Transcription of quorum-sensing system genes in clinical and environmental isolates of Pseudomonas aeruginosa

Bacteriophages and Green Synthesized Zinc Oxide Nanoparticles in Combination Are Efficient against Biofilm Formation of Pseudomonas aeruginosa

Bacteriophages (phages) are viruses that infect the bacteria within which their reproduction cycle takes place, a process that ends in the lysis and death of the bacterial cell. Some phages are also able to destroy bacterial biofilms. Due to increased antibiotics resistance, Pseudomonas aeruginosa, another biofilm-forming pathogen, is a problem in many parts of the world. Zinc oxide (ZnO) and other metal nanoparticles (NPs) are biologically active and also possess anti-biofilm properties. ZnO-NPs were prepared by the green synthesis method using orange peels. The vibrational peaks of the ZnO-NPs were analyzed using FTIR analysis, and their size and morphological properties were determined using scanning electron microscopy (SEM). The ability of the ZnO-NPs to reduce or eliminate P. aeruginosa biofilm alone or in combination with phages PB10 and PA19 was investigated. The P. aeruginosa cells were effectively killed in the preformed 48 h biofilms during a 24 h incubation with the ZnO-NP-phage combination, in comparison with the control or ZnO-NPs alone. The treatments on growing biofilms were most efficient in the final stages of biofilm development. All five treatment groups showed a significant biofilm reduction compared to the control group (p < 0.0001) at 48 h of incubation. The influence of the ZnO-NPs and phages on the quorum sensing system of P. aeruginosa was monitored by quantitative real-time PCR (qRT-PCR) of the autoinducer biosynthesis gene lasI. While the ZnO-NPs repressed the lasI gene transcription, the phages slightly activated it at 24 and 48 h of incubation. Also, the effect of the ZnO-NPs and phage PA19 on the viability of HFF2 cells was investigated and the results showed that the combination of NPs with PA19 reduced the toxic effect of ZnO-NPs and also stimulated the growth in normal cells.

Pseudomonas aeruginosa from river water: antimicrobial resistance, virulence and molecular typing

Pseudomonas aeruginosa isolates were recovered from surface river water samples in La Rioja region (Spain) to characterise their antibiotic resistance, molecular typing and virulence mechanisms. Fifty-two P. aeruginosa isolates were isolated from 15 different water samples (45.4%) and belonged to 23 different pulsed-field electrophoresis (PFGE) patterns. All isolates were susceptible to all antibiotics tested, except one carbapenem-resistant P. aeruginosa that showed a premature stop codon in OprD porin. Twenty-two sequence types (STs) (six new ones) were detected among 29 selected P. aeruginosa (one strain with a different PFGE pattern per sample), with ST274 (14%) being the most frequent one. O:6 and O:3 were the predominant serotypes (31%). Seven virulotypes were detected, being 59% exoS-exoY-exoT-exoA-lasA-lasB-lasI-lasR-rhlAB-rhlI-rhlR-aprA-positive P. aeruginosa. It is noteworthy that the exlA gene was identified in three strains (10.3%), and the exoU gene in seven (24.1%), exoS in 18 (62.1%), and both exoS and exoU genes in one strain. High motility ranges were found in these strains. Twenty-seven per cent of strains produced more biofilm biomass, 90% more pyorubin, 83% more pyocyanin and 65.5% more than twice the elastase activity compared with the PAO1 strain. These results highlight the importance of rivers as temporary reservoirs and sources of P. aeruginosa transmission, and show the importance of their epidemiological surveillance in the environment.

The end of the reign of a "master regulator''? A defect in function of the LasR quorum sensing regulator is a common feature of Pseudomonas aeruginosa isolates

Pseudomonas aeruginosa, a bacterium causing infections in immunocompromised individuals, regulates several of its virulence functions using three interlinked quorum sensing (QS) systems (las, rhl, and pqs). Despite its presumed importance in regulating virulence, dysfunction of the las system regulator LasR occurs frequently in strains isolated from various environments, including clinical infections. This newfound abundance of LasR-defective strains calls into question existing hypotheses regarding their selection. Indeed, current assumptions concerning factors driving the emergence of LasR-deficient isolates and the role of LasR in the QS hierarchy must be reconsidered. Here, we propose that LasR is not the primary master regulator of QS in all P. aeruginosa genetic backgrounds, even though it remains ecologically significant. We also revisit and complement current knowledge on the ecology of LasR-dependent QS in P. aeruginosa, discuss the hypotheses explaining the putative adaptive benefits of selecting against LasR function, and consider the implications of this renewed understanding.

Molecular Aspects of the Functioning of Pathogenic Bacteria Biofilm Based on Quorum Sensing (QS) Signal-Response System and Innovative Non-Antibiotic Strategies for Their Elimination

One of the key mechanisms enabling bacterial cells to create biofilms and regulate crucial life functions in a global and highly synchronized way is a bacterial communication system called quorum sensing (QS). QS is a bacterial cell-to-cell communication process that depends on the bacterial population density and is mediated by small signalling molecules called autoinducers (AIs). In bacteria, QS controls the biofilm formation through the global regulation of gene expression involved in the extracellular polymeric matrix (EPS) synthesis, virulence factor production, stress tolerance and metabolic adaptation. Forming biofilm is one of the crucial mechanisms of bacterial antimicrobial resistance (AMR). A common feature of human pathogens is the ability to form biofilm, which poses a serious medical issue due to their high susceptibility to traditional antibiotics. Because QS is associated with virulence and biofilm formation, there is a belief that inhibition of QS activity called quorum quenching (QQ) may provide alternative therapeutic methods for treating microbial infections. This review summarises recent progress in biofilm research, focusing on the mechanisms by which biofilms, especially those formed by pathogenic bacteria, become resistant to antibiotic treatment. Subsequently, a potential alternative approach to QS inhibition highlighting innovative non-antibiotic strategies to control AMR and biofilm formation of pathogenic bacteria has been discussed.

Collective decision-making in Pseudomonas aeruginosa involves transient segregation of quorum-sensing activities across cells

A hallmark of bacterial sociality is that groups can coordinate cooperative actions through a cell-to-cell communication process called quorum sensing (QS). QS regulates key bacterial phenotypes such as virulence in infections and digestion of extracellular compounds in the environment. Although QS responses are typically studied as group-level phenotypes, it is unclear whether individuals coordinate their actions at the single-cell level or whether group phenotypes simply reflect the sum of their noisy members. Here, we studied the behavior of Pseudomonas aeruginosa individuals by tracking their temporal commitments to the two intertwined Las and Rhl-QS systems, from low to high population density. Using chromosomally integrated fluorescent gene reporters, we found that QS gene expression (signal, receptor, and cooperative exoproduct) was noisy with heterogeneity peaking during the build-up phase of QS. Moreover, we observed the formation of discrete subgroups of cells that transiently segregate into two gene expression states: low Las-receptor expressers that instantly activate exoproduct production and high Las-receptor expressers with delayed exoproduct production. Later, gene expression activities converged with all cells fully committing to QS. We developed general mathematical models to show that gene expression segregation can mechanistically be spurred by molecular resource limitations during the initiation phase of regulatory cascades such as QS. Moreover, our models indicate that gene expression segregation across cells can operate as a built-in brake enabling a temporary bet-hedging strategy in unpredictable environments. Altogether, our work reveals that studying the behavior of bacterial individuals is key to understanding emergent collective actions at the group level.

Quorum Sensing Promotes Phage Infection in Pseudomonas aeruginosa PAO1

Quorum sensing (QS) is used to coordinate social behaviors, such as virulence and biofilm formation, across bacterial populations. However, the role of QS in regulating phage-bacterium interactions remains unclear. Preventing phage recognition and adsorption are the first steps of bacterial defense against phages; however, both phage recognition and adsorption are a prerequisite for the successful application of phage therapy. In the present study, we report that QS upregulated the expression of phage receptors, thus increasing phage adsorption and infection rates in Pseudomonas aeruginosa. In P. aeruginosa PAO1, we found that las QS, instead of rhl QS, upregulated the expression of galU for lipopolysaccharide synthesis. Lipopolysaccharides act as the receptor of the phage vB_Pae_QDWS. This las QS-mediated phage susceptibility is a dynamic process, depending on host cell density. Our data suggest that inhibiting QS may reduce the therapeutic efficacy of phages. IMPORTANCE Phage resistance is a major limitation of phage therapy, and understanding the mechanisms by which bacteria block phage infection is critical for the successful application of phage therapy. In the present study, we found that Pseudomonas aeruginosa PAO1 uses las QS to promote phage infection by upregulating the expression of galU, which is necessary for the synthesis of phage receptor lipopolysaccharides. In contrast to the results of previous reports, we showed that QS increases the efficacy of phage-mediated bacterial killing. Since QS upregulates the expression of virulence factors and promotes biofilm development, which are positively correlated with lipopolysaccharide production in P. aeruginosa, increased phage susceptibility is a novel QS-mediated trade-off. QS inhibition may increase the efficacy of antibiotic treatment, but it will reduce the effectiveness of phage therapy.

Evolution of the Quorum Sensing Regulon in Cooperating Populations of Pseudomonas aeruginosa

In the opportunistic pathogenic bacterium Pseudomonas aeruginosa acyl-homoserine lactone quorum sensing (QS) can activate expression of dozens to hundreds of genes depending on the strain under investigation. Many QS-activated genes code for extracellular products. P. aeruginosa has become a model for studies of cell-cell communication and coordination of cooperative activities, which result from production of extracellular products. We hypothesized that strain variation in the size of the QS regulon might reflect the environmental history of an isolate. We tested the hypothesis by performing long-term growth experiments with the well-studied strain PAO1, which has a relatively large QS regulon, under conditions where only limited QS-controlled functions are required. We grew P. aeruginosa for about 1000 generations in a condition where expression of QS-activated genes was required, and emergence of QS mutants was constrained and compared the QS regulons of populations after 35 generations to those after about 1000 generations in two independent lineages by using quorum quenching and RNA-seq technology. In one lineage the number of QS-activated genes identified was reduced by over 60% and in the other by about 30% in 1000-generation populations compared to 35-generation populations. Our results provide insight about the variations in the number of QS-activated genes reported for different P. aeruginosa environmental and clinical isolates and, about how environmental conditions might influence social evolution. IMPORTANCE Pseudomonas aeruginosa uses quorum sensing (QS) to activate expression of dozens of genes (the QS regulon). Because there is strain-to-strain variation in the size and content of the QS regulon, we asked how the regulon might evolve during long-term P. aeruginosa growth when cells require some but not all the functions activated by QS. We demonstrate that the P. aeruginosa QS-regulon can undergo a reductive adaptation in response to continuous QS-dependent growth. Our results provide insights into why there is strain-to-strain variability in the size and content of the P. aeruginosa QS regulon.

Recent Advance in Small Molecules Targeting RhlR of Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic gram-negative pathogen that can cause various infections, particularly in patients with compromised host defenses. P. aeruginosa forms biofilms and produces virulence factors through quorum sensing (QS) network, resulting in resistance to antibiotics. RhlI/RhlR, one of key QS systems in P. aeruginosa, is considered an attractive target for inhibiting biofilm formation and attenuating virulence factors. Several recent studies examined small molecules targeting the RhlI/RhlR system and their in vitro and in vivo biological activities. In this review, RhlR-targeted modulators, including agonists and antagonists, are discussed with particular focus on structure-activity relationship studies and outlook for next-generation anti-biofilm agents.

Combined Treatment of 6-Gingerol Analog and Tobramycin for Inhibiting Pseudomonas aeruginosa Infections

Pseudomonas aeruginosa is a ubiquitous human pathogen that causes severe infections. Although antibiotics, such as tobramycin, are currently used for infection therapy, their antibacterial activity has resulted in the emergence of multiple antibiotic-resistant bacteria. The 6-gingerol analog, a structural derivative of the main component of ginger, is a quorum sensing (QS) inhibitor. However, it has a lower biofilm inhibitory activity than antibiotics and the possibility to cause toxicity in humans. Therefore, novel and more effective approaches for decreasing dosing concentration and increasing biofilm inhibitory activity are required to alleviate P. aeruginosa infections. In this study, a 6-gingerol analog was combined with tobramycin to treat P. aeruginosa infections. The combined treatment of 6-gingerol analog and tobramycin showed strong inhibitory activities on biofilm formation and the production of QS-related virulence factors of P. aeruginosa compared to single treatments. Furthermore, the combined treatment alleviated the infectivity of P. aeruginosa in an insect model using Tenebrio molitor larvae without inducing any cytotoxic effects in human lung epithelial cells. The 6-gingerol analog showed these inhibitory activities at much lower concentrations when used in combination with tobramycin. Adjuvant effects were observed through increased QS-disrupting processes rather than through antibacterial action. In particular, improved RhlR inactivation by this combination is a possible target for therapeutic development in LasR-independent chronic infections. Therefore, the combined treatment of 6-gingerol analog and tobramycin may be considered an effective method for treating P. aeruginosa infections. IMPORTANCE Pseudomonas aeruginosa is a pathogen that causes various infectious diseases through quorum-sensing regulation. Although antibiotics are mainly used to treat P. aeruginosa infections, they cause the emergence of resistant bacteria in humans. To compensate for the disadvantages of antibiotics and increase their effectiveness, natural products were used in combination with antibiotics in this study. We discovered that combined treatment with 6-gingerol analog from naturally-derived ginger substances and tobramycin resulted in more effective reductions of biofilm formation and virulence factor production in P. aeruginosa than single treatments. Our findings support the notion that when 6-gingerol analog is combined with tobramycin, the effects of the analog can be exerted at much lower concentrations. Furthermore, its improved LasR-independent RhlR inactivation may serve as a key target for therapeutic development in chronic infections. Therefore, the combined treatment of 6-gingerol analog and tobramycin is suggested as a novel alternative for treating P. aeruginosa infections.

Pseudomonas aeruginosa isolates defective in function of the LasR quorum sensing regulator are frequent in diverse environmental niches

The saprophyte Pseudomonas aeruginosa is a versatile opportunistic pathogen causing infections in immunocompromised individuals. To facilitate its adaptation to a large variety of niches, this bacterium exploits population density-dependent gene regulation systems called quorum sensing (QS). In P. aeruginosa, three distinct but interrelated QS systems (las, rhl and pqs) regulate the production of many survival and virulence functions. In prototypical strains, the las system, through its transcriptional regulator LasR, is important for the full activation of the rhl and pqs systems. Still, LasR-deficient isolates have been reported, mostly sampled from the lungs of people with cystic fibrosis, where they are considered selected by the chronic infection environment. In this study, we show that a defect in LasR activity appears to be an actually widespread mechanism of adaptation in this bacterium. Indeed, we found abundant LasR-defective isolates sampled from hydrocarbon-contaminated soils, hospital sink drains and meat/fish market environments, using an approach based on phenotypic profiling, supported by gene sequencing. Interestingly, several LasR-defective isolates maintain an active rhl system or are deficient in pqs system signalling. The high prevalence of a LasR-defective phenotype among environmental P. aeruginosa isolates questions the role of QS in niche adaptation.

High-Throughput Approaches for the Identification of Pseudomonas aeruginosa Antivirulents

Antimicrobial resistance is a serious medical threat, particularly given the decreasing rate of discovery of new treatments. Although attempts to find new treatments continue, it has become clear that merely discovering new antimicrobials, even if they are new classes, will be insufficient. It is essential that new strategies be aggressively pursued. Toward that end, the search for treatments that can mitigate bacterial virulence and tilt the balance of host-pathogen interactions in favor of the host has become increasingly popular. In this review, we will discuss recent progress in this field, with a special focus on synthetic small molecule antivirulents that have been identified from high-throughput screens and on treatments that are effective against the opportunistic human pathogen Pseudomonas aeruginosa.

Host cystathionine-γ lyase derived hydrogen sulfide protects against Pseudomonas aeruginosa sepsis

Hydrogen sulfide (H2S) has recently been recognized as a novel gaseous transmitter with several anti-inflammatory properties. The role of host- derived H2S in infections by Pseudomonas aeruginosa was investigated in clinical and mouse models. H2S concentrations and survival was assessed in septic patients with lung infection. Animal experiments using a model of severe systemic multidrug-resistant P. aeruginosa infection were performed using mice with a constitutive knock-out of cystathionine-γ lyase (Cse) gene (Cse-/-) and wild-type mice with a physiological expression (Cse+/+). Experiments were repeated in mice after a) treatment with cyclophosphamide; b) bone marrow transplantation (BMT) from a Cse+/+ donor; c) treatment with H2S synthesis inhibitor aminooxyacetic acid (ΑΟΑΑ) or propargylglycine (PAG) and d) H2S donor sodium thiosulfate (STS) or GYY3147. Bacterial loads and myeloperoxidase activity were measured in tissue samples. The expression of quorum sensing genes (QS) was determined in vivo and in vitro. Cytokine concentration was measured in serum and incubated splenocytes. Patients survivors at day 28 had significantly higher serum H2S compared to non-survivors. A cut- off point of 5.3 μΜ discriminated survivors with sensitivity 92.3%. Mortality after 28 days was 30.9% and 93.7% in patients with H2S higher and less than 5.3 μΜ (p = 7 x 10-6). In mice expression of Cse and application of STS afforded protection against infection with multidrug-resistant P. aeruginosa. Cyclophosphamide pretreatment eliminated the survival benefit of Cse+/+ mice, whereas BMT increased the survival of Cse-/- mice. Cse-/- mice had increased pathogen loads compared to Cse+/+ mice. Phagocytic activity of leukocytes from Cse-/- mice was reduced but was restored after H2S supplementation. An H2S dependent down- regulation of quorum sensing genes of P.aeruginosa could be demonstrated in vivo and in vitro. Endogenous H2S is a potential independent parameter correlating with the outcome of P. aeruginosa. H2S provides resistance to infection by MDR bacterial pathogens.

Intraspecies Signaling between Common Variants of Pseudomonas aeruginosa Increases Production of Quorum-Sensing-Controlled Virulence Factors

The opportunistic pathogen Pseudomonas aeruginosa damages hosts through the production of diverse secreted products, many of which are regulated by quorum sensing (QS). The lasR gene, which encodes a central QS regulator, is frequently mutated in clinical isolates from chronic infections, and loss of LasR function (LasR-) generally impairs the activity of downstream QS regulators RhlR and PqsR. We found that in cocultures containing LasR+ and LasR- strains, LasR- strains hyperproduce the RhlR/RhlI-regulated antagonistic factors pyocyanin and rhamnolipids in diverse models and media and in different strain backgrounds. Diffusible QS autoinducers produced by the wild type were not required for this effect. Using transcriptomics, genetics, and biochemical approaches, we uncovered a reciprocal interaction between wild-type and lasR mutant pairs wherein the iron-scavenging siderophore pyochelin produced by the lasR mutant induced citrate release and cross-feeding from the wild type. Citrate, a metabolite often secreted in low iron environments, stimulated RhlR signaling and RhlI levels in LasR-but not in LasR+ strains. These studies reveal the potential for complex interactions between recently diverged, genetically distinct isolates within populations from single chronic infections.IMPORTANCE Coculture interactions between lasR loss-of-function and LasR+ Pseudomonas aeruginosa strains may explain the worse outcomes associated with the presence of LasR- strains. More broadly, this report illustrates how interactions within a genotypically diverse population, similar to those that frequently develop in natural settings, can promote unpredictably high virulence factor production.

PqsE Is Essential for RhlR-Dependent Quorum Sensing Regulation in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a versatile bacterium found in various environments. It can cause severe infections in immunocompromised patients and naturally resists many antibiotics. The World Health Organization listed it among the top priority pathogens for research and development of new antimicrobial compounds. Quorum sensing (QS) is a cell-cell communication mechanism, which is important for P. aeruginosa adaptation and pathogenesis. Here, we validate the central role of the PqsE protein in QS particularly by its impact on the regulator RhlR. This study challenges the traditional dogmas of QS regulation in P. aeruginosa and ties loose ends in our understanding of the traditional QS circuit by confirming RhlR to be the main QS regulator in P. aeruginosa. PqsE could represent an ideal target for the development of new control methods against the virulence of P. aeruginosa. This is especially important when considering that LasR-defective mutants frequently arise, e.g., in chronic infections.

The bacterium Pseudomonas aeruginosa has emerged as a central threat in health care settings and can cause a large variety of infections. It expresses an arsenal of virulence factors and a diversity of survival functions, many of which are finely and tightly regulated by an intricate circuitry of three quorum sensing (QS) systems. The las system is considered at the top of the QS hierarchy and activates the rhl and pqs systems. It is composed of the LasR transcriptional regulator and the LasI autoinducer synthase, which produces 3-oxo-C₁₂-homoserine lactone (3-oxo-C₁₂-HSL), the ligand of LasR. RhlR is the transcriptional regulator for the rhl system and is associated with RhlI, which produces its cognate autoinducer C₄-HSL. The third QS system is composed of the pqsABCDE operon and the MvfR (PqsR) regulator. PqsABCD synthetize 4-hydroxy-2-alkylquinolines (HAQs), which include ligands activating MvfR. PqsE is not required for HAQ production and instead is associated with the expression of genes controlled by the rhl system. While RhlR is often considered the main regulator of rhlI, we confirmed that LasR is in fact the principal regulator of C₄-HSL production and that RhlR regulates rhlI and production of C₄-HSL essentially only in the absence of LasR by using liquid chromatography-mass spectrometry quantifications and gene expression reporters. Investigating the expression of RhlR targets also clarified that activation of RhlR-dependent QS relies on PqsE, especially when LasR is not functional. This work positions RhlR as the key QS regulator and points to PqsE as an essential effector for full activation of this regulation.

IMPORTANCEPseudomonas aeruginosa is a versatile bacterium found in various environments. It can cause severe infections in immunocompromised patients and naturally resists many antibiotics. The World Health Organization listed it among the top priority pathogens for research and development of new antimicrobial compounds. Quorum sensing (QS) is a cell-cell communication mechanism, which is important for P. aeruginosa adaptation and pathogenesis. Here, we validate the central role of the PqsE protein in QS particularly by its impact on the regulator RhlR. This study challenges the traditional dogmas of QS regulation in P. aeruginosa and ties loose ends in our understanding of the traditional QS circuit by confirming RhlR to be the main QS regulator in P. aeruginosa. PqsE could represent an ideal target for the development of new control methods against the virulence of P. aeruginosa. This is especially important when considering that LasR-defective mutants frequently arise, e.g., in chronic infections.

Itaconimides as Novel Quorum Sensing Inhibitors of Pseudomonas aeruginosa

Pseudomonas aeruginosa is known as an opportunistic pathogen that often causes persistent infections associated with high level of antibiotic-resistance and biofilms formation. Chemical interference with bacterial cell-to-cell communication, termed quorum sensing (QS), has been recognized as an attractive approach to control infections and address the drug resistance problems currently observed worldwide. Instead of imposing direct selective pressure on bacterial growth, the right bioactive compounds can preferentially block QS-based communication and attenuate cascades of bacterial gene expression and production of virulence factors, thus leading to reduced pathogenicity. Herein, we report on the potential of itaconimides as quorum sensing inhibitors (QSI) of P. aeruginosa. An initial hit was discovered in a screening program of an in-house compound collection, and subsequent structure-activity relationship (SAR) studies provided analogs that could reduce expression of central QS-regulated virulence factors (elastase, rhamnolipid, and pyocyanin), and also successfully lead to the eradication of P. aeruginosa biofilms in combination with tobramycin. Further studies on the cytotoxicity of compounds using murine macrophages indicated no toxicity at common working concentrations, thereby pointing to the potential of these small molecules as promising entities for antimicrobial drug development.

Chlorate Specifically Targets Oxidant-Starved, Antibiotic-Tolerant Populations of Pseudomonas aeruginosa Biofilms

The anaerobic growth and survival of bacteria are often correlated with physiological tolerance to conventional antibiotics, motivating the development of novel strategies targeting pathogens in anoxic environments. A key challenge is to identify drug targets that are specific to this metabolic state. Chlorate is a nontoxic compound that can be reduced to toxic chlorite by a widespread enzyme of anaerobic metabolism. We tested the antibacterial properties of chlorate against Pseudomonas aeruginosa, a pathogen that can inhabit hypoxic or anoxic microenvironments, including those that arise in human infection. Chlorate and the antibiotic tobramycin kill distinct metabolic populations in P. aeruginosa biofilms, where chlorate targets anaerobic cells that tolerate tobramycin. Chlorate is particularly effective against P. aeruginosalasR mutants, which are frequently isolated from human infections and more resistant to some antibiotics. This work suggests that chlorate may hold potential as an anaerobic prodrug.

Nitrate respiration is a widespread mode of anaerobic energy generation used by many bacterial pathogens, and the respiratory nitrate reductase, Nar, has long been known to reduce chlorate to the toxic oxidizing agent chlorite. Here, we demonstrate the antibacterial activity of chlorate against Pseudomonas aeruginosa, a representative pathogen that can inhabit hypoxic or anoxic host microenvironments during infection. Aerobically grown P. aeruginosa cells are tobramycin sensitive but chlorate tolerant. In the absence of oxygen or an alternative electron acceptor, cells are tobramycin tolerant but chlorate sensitive via Nar-dependent reduction. The fact that chlorite, the product of chlorate reduction, is not detected in culture supernatants suggests that it may react rapidly and be retained intracellularly. Tobramycin and chlorate target distinct populations within metabolically stratified aggregate biofilms; tobramycin kills cells on the oxic periphery, whereas chlorate kills hypoxic and anoxic cells in the interior. In a matrix populated by multiple aggregates, tobramycin-mediated death of surface aggregates enables deeper oxygen penetration into the matrix, benefiting select aggregate populations by increasing survival and removing chlorate sensitivity. Finally, lasR mutants, which commonly arise in P. aeruginosa infections and are known to withstand conventional antibiotic treatment, are hypersensitive to chlorate. A lasR mutant shows a propensity to respire nitrate and reduce chlorate more rapidly than the wild type does, consistent with its heightened chlorate sensitivity. These findings illustrate chlorate’s potential to selectively target oxidant-starved pathogens, including physiological states and genotypes of P. aeruginosa that represent antibiotic-tolerant populations during infections.

Options and Limitations in Clinical Investigation of Bacterial Biofilms

Bacteria can form single- and multispecies biofilms exhibiting diverse features based upon the microbial composition of their community and microenvironment. The study of bacterial biofilm development has received great interest in the past 20 years and is motivated by the elegant complexity characteristic of these multicellular communities and their role in infectious diseases. Biofilms can thrive on virtually any surface and can be beneficial or detrimental based upon the community's interplay and the surface. Advances in the understanding of structural and functional variations and the roles that biofilms play in disease and host-pathogen interactions have been addressed through comprehensive literature searches. In this review article, a synopsis of the methodological landscape of biofilm analysis is provided, including an evaluation of the current trends in methodological research. We deem this worthwhile because a keyword-oriented bibliographical search reveals that less than 5% of the biofilm literature is devoted to methodology. In this report, we (i) summarize current methodologies for biofilm characterization, monitoring, and quantification; (ii) discuss advances in the discovery of effective imaging and sensing tools and modalities; (iii) provide an overview of tailored animal models that assess features of biofilm infections; and (iv) make recommendations defining the most appropriate methodological tools for clinical settings.

Pseudomonas aeruginosa Isolates from Spanish Children: Occurrence in Faecal Samples, Antimicrobial Resistance, Virulence, and Molecular Typing

Pseudomonas aeruginosa is a major opportunistic human pathogen, responsible for nosocomial infections and infections in patients with impaired immune systems. Little data exist about the faecal colonisation by P. aeruginosa isolates in healthy humans. The occurrence, antimicrobial resistance phenotype, virulence genotype, and genetic lineages of P. aeruginosa from faecal samples of children from two different Spanish regions were characterised. Seventy-two P. aeruginosa were isolated from 1,443 faecal samples. Low antimicrobial resistance levels were detected: ceftazidime (8%), cefepime (7%), aztreonam (7%), gentamicin (3%), ciprofloxacin (1%), and imipenem (1%); susceptibility to meropenem, amikacin, tobramycin, levofloxacin, and colistin. Four multidrug-resistant strains were found. Important differences were detected between both geographical regions. Forty-one sequence types were detected among the 48 tested strains. Virulence and quorum sensing genes were analysed and 13 virulotypes were detected, being 26 exoU-positive strains. Alteration in protein OprD showed eight different patterns. The unique imipenem-resistant strain showed a premature stop codon in OprD. Intestinal colonisation by P. aeruginosa, mainly by international clones (as ST244, ST253, and ST274), is an important factor for the systemic infections development and the environmental dissemination. Periodic active surveillance is useful to identify these community human reservoirs and to control the evolution of antibiotic resistance and virulence activity.

Bacterial Quorum Sensing and Microbial Community Interactions

Many bacteria use a cell-cell communication system called quorum sensing to coordinate population density-dependent changes in behavior. Quorum sensing involves production of and response to diffusible or secreted signals, which can vary substantially across different types of bacteria. In many species, quorum sensing modulates virulence functions and is important for pathogenesis. Over the past half-century, there has been a significant accumulation of knowledge of the molecular mechanisms, signal structures, gene regulons, and behavioral responses associated with quorum-sensing systems in diverse bacteria. More recent studies have focused on understanding quorum sensing in the context of bacterial sociality. Studies of the role of quorum sensing in cooperative and competitive microbial interactions have revealed how quorum sensing coordinates interactions both within a species and between species. Such studies of quorum sensing as a social behavior have relied on the development of "synthetic ecological" models that use nonclonal bacterial populations. In this review, we discuss some of these models and recent advances in understanding how microbes might interact with one another using quorum sensing. The knowledge gained from these lines of investigation has the potential to guide studies of microbial sociality in natural settings and the design of new medicines and therapies to treat bacterial infections.

Quorum-sensing control of antibiotic resistance stabilizes cooperation in Chromobacterium violaceum

Many Proteobacteria use quorum sensing to regulate production of public goods, such as antimicrobials and proteases, that are shared among members of a community. Public goods are vulnerable to exploitation by cheaters, such as quorum sensing-defective mutants. Quorum sensing- regulated private goods, goods that benefit only producing cells, can prevent the emergence of cheaters under certain growth conditions. Previously, we developed a laboratory co-culture model to investigate the importance of quorum-regulated antimicrobials during interspecies competition. In our model, Burkholderia thailandensis and Chromobacterium violaceum each use quorum sensing-controlled antimicrobials to inhibit the other species' growth. Here, we show that C. violaceum uses quorum sensing to increase resistance to bactobolin, a B. thailandensis antibiotic, by increasing transcription of a putative antibiotic efflux pump. We demonstrate conditions where C. violaceum quorum-defective cheaters emerge and show that in these conditions, bactobolin restrains cheaters. We also demonstrate that bactobolin restrains quorum-defective mutants in our co-culture model, and the increase in antimicrobial-producing cooperators drives the C. violaceum population to become more competitive. Our results describe a mechanism of cheater restraint involving quorum control of efflux pumps and demonstrate that interspecies competition can reinforce cooperative behaviors by placing constraints on quorum sensing-defective mutants.

Combination Therapy Strategy of Quorum Quenching Enzyme and Quorum Sensing Inhibitor in Suppressing Multiple Quorum Sensing Pathways of P. aeruginosa

The threat of antibiotic resistant bacteria has called for alternative antimicrobial strategies that would mitigate the increase of classical resistance mechanism. Many bacteria employ quorum sensing (QS) to govern the production of virulence factors and formation of drug-resistant biofilms. Targeting the mechanism of QS has proven to be a functional alternative to conventional antibiotic control of infections. However, the presence of multiple QS systems in individual bacterial species poses a challenge to this approach. Quorum sensing inhibitors (QSI) and quorum quenching enzymes (QQE) have been both investigated for their QS interfering capabilities. Here, we first simulated the combination effect of QQE and QSI in blocking bacterial QS. The effect was next validated by experiments using AiiA as QQE and G1 as QSI on Pseudomonas aeruginosa LasR/I and RhlR/I QS circuits. Combination of QQE and QSI almost completely blocked the P. aeruginosa las and rhl QS systems. Our findings provide a potential chemical biology application strategy for bacterial QS disruption.

Environmentally Endemic Pseudomonas aeruginosa Strains with Mutations in lasR Are Associated with Increased Disease Severity in Corneal Ulcers

The Steroids for Corneal Ulcers Trial (SCUT) was a multicenter, international study of bacterial keratitis in which 101 Pseudomonas aeruginosa infections were treated. Twenty-two of 101 P. aeruginosa isolates collected had a colony morphology characteristic of a loss-of-function mutation in lasR, the gene encoding a quorum-sensing master regulator. Ulcers caused by these 22 strains were associated with larger areas of corneal opacification, worse vision, and a lower rate of vision recovery in response to treatment than ulcers caused by the other isolates. The lasR sequences from these isolates each contained one of three nonsynonymous substitutions, and these strains were deficient in production of LasR-regulated protease and rhamnolipids. Replacement of lasR with either of the two most common lasR alleles from the SCUT isolates was sufficient to decrease protease and rhamnolipid production in PA14. Loss of LasR function is associated with increased production of CupA fimbriae, and the LasR-defective isolates exhibited higher production of CupA fimbriae than LasR-intact isolates. Strains with the same lasR mutation were of the same multilocus sequence type, suggesting that LasR-deficient, environmental P. aeruginosa strains were endemic to the area, and infections caused by these strains were associated with worse patient outcomes in the SCUT study. (This study has been registered at ClinicalTrials.gov under registration no. NCT00324168.) IMPORTANCE The LasR transcription factor is an important regulator of quorum sensing in P. aeruginosa and positively controls multiple virulence-associated pathways. The emergence of strains with lasR loss-of-function alleles in chronic disease is well described and is thought to represent a specific adaptation to the host environment. However, the prevalence and virulence of these strains in acute infections remain unclear. This report describes observations revealing that lasR mutants were common among isolates from a large, multicenter clinical study of keratitis and were associated with worse clinical outcomes than LasR-intact strains despite reduced production of LasR-regulated factors. Additionally, these lasR mutants were closely related strains or clones, as determined by molecular analysis. Because bacterial keratitis is community acquired, these data indicate infection by endemic, LasR-deficient strains in the environment. These results suggest that the conventional paradigm regarding the role for LasR-mediated regulation of virulence is more complex than previously appreciated.

Cystic fibrosis-adapted Pseudomonas aeruginosa quorum sensing lasR mutants cause hyperinflammatory responses

Cystic fibrosis lung disease is characterized by chronic airway infections with the opportunistic pathogen Pseudomonas aeruginosa and severe neutrophilic pulmonary inflammation. P. aeruginosa undergoes extensive genetic adaptation to the cystic fibrosis (CF) lung environment, and adaptive mutations in the quorum sensing regulator gene lasR commonly arise. We sought to define how mutations in lasR alter host-pathogen relationships. We demonstrate that lasR mutants induce exaggerated host inflammatory responses in respiratory epithelial cells, with increased accumulation of proinflammatory cytokines and neutrophil recruitment due to the loss of bacterial protease- dependent cytokine degradation. In subacute pulmonary infections, lasR mutant-infected mice show greater neutrophilic inflammation and immunopathology compared with wild-type infections. Finally, we observed that CF patients infected with lasR mutants have increased plasma interleukin-8 (IL-8), a marker of inflammation. These findings suggest that bacterial adaptive changes may worsen pulmonary inflammation and directly contribute to the pathogenesis and progression of chronic lung disease in CF patients.

The Pseudomonas aeruginosa Transcriptional Landscape Is Shaped by Environmental Heterogeneity and Genetic Variation

Phenotypic variability among bacteria depends on gene expression in response to different environments, and it also reflects differences in genomic structure. In this study, we analyzed transcriptome sequencing (RNA-seq) profiles of 151 Pseudomonas aeruginosa clinical isolates under standard laboratory conditions and of one P. aeruginosa type strain under 14 different environmental conditions. Our approach allowed dissection of the impact of the genetic background versus environmental cues on P. aeruginosa gene expression profiles and revealed that phenotypic variation was larger in response to changing environments than between genomically different isolates. We demonstrate that mutations within the global regulator LasR affect more than one trait (pleiotropy) and that the interaction between mutations (epistasis) shapes the P. aeruginosa phenotypic plasticity landscape. Because of pleiotropic and epistatic effects, average genotype and phenotype measures appeared to be uncorrelated in P. aeruginosa.

This work links experimental data of unprecedented complexity with evolution theory and delineates the transcriptional landscape of the opportunistic pathogen Pseudomonas aeruginosa. We found that gene expression profiles are most strongly influenced by environmental cues, while at the same time the transcriptional profiles were also shaped considerably by genetic variation within global regulators. The comprehensive set of transcriptomic and genomic data of more than 150 clinical P. aeruginosa isolates will be made publically accessible to all researchers via a dedicated web interface. Both Pseudomonas specialists interested in expression and regulation of specific genes and researchers from other fields with more global interest in the phenotypic and genotypic variation of this important model species can access all information on various levels of detail.

The Pseudomonas aeruginosa RhlR-controlled aegerolysin RahU is a low-affinity rhamnolipid-binding protein

The opportunistic pathogen Pseudomonas aeruginosa uses quorum-sensing systems to regulate collective behaviour in response to the environment, by linking the expression of particular genes to population density. The quorum-sensing transcription factors LasR and RhlR and their cognate N-acyl-homoserine lactone (HSL) signals N-3-oxo-dodecanoyl-L-HSL (3OC12-HSL) and N-butanoyl-L-HSL (C4-HSL) control the expression of several hundred genes, which include those involved in virulence and biofilm formation. Here, we have focused on regulation of the expression of the putative virulence factor gene, rahU. We show that the intact las-rhl box immediately upstream of the -35 promoter element is needed for rahU expression in P. aeruginosa. Using β-galactosidase assays and quantification of the mRNA levels for rahU, lasR and rhlR, we provide evidence that for rahU promoter activity, 3OC12-HSL-LasR is not sufficient, and instead C4-HSL-RhlR is the trigger. Furthermore, surface plasmon resonance analysis revealed that RahU binds the biosurfactant rhamnolipids. Thus, this is the first report of a bacterial molecule that interacts with RahU.

Enhanced in vitro formation and antibiotic resistance of nonattached Pseudomonas aeruginosa aggregates through incorporation of neutrophil products

Pseudomonas aeruginosa is a major pathogen in cystic fibrosis (CF) lung disease. Children with CF are routinely exposed to P. aeruginosa from the natural environment, and by adulthood, 80% of patients are chronically infected. P. aeruginosa in the CF airway exhibits a unique biofilm-like structure, where it grows in small clusters or aggregates of bacteria in association with abundant polymers of neutrophil-derived components F-actin and DNA, among other components. These aggregates differ substantially in size and appearance compared to surface-attached in vitro biofilm models classically utilized for studies but are believed to share properties of surface-attached biofilms, including antibiotic resistance. However, little is known about the formation and function of surface-independent modes of biofilm growth, how they might be eradicated, and quorum sensing communication. To address these issues, we developed a novel in vitro model of P. aeruginosa aggregates incorporating human neutrophil-derived products. Aggregates grown in vitro and those found in CF patients' sputum samples were morphologically similar; viable bacteria were distributed in small pockets throughout the aggregate. The lasA quorum sensing gene was differentially expressed in the presence of neutrophil products. Importantly, aggregates formed in the presence of neutrophils acquired resistance to tobramycin, which was lost when the aggregates were dispersed with DNase, and antagonism of tobramycin and azithromycin was observed. This novel yet simple in vitro system advances our ability to model infection of the CF airway and will be an important tool to study virulence and test alternative eradication strategies against P. aeruginosa.

The hierarchy quorum sensing network in Pseudomonas aeruginosa

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

The correlation between the presence of quorum sensing, toxin-antitoxin system genes and MIC values with ability of biofilm formation in clinical isolates of Pseudomonas aeruginosa

INTRODUCTION

Pseudomonas aeruginosa is a Gram-negative bacterium that considered as important opportunistic human pathogen. One of the mechanisms that help bacteria to tolerate survival in adverse conditions and resistance to antibiotics is biofilm formation through quorum sensing (QS) signals and toxin-antitoxin (TA) systems. QS and TA are two systems that have important roles in biofilm formation. QS is a global regulatory mechanism that enable bacteria to communicate with each other by production of auto inducers (AI) molecules in population. Because of importance biofilm formation in P. aeruginosa infections, here, we studied frequency of QS and TA genes among clinical isolates of P. aeruginosa with ability of biofilm formation.

MATERIALS AND METHODS

One hundred and forty clinical isolates of P. aeruginosa were collected from Tehran and Ilam hospitals. The isolates were identified by biochemical tests. Biofilm formation was evaluated by microplate method. After DNA extraction by boiling method, the frequency of QS genes (lasIR, rhlIR), and TA genes (mazEF, relBE, hipBA, ccdAB and mqsR) were analyzed by PCR.

RESULTS

Our results showed that maximum resistance is related to aztreonam (72.85%) antibiotic. Most of isolates were able to produce biofilm (87.15%) and the majority of them formed strong biofilm (56.42%). PCR results showed that frequency of mazEF, relBE, hipBA, ccdAB, mqsR, lasIR and rhlIR genes were 85.71, 100, 1.42, 100, 57.14, 93.57 and 83.57 percent, respectively.

CONCLUSION

Clinical isolates of P. aeruginosa had high ability to form biofilm, and QS and TA system genes among these isolates were very high (except hipBA genes). There are significaut correlation between biofilm for mation and present of QS and TA system genes.

QsrO a novel regulator of quorum-sensing and virulence in Pseudomonas aeruginosa

In Pseudomonas aeruginosa, the production of many secreted virulence factors is controlled by a quorum-sensing (QS) circuit, constituted of transcriptional activators (LasR, RhlR, PqsR) and their cognate signaling molecules (3-oxo-C12-HSL, C4-HSL, PQS). QS is a cooperative behavior that is beneficial to a population but can be exploited by “QS-cheaters”, individuals which do not respond to the QS-signal, but can use public goods produced by QS-cooperators. In order to identify QS-deficient clones we designed a genetic screening based on a lasB-lacZ fusion. We isolated one clone (PT1617) deficient in QS-dependent gene expression and virulence factor production despite wild type lasR, rhlR and pqsR alleles. Whole genome sequencing of PT1617 revealed a 3,552 bp deletion encompassing ORFs PA2228-PA2229-PA2230 and the pslA gene. However, complementation of PT1617 by plasmid-encoded copies of these ORFs, did not restore QS. Unexpectedly, gene expression levels of ORFs PA2228, PA2227 (vqsM) and PA2222, located adjacent to the deletion, were 10 to 100 fold higher in mutant PT1617 than in PAO1. When expressed from a constitutive promoter on a plasmid, PA2226, alone was found to be sufficient to confer a QS-negative phenotype on PAO1 as well as on PA14. Co-expression of PA2226 and PA2225 in PAO1 further prevented induction of the type III secretion system. In summary, we have identified a novel genetic locus including ORF2226 termed qsrO (QS-repressing ORF), capable of down-regulating all three known QS-systems in P. aeruginosa.

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.

Disruption of the endothelial barrier by proteases from the bacterial pathogen Pseudomonas aeruginosa: implication of matrilysis and receptor cleavage

Within the vasculature, uncontrolled pericellular proteolysis can lead to disruption of cell-to-cell and cell-to-matrix interactions and subsequent detachment-induced cell apoptosis, or anoikis, contributing to inflammatory vascular diseases, with the endothelium as the major target. Most studies so far have focused on endogenous proteinases. However, during bloodstream infections, bacterial proteinases may also trigger endothelial anoikis. We thus investigated the potential apoptotic activity of the proteinases secreted by the haematotropic opportunistic pathogen, Pseudomonas aeruginosa, and particularly its predominant metalloproteinase, LasB. For this, we used the secretome of the LasB-expressing pseudomonal strain, PAO1, and compared it with that from the isogenic, LasB-deficient strain (PAO1∆lasB), as well as with purified LasB. Secretomes were tested for apoptotic activity on cultured human endothelial cells derived from the umbilical vein or from the cerebral microvasculature. We found that the PAO1 secretome readily induced endothelial cell anoikis, as did secretomes of LasB-positive clinical pseudomonal isolates, while the PAO1∆lasB secretome had only a limited impact on endothelial adherence and viability. Notably, purified LasB reproduced most of the effects of the LasB-containing secretomes, and these were drastically reduced in the presence of the LasB-selective inhibitor, phosphoramidon. A precocious and extensive LasB-dependent degradation of several proteins associated with the endothelial extracellular matrix, fibronectin and von Willebrand factor, was observed by immunofluorescence and/or immunoblotting analysis of cell cultures. Moreover, the PAO1 secretome, but not that from PAO1∆lasB, specifically induced rapid endoproteolysis of two major interendothelial junction components, VE-cadherin and occludin, as well as of the anti-anoikis, integrin-associated urokinase receptor, uPAR. Taken as a prototype for exogenous haemorrhagic proteinases, pseudomonal LasB thus appears to induce endothelial anoikis not only via matrilysis, as observed for many pro-apoptotic proteinases, but also via cleavage of some essential cell-to-cell and cell-to-matrix adhesion receptors implicated in the maintenance of the endothelial barrier.

Acyl-homoserine lactone quorum sensing: from evolution to application

Quorum sensing (QS) is a widespread process in bacteria that employs autoinducing chemical signals to coordinate diverse, often cooperative activities such as bioluminescence, biofilm formation, and exoenzyme secretion. Signaling via acyl-homoserine lactones is the paradigm for QS in Proteobacteria and is particularly well understood in the opportunistic pathogen Pseudomonas aeruginosa. Despite thirty years of mechanistic research, empirical studies have only recently addressed the benefits of QS and provided support for the traditional assumptions regarding its social nature and its role in optimizing cell-density-dependent group behaviors. QS-controlled public-goods production has served to investigate principles that explain the evolution and stability of cooperation, including kin selection, pleiotropic constraints, and metabolic prudence. With respect to medical application, appreciating social dynamics is pertinent to understanding the efficacy of QS-inhibiting drugs and the evolution of resistance. Future work will provide additional insight into the foundational assumptions of QS and relate laboratory discoveries to natural ecosystems.

Interference of bacterial cell-to-cell communication: a new concept of antimicrobial chemotherapy breaks antibiotic resistance

Bacteria use a cell-to-cell communication activity termed "quorum sensing" to coordinate group behaviors in a cell density dependent manner. Quorum sensing influences the expression profile of diverse genes, including antibiotic tolerance and virulence determinants, via specific chemical compounds called "autoinducers". During quorum sensing, Gram-negative bacteria typically use an acylated homoserine lactone (AHL) called autoinducer 1. Since the first discovery of quorum sensing in a marine bacterium, it has been recognized that more than 100 species possess this mechanism of cell-to-cell communication. In addition to being of interest from a biological standpoint, quorum sensing is a potential target for antimicrobial chemotherapy. This unique concept of antimicrobial control relies on reducing the burden of virulence rather than killing the bacteria. It is believed that this approach will not only suppress the development of antibiotic resistance, but will also improve the treatment of refractory infections triggered by multi-drug resistant pathogens. In this paper, we review and track recent progress in studies on AHL inhibitors/modulators from a biological standpoint. It has been discovered that both natural and synthetic compounds can disrupt quorum sensing by a variety of means, such as jamming signal transduction, inhibition of signal production and break-down and trapping of signal compounds. We also focus on the regulatory elements that attenuate quorum sensing activities and discuss their unique properties. Understanding the biological roles of regulatory elements might be useful in developing inhibitor applications and understanding how quorum sensing is controlled.

Antibiotic-mediated selection of quorum-sensing-negative Staphylococcus aureus

Staphylococcus aureus is a human commensal that at times turns into a serious bacterial pathogen causing life-threatening infections. For the delicate control of virulence, S. aureus employs the agr quorum-sensing system that, via the intracellular effector molecule RNAIII, regulates virulence gene expression. We demonstrate that the presence of the agr locus imposes a fitness cost on S. aureus that is mediated by the expression of RNAIII. Further, we show that exposure to sublethal levels of the antibiotics ciprofloxacin, mupirocin, and rifampin, each targeting separate cellular functions, markedly increases the agr-mediated fitness cost by inducing the expression of RNAIII. Thus, the extensive use of antibiotics in hospitals may explain why agr-negative variants are frequently isolated from hospital-acquired S. aureus infections but rarely found among community-acquired S. aureus strains. Importantly, agr deficiency correlates with increased duration of and mortality due to bacteremia during antibiotic treatment and with a higher frequency of glycopeptide resistance than in agr-carrying strains. Our results provide an explanation for the frequent isolation of agr-defective strains from hospital-acquired S. aureus infections and suggest that the adaptability of S. aureus to antibiotics involves the agr locus.

Staphylococcus aureus is the most frequently isolated pathogen in intensive care units and a common cause of nosocomial infections, resulting in a high degree of morbidity and mortality. Surprisingly, a large fraction (15 to 60%) of hospital-isolated S. aureus strains are agr defective and lack the main quorum-sensing-controlled virulence regulatory system. This is a problem, as agr-defective strains are associated with a mortality level in bacteremic infections and a probability of glycopeptide resistance greater than those of other strains. We show here that agr-negative strains have a fitness advantage over agr-positive strains in the presence of sublethal concentrations of some antibiotics and that the fitness defect of agr-positive cells is caused by antibiotic-mediated expression of the agr effector molecule RNAIII. These results offer an explanation of the frequent isolation of agr-defective S. aureus strains in hospitals and will influence how we treat S. aureus infections.

Quorum sensing and the confusion about diffusion

Two hypotheses, termed quorum sensing (QS) and diffusion sensing (DS), have been suggested as competing explanations for why bacterial cells use the local concentration of small molecules to regulate numerous extracellular behaviours. Here, we show that: (i) although there are important differences between QS and DS, they are not diametrically opposed; (ii) empirical attempts to distinguish between QS and DS are misguided and will lead to confusion; (iii) the fundamental distinction is not between QS and DS, but whether or not the trait being examined is social; (iv) empirical data are consistent with both social interactions and a role of diffusion; (v) alternate hypotheses, such as efficiency sensing (ES), are not required to unite QS and DS. More generally, work in this area illustrates how the use of jargon can obscure the underlying concepts and key questions.

Strain-dependent diversity in the Pseudomonas aeruginosa quorum-sensing regulon

Quorum sensing allows bacteria to sense and respond to changes in population density. Acyl-homoserine lactones serve as quorum-sensing signals for many Proteobacteria, and acyl-homoserine lactone signaling is known to control cooperative activities. Quorum-controlled activities vary from one species to another. Quorum-sensing controls a constellation of genes in the opportunistic pathogen Pseudomonas aeruginosa, which thrives in a number of habitats ranging from soil and water to animal hosts. We hypothesized that there would be significant variation in quorum-sensing regulons among strains of P. aeruginosa isolated from different habitats and that differences in the quorum-sensing regulons might reveal insights about the ecology of P. aeruginosa. As a test of our hypothesis we used RNA-seq to identify quorum-controlled genes in seven P. aeruginosa isolates of diverse origins. Although our approach certainly overlooks some quorum-sensing-regulated genes we found a shared set of genes, i.e., a core quorum-controlled gene set, and we identified distinct, strain-variable sets of quorum-controlled genes, i.e., accessory genes. Some quorum-controlled genes in some strains were not present in the genomes of other strains. We detected a correlation between traits encoded by some genes in the strain-variable subsets of the quorum regulons and the ecology of the isolates. These findings indicate a role for quorum sensing in extension of the range of habitats in which a species can thrive. This study also provides a framework for understanding the molecular mechanisms by which quorum-sensing systems operate, the evolutionary pressures by which they are maintained, and their importance in disparate ecological contexts.

Fitness landscape of antibiotic tolerance in Pseudomonas aeruginosa biofilms

Bacteria in biofilms have higher antibiotic tolerance than their planktonic counterparts. A major outstanding question is the degree to which the biofilm-specific cellular state and its constituent genetic determinants contribute to this hyper-tolerant phenotype. Here, we used genome-wide functional profiling of a complex, heterogeneous mutant population of Pseudomonas aeruginosa MPAO1 in biofilm and planktonic growth conditions with and without tobramycin to systematically quantify the contribution of each locus to antibiotic tolerance under these two states. We identified large sets of mutations that contribute to antibiotic tolerance predominantly in the biofilm or planktonic setting only, offering global insights into the differences and similarities between biofilm and planktonic antibiotic tolerance. Our mixed population-based experimental design recapitulated the complexity of natural biofilms and, unlike previous studies, revealed clinically observed behaviors including the emergence of quorum sensing-deficient mutants. Our study revealed a substantial contribution of the cellular state to the antibiotic tolerance of biofilms, providing a rational foundation for the development of novel therapeutics against P. aeruginosa biofilm-associated infections.

Biofilms, matrix-enclosed surface-colonized communities of bacteria, are extremely resistant to antimicrobial agents, withstanding concentrations of antibiotics orders of magnitude higher compared to free-swimming planktonic cells. This is a well-established characteristic of infections caused by the opportunistic pathogen Pseudomonas aeruginosa, the major cause of morbidity in cystic fibrosis patients and a frequent cause of nosocomial infections, and Pseudomonas infections generally persist despite the use of long-term antibiotic therapy. Nonetheless, the genetic basis of the hyper-tolerance of biofilms to antimicrobial agents is poorly understood. In this study, we use a genome-wide genetic footprinting technology to systematically quantify the contribution of each locus in P. aeruginosa to antibiotic tolerance in both biofilm and planktonic states. Comparing and contrasting the genome-wide genetic profile of these two physiological states revealed that large sets of genes modulate antibiotic tolerance as a function of the cellular state.

Determination of acyl homoserine lactone and tetramic acid concentrations in biological samples

Within environmental communities, there is a constant struggle for survival, as nutrients are often limited. In response, bacteria have developed elaborate methods to deal with competitors. One such mechanism is the coordination of behaviors and function via the exchange of small chemical signals in a process known as quorum sensing. This process is especially prominent in the pathogenicity of Pseudomonas aeruginosa, an opportunistic human pathogen that forms sessile communities known as biofilms. These biofilms play an important role in the lifestyle of P. aeruginosa, either in their natural environment or during establishment and maintenance of infection in human hosts; thus, they often have grievous effects on human health. As such, a method for the detection of these QS signals may provide insights into the pathogenicity and survival of P. aeruginosa. In this chapter, we present a method for the extraction and quantitation of the P. aeruginosa QS signal N-3-oxo-dodecanoyl-homoserine lactone, and its rearranged tetramic acid product, C12-TA, which itself has implications as a survival tactic used by P. aeruginosa.

Microevolution of Pseudomonas aeruginosa to a chronic pathogen of the cystic fibrosis lung

Pseudomonas aeruginosa is the leading pathogen of chronic cystic fibrosis (CF) lung infection. Life-long persistance of P. aeruginosa in the CF lung requires a sophisticated habitat-specific adaptation of this pathogen to the heterogeneous and fluctuating lung environment. Due to the high selective pressure of inflamed CF lungs, P. aeruginosa increasingly experiences complex physiological and morphological changes. Pulmonary adaptation of P. aeruginosa is mediated by genetic variations that are fixed by the repeating interplay of mutation and selection. In this context, the emergence of hypermutable phenotypes (mutator strains) obviously improves the microevolution of P. aeruginosa to the diverse microenvironments of the CF lung. Mutator phenotypes are amplified during CF lung disease and accelerate the intraclonal diversification of P. aeruginosa. The resulting generation of numerous subclonal variants is advantegous to prepare P. aeruginosa population for unpredictable stresses (insurance hypothesis) and thus supports long-term survival of this pathogen. Oxygen restriction within CF lung environment further promotes persistence of P. aeruginosa due to increased antibiotic tolerance, alginate production and biofilm formation. Finally, P. aeruginosa shifts from an acute virulent pathogen of early infection to a host-adapted chronic virulent pathogen of end-stage infection of the CF lung. Common changes that are observed among chronic P. aeruginosa CF isolates include alterations in surface antigens, loss of virulence-associated traits, increasing antibiotic resistances, the overproduction of the exopolysaccharide alginate and the modulation of intermediary and micro-aerobic metabolic pathways (Hogardt and Heesemann, Int J Med Microbiol 300(8):557-562, 2010). Loss-of-function mutations in mucA and lasR genes determine the transition to mucoidity and loss of quorum sensing, which are hallmarks of the chronic virulence potential of P. aeruginosa. Metabolic factors that are positively selected in response to the specific environment of CF lung include the outer membrane protein OprF, the microaerophilic oxidase Cbb3-2, the blue copper protein azurin, the cytochrome c peroxidase c551 and the enzymes of the arginine deiminase pathway ArcA-ArcD. These metabolic adaptations probably support the growth of P. aeruginosa within oxygen-depleted CF mucus. The deeper understanding of the physiological mechanisms of niche specialization of P. aeruginosa during CF lung infection will help to identify new targets for future anti-pseudomonal treatment strategies to prevent the selection of mutator isolates and the establishment of chronic CF lung infection.

Candida albicans-produced farnesol stimulates Pseudomonas quinolone signal production in LasR-defective Pseudomonas aeruginosa strains

Candida albicans has been previously shown to stimulate the production of Pseudomonas aeruginosa phenazine toxins in dual-species colony biofilms. Here, we report that P. aeruginosa lasR mutants, which lack the master quorum sensing system regulator, regain the ability to produce quorum-sensing-regulated phenazines when cultured with C. albicans. Farnesol, a signalling molecule produced by C. albicans, was sufficient to stimulate phenazine production in LasR(-) laboratory strains and clinical isolates. P. aeruginosa ΔlasR mutants are defective in production of the Pseudomonas quinolone signal (PQS) due to their inability to properly induce pqsH, which encodes the enzyme necessary for the last step in PQS biosynthesis. We show that expression of pqsH in a ΔlasR strain was sufficient to restore PQS production, and that farnesol restored pqsH expression in ΔlasR mutants. The farnesol-mediated increase in pqsH required RhlR, a transcriptional regulator downstream of LasR, and farnesol led to higher levels of N-butyryl-homoserine lactone, the small molecule activator of RhlR. Farnesol promotes the production of reactive oxygen species (ROS) in a variety of species. Because the antioxidant N-acetylcysteine suppressed farnesol-induced RhlR activity in LasR(-) strains, and hydrogen peroxide was sufficient to restore PQS production in las mutants, we propose that ROS are responsible for the activation of downstream portions of this quorum sensing pathway. LasR mutants frequently arise in the lungs of patients chronically infected with P. aeruginosa. The finding that C. albicans, farnesol or ROS stimulate virulence factor production in lasR strains provides new insight into the virulence potential of these strains.

Nutrient availability as a mechanism for selection of antibiotic tolerant Pseudomonas aeruginosa within the CF airway

Microbes are subjected to selective pressures during chronic infections of host tissues. Pseudomonas aeruginosa isolates with inactivating mutations in the transcriptional regulator LasR are frequently selected within the airways of people with cystic fibrosis (CF), and infection with these isolates has been associated with poorer lung function outcomes. The mechanisms underlying selection for lasR mutation are unknown but have been postulated to involve the abundance of specific nutrients within CF airway secretions. We characterized lasR mutant P. aeruginosa strains and isolates to identify conditions found in CF airways that select for growth of lasR mutants. Relative to wild-type P. aeruginosa, lasR mutants exhibited a dramatic metabolic shift, including decreased oxygen consumption and increased nitrate utilization, that is predicted to confer increased fitness within the nutrient conditions known to occur in CF airways. This metabolic shift exhibited by lasR mutants conferred resistance to two antibiotics used frequently in CF care, tobramycin and ciprofloxacin, even under oxygen-dependent growth conditions, yet selection for these mutants in vitro did not require preceding antibiotic exposure. The selection for loss of LasR function in vivo, and the associated adverse clinical impact, could be due to increased bacterial growth in the oxygen-poor and nitrate-rich CF airway, and from the resulting resistance to therapeutic antibiotics. The metabolic similarities among diverse chronic infection-adapted bacteria suggest a common mode of adaptation and antibiotic resistance during chronic infection that is primarily driven by bacterial metabolic shifts in response to nutrient availability within host tissues.

Chronic infections are distinguished from many other infections in that they are difficult to eradicate with antibiotics. Thus, the microbes that cause chronic infections persist within host tissues for long periods despite our best treatment efforts. During the course of these chronic infections, the causative microbes often change genetically. For example, a bacterium that commonly infects the lungs of people with the genetic disease cystic fibrosis (CF) undergoes several known changes that affect the growth of this pathogen. However, the causes and clinical impact of the changes undergone by this and other chronically infecting microbes are unclear. We show that a common, early mutation found in bacteria isolated from chronically infected CF airways renders these bacteria better able to grow in the nutrients found in CF lung secretions. Interestingly, these same changes also confer resistance to several antibiotics used commonly to treat CF patients. Many of the characteristics conferred by this mutation are exhibited by other microbes found in chronic infections, suggesting that adaptation of these microbes to host tissue nutrient environments may be a common mechanism of antibiotic resistance in chronic infections.

Defining the mode of action of tetramic acid antibacterials derived from Pseudomonas aeruginosa quorum sensing signals

In nature, bacteria rarely exist as single, isolated entities, but rather as communities comprised of many other species including higher host organisms. To survive in these competitive environments, microorganisms have developed elaborate tactics such as the formation of biofilms and the production of antimicrobial toxins. Recently, it was discovered that the gram-negative bacterium Pseudomonas aeruginosa , an opportunistic human pathogen, produces an antibiotic, 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione (C(12)-TA), derived from one of its quorum sensing molecules. Here, we present a comprehensive study of the expanded spectrum of C(12)-TA antibacterial activity against microbial competitors encountered by P. aeruginosa in nature as well as significant human pathogens. The mechanism of action of C(12)-TA was also elucidated, and C(12)-TA was found to dissipate both the membrane potential and the pH gradient of Gram-positive bacteria, correlating well with cell death. Notably, in stark contrast to its parent molecule 3-oxo-dodecanoyl homoserine lactone (3-oxo-C(12)-HSL), neither activation of cellular stress pathways nor cytotoxicity was observed in human cells treated with C(12)-TA. Our results suggest that the QS machinery of P. aeruginosa has evolved for a dual-function, both to signal others of the same species and also to defend against host immunity and competing bacteria. Because of the broad-spectrum antibacterial activity, established mode of action, lack of rapid resistance development, and tolerance by human cells, the C(12)-TA scaffold may also serve as a new lead compound for the development of antimicrobial therapeutics.

Instantaneous within-patient diversity of Pseudomonas aeruginosa quorum-sensing populations from cystic fibrosis lung infections

In the opportunistic pathogen Pseudomonas aeruginosa, acyl-homoserine lactone (acyl-HSL) quorum sensing (QS) regulates biofilm formation and expression of many extracellular virulence factors. Curiously, QS-deficient variants, often carrying mutations in the central QS regulator LasR, are frequently isolated from infections, particularly from cystic fibrosis (CF) lung infections. Very little is known about the proportion and diversity of these QS variants in individual infections. Such information is desirable to better understand the selective forces that drive the evolution of QS phenotypes, including social cheating and innate (nonsocial) benefits. To obtain insight into the instantaneous within-patient diversity of QS, we assayed a panel of 135 concurrent P. aeruginosa isolates from eight different adult CF patients (9 to 20 isolates per patient) for various QS-controlled phenotypes. Most patients contained complex mixtures of QS-proficient and -deficient isolates. Among all patients, deficiency in individual phenotypes ranged from 0 to about 90%. Acyl-HSL, sequencing, and complementation analyses of variants with global loss-of-function phenotypes revealed dependency upon the central QS circuitry genes lasR, lasI, and rhlI. Deficient and proficient isolates were clonally related, implying evolution from a common ancestor in vivo. Our results show that the diversity of QS types is high within and among patients, suggesting diverse selection pressures in the CF lung. A single selective mechanism, be it of a social or nonsocial nature, is unlikely to account for such heterogeneity. The observed diversity also shows that conclusions about the properties of P. aeruginosa QS populations in individual CF infections cannot be drawn from the characterization of one or a few selected isolates.

Microbial quorum sensing: a tool or a target for antimicrobial therapy?

Inter-cell communication aided by released chemical signals when cell density reaches a critical concentration has been investigated for over 30 years as quorum sensing. Originally discovered in Gram-negative bacteria, quorum-sensing systems have also been studied extensively in Gram-positive bacteria and dimorphic fungi. Microbial communities communicating via quorum sensing employ various chemical signals to supervise their surrounding environment, alter genetic expression and gain advantage over their competitors. These signals vary from acylhomoserine lactones to small modified or unmodified peptides to complex gamma-butyrolactone molecules. The scope of this review is to give an insight into some of the quorum-sensing systems now known and to explore their role in microbial physiology and development of pathogenesis. Particular attention will be dedicated to the signalling molecules involved in quorum-sensing-mediated processes and the potential shown by some of their natural and synthetic analogues in the treatment of infections triggered by quorum sensing.

LasI/R and RhlI/R quorum sensing in a strain of Pseudomonas aeruginosa beneficial to plants

Pseudomonas aeruginosa possesses three quorum-sensing (QS) systems which are key in the expression of a large number of genes, including many virulence factors. Most studies of QS in P. aeruginosa have been performed in clinical isolates and have therefore focused on its role in pathogenicity. P. aeruginosa, however, is regarded as a ubiquitous organism capable of colonizing many different environments and also of establishing beneficial associations with plants. In this study we examined the role of the two N-acyl homoserine lactone systems known as RhlI/R and LasI/R in the environmental rice rhizosphere isolate P. aeruginosa PUPa3. Both the Rhl and Las systems are involved in the regulation of plant growth-promoting traits. The environmental P. aeruginosa PUPa3 is pathogenic in two nonmammalian infection models, and only the double las rhl mutants are attenuated for virulence. In fact it was established that the two QS systems are not hierarchically organized and that they are both important for the colonization of the rice rhizosphere. This is an in-depth genetic and molecular study of QS in an environmental P. aeruginosa strain and highlights several differences with QS regulation in the clinical isolate PAO1.