivator of QS. QS is a major regulator of virulence factors in P. aeruginosa; therefore, we propose that the hypervirulent phenotype of the PA14 strain is, at least partially, due to the lack of QslA expression. This mechanism could be of great importance, as it could be conserved among other P. aeruginosa isolates.

Synthesis of New Fused Heterocyclic 2-Quinolones and 3-Alkanonyl-4-Hydroxy-2-Quinolones

Herein, we report the synthesis of 5,12-dihydropyrazino[2,3-c:5,6-c′]difuro[2,3-c:4,5-c′]-diquinoline-6,14(5H,12H)diones, 2-(4-hydroxy-2-oxo-1,2-dihydroquinolin-3-yl)-1,4-diphenyl- butane-1,4-diones and 4-(benzo-[d]oxazol-2-yl)-3-hydroxy-1H-[4,5]oxazolo[3,2-a]pyridine-1-one. The new candidates were synthesized and identified by different spectroscopic techniques, and X-ray crystallography.

Beyond nitrogen metabolism: nitric oxide, cyclic-di-GMP and bacterial biofilms

The nitrogen cycle pathways are responsible for the circulation of inorganic and organic N-containing molecules in nature. Among these pathways, those involving amino acids, N-oxides and in particular nitric oxide (NO) play strategic roles in the metabolism of microorganisms in natural environments and in host-pathogen interactions. Beyond their role in the N-cycle, amino acids and NO are also signalling molecules able to influence group behaviour in microorganisms and cell-cell communication in multicellular organisms, including humans. In this minireview, we summarise the role of these compounds in the homeostasis of the bacterial communities called biofilms, commonly found in environmental, industrial and medical settings. Biofilms are difficult to eradicate since they are highly resistant to antimicrobials and to the host immune system. We highlight the effect of amino acids such as glutamate, glutamine and arginine and of NO on the signalling pathways involved in the metabolism of 3',5'-cyclic diguanylic acid (c-di-GMP), a master regulator of motility, attachment and group behaviour in bacteria. The study of the metabolic routes involving these N-containing compounds represents an attractive topic to identify targets for biofilm control in both natural and medical settings.

Small Is Mighty—Chemical Communication Systems in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes a variety of acute and chronic infections. Usually a commensal on the host body, P. aeruginosa is capable of transforming into a virulent pathogen upon sensing favorable changes in the host immune system or stress cues. P. aeruginosa infections are hard to eradicate, because this pathogen has developed strong resistance to most conventional antibiotics; in addition, in chronic infections it commonly forms a biofilm matrix, which provides bacterial cells a protected environment to withstand various stresses including antibiotics. Given its importance as a human pathogen and its notorious antimicrobial tolerance, P. aeruginosa has been the subject of intensive investigations internationally. Research progress over the last two decades has unveiled a range of chemical communication systems in this pathogen. These diversified chemical communication systems endow P. aeruginosa a superb ability and remarkable flexibility to coordinate and modulate accordingly the transcriptional expression of various sets of genes associated with virulence and other physiologic activities in response to environmental changes. A fair understanding of the chemical signaling mechanisms with which P. aeruginosa governs virulence gene expression may hold the key to developing alternative therapeutic interventions that control and prevent bacterial infections.