Isolation and characterization of a regulatory gene affecting rhamnolipid biosurfactant synthesis in Pseudomonas aeruginosa

Intercellular communication in bacteria

Bacteria have been long considered primitive organisms, with a lifestyle focused on the survival and propagation of single cells. However, in the past few decades it became obvious that bacteria can display sophisticated group behaviors. For instance, bacteria can communicate amongst themselves and with their hosts, by producing, sensing, and responding to chemical signals. By doing so, they can sense their surroundings and adapt as to increase their chances of survival and propagation. Here, we review the discovery of bacterial intercellular communication, some of the signaling molecules identified to date, the role of intercellular signaling in symbiotic and pathogenic relationships between bacteria and their hosts and its implications for the development of new therapeutic strategies against human disease.

Isolation, characterization, and utilization of a temperature-sensitive allele of a Pseudomonas replicon

In order to facilitate genetic study of the opportunistic bacterial pathogen Pseudomonas aeruginosa, we isolated a conditional, temperature-sensitive plasmid origin of replication. We mutagenized the popular Pseudomonas stabilizing fragment from pRO1610 in vitro using the Taq thermostable DNA polymerase in a polymerase chain reaction (PCR). Out of approximately 23,000 potential clones, 48 temperature-sensitive mutants were isolated. One mutant was further characterized and the origin of replication was designated as mSF(ts1). The mutations that resulted in a temperature-sensitive phenotype in mSF(ts1) were localized to the 1.2 kb of minimum sequence required for replication in P. aeruginosa. The DNA sequence analysis revealed two mutations within the coding sequence of the Replication control (Rep) protein. Growth of P. aeruginosa carrying the temperature-sensitive plasmid at the non-permissive temperature of 42 degrees C resulted in loss of the plasmid by greater than 99.9999% of the cells after 16 h of growth. In order to facilitate its utilization, the mSF(ts1) was converted into a genetic cassette flanked by mirrored restriction endonuclease digestion sites of a pUC1918 derivative. We demonstrate utilization of the mSF(ts1) for genetic studies involving complementation and regeneration of a mutant in P. aeruginosa research.

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.

GidA posttranscriptionally regulates rhl quorum sensing in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa utilizes two interconnected acyl-homoserine lactone quorum-sensing (acyl-HSL QS) systems, LasRI and RhlRI, to regulate the expression of hundreds of genes. The QS circuitry itself is integrated into a complex network of regulation by other factors. However, our understanding of this network is still unlikely to be complete, as a comprehensive, saturating approach to identifying regulatory components has never been attempted. Here, we utilized a nonredundant P. aeruginosa PA14 transposon library to identify additional genes that regulate QS at the level of LasRI/RhlRI. We initially screened all 5,459 mutants for loss of function in one QS-controlled trait (skim milk proteolysis) and then rescreened attenuated candidates for defects in other QS phenotypes (LasA protease, rhamnolipid, and pyocyanin production) to exclude mutants defective in functions other than QS. We identified several known and novel genes, but only two novel genes, gidA and pcnB, affected all of the traits assayed. We characterized gidA, which exhibited the most striking QS phenotypes, further. This gene is predicted to encode a conserved flavin adenine dinucleotide-binding protein involved in tRNA modification. Inactivation of the gene primarily affected rhlR-dependent QS phenotypes such as LasA, pyocyanin, and rhamnolipid production. GidA affected RhlR protein but not transcript levels and also had no impact on LasR and acyl-HSL production. Overexpression of rhlR in a gidA mutant partially restored QS-dependent phenotypes. Taken together, these results indicate that GidA selectively controls QS gene expression posttranscriptionally via RhlR-dependent and -independent pathways.

Pseudomonas aeruginosa las and rhl quorum-sensing systems are important for infection and inflammation in a rat prostatitis model

Pseudomonas aeruginosa frequently acts as an opportunistic pathogen of mucosal surfaces; yet, despite causing aggressive prostatitis in some men, its role as a pathogen in the prostate has not been investigated. Consequently, we developed a Ps. aeruginosa infection model in the rat prostate by instilling wild-type (WT) Ps. aeruginosa strain PAO1 into the rat prostate. It was found that Ps. aeruginosa produced acute and chronic infections in this mucosal tissue as determined by bacterial colonization, gross morphology, tissue damage and inflammatory markers. WT strain PAO1 and its isogenic mutant PAO-JP2, in which both the lasI and rhlI quorum-sensing signal systems have been silenced, were compared during both acute and chronic prostate infections. In acute infections, bacterial numbers and inflammatory markers were comparable between WT PA01 and PAO-JP2; however, considerably less tissue damage occurred in infections with PAO-JP2. Chronic infections with PAO-JP2 resulted in reduced bacterial colonization, tissue damage and inflammation as compared to WT PAO1 infections. Therefore, the quorum-sensing lasI and rhlI genes in Ps. aeruginosa affect acute prostate infections, but play a considerably more important role in maintaining chronic infections. We have thus developed a highly reproducible model for the study of Ps. aeruginosa virulence in the prostate.

Pseudomonas aeruginosa virulence and therapy: evolving translational strategies

OBJECTIVE

Although most reviews of Pseudomonas aeruginosa therapeutics focus on antibiotics currently in use or in the pipeline, we review evolving translational strategies aimed at using virulence factor antagonists as adjunctive therapies.

DATA SOURCE

Current literature regarding P. aeruginosa virulence determinants and approaches that target them, with an emphasis on type III secretion, quorum-sensing, biofilms, and flagella.

DATA EXTRACTION AND SYNTHESIS

P. aeruginosa remains one of the most important pathogens in nosocomial infections, with high associated morbidity and mortality. Its predilection to develop resistance to antibiotics and expression of multiple virulence factors contributes to the frequent ineffectiveness of current therapies. Among the many P. aeruginosa virulence determinants that impact infections, type III secretion, quorum sensing, biofilm formation, and flagella have been the focus on much recent investigation. Here we review how increased understanding of these important bacterial structures and processes has enabled the development of novel approaches to inhibit each. These promising translational strategies may lead to the development of adjunctive therapies capable of improving outcomes.

CONCLUSIONS

Adjuvant therapies directed against virulence factors have the potential to improve outcomes in P. aeruginosa infections.

Quorum sensing in Pseudomonas aeruginosa biofilms

In nature, the bulk of bacterial biomass is believed to exist as an adherent community of cells called a biofilm. Pseudomonas aeruginosa has become a model organism for studying this mode of growth. Over the past decade, significant strides have been made towards understanding biofilm development in P. aeruginosa and we now have a clearer picture of the mechanisms involved. Available evidence suggests that construction of these sessile communities proceeds by many different pathways, rather than a specific programme of biofilm development. A cell-to-cell communication mechanism known as quorum sensing (QS) has been found to play a role in P. aeruginosa biofilm formation. Because both QS and biofilms are impacted by the surrounding environment, understanding the full involvement of cell-to-cell signalling in establishing these complex communities represents a challenge. Nevertheless, under set conditions, several links between QS and biofilm formation have been recognized, which is the focus of this review. A role for antibiotics as alternative QS signalling molecules influencing biofilm development is also discussed.

Functional role of bacterial multidrug efflux pumps in microbial natural ecosystems

Multidrug efflux pumps have emerged as relevant elements in the intrinsic and acquired antibiotic resistance of bacterial pathogens. In contrast with other antibiotic resistance genes that have been obtained by virulent bacteria through horizontal gene transfer, genes coding for multidrug efflux pumps are present in the chromosomes of all living organisms. In addition, these genes are highly conserved (all members of the same species contain the same efflux pumps) and their expression is tightly regulated. Together, these characteristics suggest that the main function of these systems is not resisting the antibiotics used in therapy and that they should have other roles relevant to the behavior of bacteria in their natural ecosystems. Among the potential roles, it has been demonstrated that efflux pumps are important for processes of detoxification of intracellular metabolites, bacterial virulence in both animal and plant hosts, cell homeostasis and intercellular signal trafficking.

PqsE functions independently of PqsR-Pseudomonas quinolone signal and enhances the rhl quorum-sensing system

Pseudomonas aeruginosa is an opportunistic pathogen that causes both acute and chronic infections in immunocompromised individuals. This gram-negative bacterium produces a battery of virulence factors that allow it to infect and survive in many different hostile environments. The control of many of these virulence factors falls under the influence of one of three P. aeruginosa cell-to-cell signaling systems. The focus of this study, the quinolone signaling system, functions through the Pseudomonas quinolone signal (PQS), previously identified as 2-heptyl-3-hydroxy-4-quinolone. This signal binds to and activates the LysR-type transcriptional regulator PqsR (also known as MvfR), which in turn induces the expression of the pqsABCDE operon. The first four genes of this operon are required for PQS synthesis, but the fifth gene, pqsE, is not. The function of the pqsE gene is not known, but it is required for the production of multiple PQS-controlled virulence factors and for virulence in multiple models of infection. In this report, we show that PqsE can activate PQS-controlled genes in the absence of PqsR and PQS. Our data also suggest that the regulatory activity of PqsE requires RhlR and indicate that a pqsE mutant can be complemented for pyocyanin production by a large excess of exogenous N-butyryl homoserine lactone (C4-HSL). Finally, we show that PqsE enhances the ability of Escherichia coli expressing RhlR to respond to C4-HSL. Overall, our data lead us to conclude that PqsE functions as a regulator that is independent of PqsR and PQS but dependent on the rhl quorum-sensing system.

Pseudomonas aeruginosa quorum-sensing signal molecules induce IL-8 production by human corneal epithelial cells

OBJECTIVES

The aim of the present study was to evaluate whether the quorum-sensing molecules of Pseudomonas aeruginosa could induce the production of interleukin-8 (IL-8) in human corneal epithelial (HCE) cells in vitro.

METHODS

A confluent monolayer of immortalized HCE cells was treated with 12.5 to 50 microM n-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) or n-butanoyl-L-homoserine lactone (BHL) for 18 hours, or challenged with a wild-type P. aeruginosa PAO1 and its quorum-sensing mutants PAO-JP1 (lasI(-)), PDO100 (rhlI(-)), and PAO-JP2 (lasI(-)/rhlI(-)) for 1 hour. The levels of IL-8 in the culture supernatants were determined using an enzyme-linked immunosorbent assay.

RESULTS

OdDHL stimulated the production of IL-8 in HCE cells in a dose dependent manner. IL-8 production was seen with low concentrations of BHL (12.5 and 25 microM), but not at higher levels. There was significantly less IL-8 production in the HCE cells challenged with quorum-sensing mutants compared with the wild-type strain PAO1-challenged cells.

CONCLUSIONS

These findings suggest that quorum-sensing signal molecules OdDHL and BHL may directly contribute to the induction of the inflammatory response in Pseudomonas keratitis.

Heterologous production of Pseudomonas aeruginosa EMS1 biosurfactant in Pseudomonas putida

A new bacterial strain isolated from activated sludge, identified as Pseudomonas aeruginosa EMS1, produced a biosurfactant when grown on acidified soybean oil as the sole carbon source. An optimum biosurfactant production of 5 g/L was obtained with the following medium composition: 2% acidified soybean oil, 0.3% NH4NO3, 0.03% KH2PO4, 0.03% K2HPO4, 0.02% MgSO4.7H2O and 0.025% CaCl2.2H2O, with shaking at 200 rpm for an incubation period of 100 h at 30 degrees C. The production of the biosurfactant was found to be a function of cell growth, with maximum production occurring during the exponential phase. Hemolysis of erythrocytes and thin-layer chromatography studies revealed that the secreted biosurfactant was rhamnolipid. To overcome the complex environmental regulation with respect to rhamnolipid biosynthesis, and to replace the opportunistic pathogen P. aeruginosa with a safe industrial strain, attempts were made to achieve rhamnolipid production in a heterologous host, Pseudomonas putida, using molecular cloning of rhlAB rhamnosyltransferase genes with the rhlRI quorum sensing system, assuming that a functional rhamnosyltransferase would catalyze the formation of rhamnosyl-6-hydroxydecanoyl-6-hydroxydecanoate (mono-rhamnolipid) in P. putida. It was shown that rhamnolipid can be produced in the heterologous strain, P. putida, when provided with the rhamnosyltransferase genes.

Central role of quorum sensing in regulating the production of pathogenicity factors in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an important opportunistic human pathogen, causing various infections that are often very persistent. P. aeruginosa infections are the major cause of death in cystic fibrosis patients. Infections are difficult to treat since P. aeruginosa is resistant to most antibiotics and its antibiotic susceptibility is decreased when it is present in biofilms. P. aeruginosa produces many exoproducts (including toxins and hydrolytic enzymes) that are involved in virulence. Recent research has elucidated many mechanisms and pathways that regulate the production of these virulence factors. The regulation is extremely complex and many components are influenced by environmental conditions. Quorum sensing is a key regulatory system, which itself is affected by many other regulators. Targeting the regulation of pathogenicity factors provides a novel strategy for combating P. aeruginosa infections. Degradation of acyl homoserine lactones, the signaling molecules of the quorum-sensing system, is a promising therapeutic treatment option.

Self-produced extracellular stimuli modulate the Pseudomonas aeruginosa swarming motility behaviour

Pseudomonas aeruginosa presents three types of motilities: swimming, twitching and swarming. The latter is characterized by rapid and coordinated group movement over a semisolid surface resulting from morphological differentiation and intercellular interactions. A striking feature of P. aeruginosa swarming motility is the formation of migrating tendrils producing colonies with complex fractal-like patterns. Previous studies have shown that normal swarming motility is intimately related to the production of extracellular surface-active molecules: rhamnolipids (RLs), composed of monorhamnolipids (mono-RLs) and dirhamnolipids (di-RLs), and 3-(3-hydroxyalkanoyloxy) alkanoic acids (HAAs). Here, we report that (i) di-RLs attract active swarming cells while HAAs behave as strong repellents, (ii) di-RLs promote and HAAs inhibit tendril formation and migration, (iii) di-RLs and HAAs display different diffusion kinetics on a surface as di-RLs spread faster than HAAs in agar, (iv) di-RLs and HAAs have no effect on swimming cells, suggesting that swarming cells are different from swimming cells not only in morphology but also at the regulatory level and (v) mono-RLs act as wetting agents. We propose a model explaining how HAAs and di-RLs together modulate the behaviour of swarming migrating cells by acting as self-produced negative and positive chemotactic-like stimuli.

Suppression of Pseudomonas aeruginosa quorum-sensing systems by macrolides: a promising strategy or an oriental mystery?

A breakthrough in antibiotic chemotherapy for patients with chronic Pseudomonas aeruginosa pulmonary infections was brought about by findings in a patient with diffuse panbronchiolitis (DPB), who had been treated with erythromycin over a period of years. Recent clinical trials have demonstrated that long-term macrolide therapy can be used not only for DPB patients but also for those with other chronic infections, including patients with cystic fibrosis (CF). The pathogenesis of chronic P. aeruginosa infection is considered to arise from a bacterial cell-to-cell signaling mechanism, named "quorum-sensing", which enables the bacteria to coordinately turn on and off their virulence genes through the production of autoinducer molecules. Accumulating evidence from clinical and basic science fields suggests the potential of macrolides as Pseudomonas quorum-sensing inhibitors. In this review, we briefly summarize the data on the clinical efficacy of macrolides in DPB and CF patients. Then we discuss the mechanisms of action of macrolides from the viewpoint of sub-minimum inhibitory concentration (sub-MIC) macrolide effects on P. aeruginosa, particularly the potential activity of this antibiotic to suppress the bacterial quorum-sensing system.

Pseudomonas aeruginosa PqsA is an anthranilate-coenzyme A ligase

Pseudomonas aeruginosa is an opportunistic human pathogen which relies on several intercellular signaling systems for optimum population density-dependent regulation of virulence genes. The Pseudomonas quinolone signal (PQS) is a 3-hydroxy-4-quinolone with a 2-alkyl substitution which is synthesized by the condensation of anthranilic acid with a 3-keto-fatty acid. The pqsABCDE operon has been identified as being necessary for PQS production, and the pqsA gene encodes a predicted protein with homology to acyl coenzyme A (acyl-CoA) ligases. In order to elucidate the first step of the 4-quinolone synthesis pathway in P. aeruginosa, we have characterized the function of the pqsA gene product. Extracts prepared from Escherichia coli expressing PqsA were shown to catalyze the formation of anthraniloyl-CoA from anthranilate, ATP, and CoA. The PqsA protein was purified as a recombinant His-tagged polypeptide, and this protein was shown to have anthranilate-CoA ligase activity. The enzyme was active on a variety of aromatic substrates, including benzoate and chloro and fluoro derivatives of anthranilate. Inhibition of PQS formation in vivo was observed for the chloro- and fluoroanthranilate derivatives, as well as for several analogs which were not PqsA enzymatic substrates. These results indicate that the PqsA protein is responsible for priming anthranilate for entry into the PQS biosynthetic pathway and that this enzyme may serve as a useful in vitro indicator for potential agents to disrupt quinolone signaling in P. aeruginosa.

Pseudomonas aeruginosa virulence genes identified in a Dictyostelium host model

The human pathogen Pseudomonas aeruginosa has been shown previously to use similar virulence factors when infecting mammalian hosts or Dictyostelium amoebae. Here we randomly mutagenized a clinical isolate of P. aeruginosa, and identified mutants with attenuated virulence towards Dictyostelium. These mutant strains also exhibited a strong decrease in virulence when infecting Drosophila and mice, confirming that P. aeruginosa makes use of similar virulence traits to confront these very different hosts. Further characterization of these bacterial mutants showed that TrpD is important for the induction of the quorum-sensing circuit, while PchH and PchI are involved in the induction of the type III secretion system. These results demonstrate the usefulness and the relevance of the Dictyostelium host model to identify and analyse new virulence genes in P. aeruginosa.

Pseudomonas aeruginosa AlgR represses the Rhl quorum-sensing system in a biofilm-specific manner

AlgR controls numerous virulence factors in Pseudomonas aeruginosa, including alginate, hydrogen cyanide production, and type IV pilus-mediated twitching motility. In this study, the role of AlgR in biofilms was examined in continuous-flow and static biofilm assays. Strain PSL317 (DeltaalgR) produced one-third the biofilm biomass of wild-type strain PAO1. Complementation with algR, but not fimTU-pilVWXY1Y2E, restored PSL317 to the wild-type biofilm phenotype. Comparisons of the transcriptional profiles of biofilm-grown PAO1 and PSL317 revealed that a number of quorum-sensing genes were upregulated in the algR deletion strain. Measurement of rhlA::lacZ and rhlI::lacZ promoter fusions confirmed the transcriptional profiling data when PSL317 was grown as a biofilm, but not planktonically. Increased amounts of rhamnolipids and N-butyryl homoserine lactone were detected in the biofilm effluent but not the planktonic supernatants of the algR mutant. Additionally, AlgR specifically bound to the rhlA and rhlI promoters in mobility shift assays. Moreover, PAO1 containing a chromosomal mutated AlgR binding site in its rhlI promoter formed biofilms and produced increased amounts of rhamnolipids similarly to the algR deletion strain. These observations indicate that AlgR specifically represses the Rhl quorum-sensing system during biofilm growth and that such repression is necessary for normal biofilm development. These data also suggest that AlgR may control transcription in a contact-dependent or biofilm-specific manner.

The autotransporter esterase EstA of Pseudomonas aeruginosa is required for rhamnolipid production, cell motility, and biofilm formation

Pseudomonas aeruginosa PAO1 produces the biodetergent rhamnolipid and secretes it into the extracellular environment. The role of rhamnolipids in the life cycle and pathogenicity of P. aeruginosa has not been completely understood, but they are known to affect outer membrane composition, cell motility, and biofilm formation. This report is focused on the influence of the outer membrane-bound esterase EstA of P. aeruginosa PAO1 on rhamnolipid production. EstA is an autotransporter protein which exposes its catalytically active esterase domain on the cell surface. Here we report that the overexpression of EstA in the wild-type background of P. aeruginosa PAO1 results in an increased production of rhamnolipids whereas an estA deletion mutant produced only marginal amounts of rhamnolipids. Also the known rhamnolipid-dependent cellular motility and biofilm formation were affected. Although only a dependence of swarming motility on rhamnolipids has been known so far, the other kinds of motility displayed by P. aeruginosa PAO1, swimming and twitching, were also affected by an estA mutation. In order to demonstrate that EstA enzyme activity is responsible for these effects, inactive variant EstA* was constructed by replacement of the active serine by alanine. None of the mutant phenotypes could be complemented by expression of EstA*, demonstrating that the phenotypes affected by the estA mutation depend on the enzymatically active protein.

Nitrate sensing and metabolism modulate motility, biofilm formation, and virulence in Pseudomonas aeruginosa

Infection by the bacterial opportunist Pseudomonas aeruginosa frequently assumes the form of a biofilm, requiring motility for biofilm formation and dispersal and an ability to grow in nutrient- and oxygen-limited environments. Anaerobic growth by P. aeruginosa is accomplished through the denitrification enzyme pathway that catalyzes the sequential reduction of nitrate to nitrogen gas. Mutants mutated in the two-component nitrate sensor-response regulator and in membrane nitrate reductase displayed altered motility and biofilm formation compared to wild-type P. aeruginosa PAO1. Analysis of additional nitrate dissimilation mutants demonstrated a second level of regulation in P. aeruginosa motility that is independent of nitrate sensor-response regulator function and is associated with nitric oxide production. Because motility and biofilm formation are important for P. aeruginosa pathogenicity, we examined the virulence of selected regulatory and structural gene mutants in the surrogate model host Caenorhabditis elegans. Interestingly, the membrane nitrate reductase mutant was avirulent in C. elegans, while nitrate sensor-response regulator mutants were fully virulent. The data demonstrate that nitrate sensing, response regulation, and metabolism are linked directly to factors important in P. aeruginosa pathogenesis.

Quorum sensing regulates denitrification in Pseudomonas aeruginosa PAO1

Anaerobic growth of Pseudomonas aeruginosa PAO1 was affected by quorum sensing. Deletion of genes that produce N-acyl-l-homoserine lactone signals resulted in an increase in denitrification activity, which was repressed by exogenous signal molecules. The effect of the las quorum-sensing system was dependent on the rhl quorum-sensing system in regulating denitrification.

An rhl-like quorum-sensing system negatively regulates pyoluteorin production in Pseudomonas sp. M18

Pseudomonas sp. M18, isolated from the watermelon rhizosphere, is antagonistic against a number of soil-borne pathogens. This strain produces an uncharacterized red pigment, pyoluteorin (Plt), and two N-acylhomoserine lactones (AHLs). A previously isolated red-pigment-defective mutant, M18-T510, contains an insert within a gene similar to rhlI in P. aeruginosa PAO1. The M18 rhlI gene product is responsible for the production of two AHL signals: N-butyryl-homoserine lactone and N-hexanoylhomoserine lactone. Mutants defective in either rhlI or rhlR showed enhanced Plt biosynthesis due to loss of transcriptional repression, which was mediated, at least in part, by suppressed expression of the activator PltR. A Plt-specific ABC transporter was also upregulated in the rhl mutants in a Plt-dependent manner. In comparison with the wild-type strain, the rhl mutants survived longer during stationary-phase growth.

The impact of quorum sensing and swarming motility on Pseudomonas aeruginosa biofilm formation is nutritionally conditional

The role of quorum sensing in Pseudomonas aeruginosa biofilm formation is unclear. Some researchers have shown that quorum sensing is important for biofilm development, while others have indicated it has little or no role. In this study, the contribution of quorum sensing to biofilm development was found to depend upon the nutritional environment. Depending upon the carbon source, quorum-sensing mutant strains (lasIrhlI and lasRrhlR) either exhibited a pronounced defect early in biofilm formation or formed biofilms identical to the wild-type strain. Quorum sensing was then shown to exert its nutritionally conditional control of biofilm development through regulation of swarming motility. Examination of pilA and fliM mutant strains further supported the role of swarming motility in biofilm formation. These data led to a model proposing that the prevailing nutritional conditions dictate the contributions of quorum sensing and swarming motility at a key juncture early in biofilm development.

Effect of site-specific mutations in different phosphotransfer domains of the chemosensory protein ChpA on Pseudomonas aeruginosa motility

The virulence of Pseudomonas aeruginosa and other surface pathogens involves the coordinate expression of a wide range of virulence determinants, including type IV pili. These surface filaments are important for the colonization of host epithelial tissues and mediate bacterial attachment to, and translocation across, surfaces by a process known as twitching motility. This process is controlled in part by a complex signal transduction system whose central component, ChpA, possesses nine potential sites of phosphorylation, including six histidine-containing phosphotransfer (HPt) domains, one serine-containing phosphotransfer domain, one threonine-containing phosphotransfer domain, and one CheY-like receiver domain. Here, using site-directed mutagenesis, we show that normal twitching motility is entirely dependent on the CheY-like receiver domain and partially dependent on two of the HPt domains. Moreover, under different assay conditions, point mutations in several of the phosphotransfer domains of ChpA give rise to unusual "swarming" phenotypes, possibly reflecting more subtle perturbations in the control of P. aeruginosa motility that are not evident from the conventional twitching stab assay. Together, these results suggest that ChpA plays a central role in the complex regulation of type IV pilus-mediated motility in P. aeruginosa.

Identification of QuiP, the product of gene PA1032, as the second acyl-homoserine lactone acylase of Pseudomonas aeruginosa PAO1

The relevance of the acyl homoserine lactone (acyl-HSL) quorum signals N-3-oxododecanoyl-homoserine lactone (3OC12HSL) and N-butanoyl-homoserine lactone to the biology and virulence of Pseudomonas aeruginosa is well investigated. Previously, P. aeruginosa was shown to degrade long-chain, but not short-chain, acyl-HSLs as sole carbon and energy sources (J. J. Huang, J.-I. Han, L.-H. Zhang, and J. R. Leadbetter, Appl. Environ. Microbiol. 69:5941-5949, 2003). A gene encoding an enzyme with acyl-HSL acylase activity, pvdQ (PA2385), was identified, but it was not required for acyl-HSL utilization. This indicated that P. aeruginosa encodes another acyl-HSL acylase, which we identify here. A comparison of total cell proteins of cultures grown with long-acyl acyl-HSLs versus other substrates implicated the involvement of a homolog of PvdQ, the product of gene PA1032, for which we propose the name QuiP. Transposon mutants of quiP were defective for growth when P. aeruginosa was cultured in medium containing decanoyl-HSL as a sole carbon and energy source. Complementation with a functional copy of quiP rescued this growth defect. When P. aeruginosa was grown in buffered lysogeny broth, constitutive expression of QuiP in P. aeruginosa led to decreased accumulations of the quorum signal 3OC12HSL, relative to the wild type. Heterologous expression of QuiP was sufficient to confer long-chain acyl-HSL acylase activity upon Escherichia coli. Examination of gene expression patterns during acyl-HSL-dependent growth of P. aeruginosa further supported the involvement of quiP in signal decay and revealed other genes also possibly involved. It is not yet known under which "natural" conditions quiP is expressed or how P. aeruginosa balances the expression of its quorum-sensing systems with the expression of its acyl-HSL acylase activities.

A network of networks: quorum-sensing gene regulation in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa possesses two N-acyl-homoserine lactone quorum-sensing systems that regulate large, overlapping sets of genes. Recent global transcriptome and proteome analyses provided a wealth of information about the identity of the regulated genes, N-acyl-homoserine lactone signal specificity, timing of induction, and environmental effects on gene expression. Quorum-sensing gene expression in P. aeruginosa is also embedded in a highly interconnected network of other regulatory systems with a high potential for integrating and responding to multiple environmental signals. Such epigenetic complexity may constitute the basis for the exceptional adaptability of P. aeruginosa to diverse environments.

Quorum sensing: cell-to-cell communication in bacteria

Bacteria communicate with one another using chemical signal molecules. As in higher organisms, the information supplied by these molecules is critical for synchronizing the activities of large groups of cells. In bacteria, chemical communication involves producing, releasing, detecting, and responding to small hormone-like molecules termed autoinducers . This process, termed quorum sensing, allows bacteria to monitor the environment for other bacteria and to alter behavior on a population-wide scale in response to changes in the number and/or species present in a community. Most quorum-sensing-controlled processes are unproductive when undertaken by an individual bacterium acting alone but become beneficial when carried out simultaneously by a large number of cells. Thus, quorum sensing confuses the distinction between prokaryotes and eukaryotes because it enables bacteria to act as multicellular organisms. This review focuses on the architectures of bacterial chemical communication networks; how chemical information is integrated, processed, and transduced to control gene expression; how intra- and interspecies cell-cell communication is accomplished; and the intriguing possibility of prokaryote-eukaryote cross-communication.

Pseudomonas aeruginosa rhamnolipids disperse Bordetella bronchiseptica biofilms

We have previously reported that the respiratory pathogen Bordetella bronchiseptica can form biofilms in vitro. In this report, we demonstrate the disruption of B. bronchiseptica biofilms by rhamnolipids secreted from Pseudomonas aeruginosa. This suggests that biosurfactants such as rhamnolipids may be utilized as antimicrobial agents for removing Bordetella biofilms.

Rhamnolipids modulate swarming motility patterns of Pseudomonas aeruginosa

Pseudomonas aeruginosa is capable of twitching, swimming, and swarming motility. The latter form of translocation occurs on semisolid surfaces, requires functional flagella and biosurfactant production, and results in complex motility patterns. From the point of inoculation, bacteria migrate as defined groups, referred to as tendrils, moving in a coordinated manner capable of sensing and responding to other groups of cells. We were able to show that P. aeruginosa produces extracellular factors capable of modulating tendril movement, and genetic analysis revealed that modulation of these movements was dependent on rhamnolipid biosynthesis. An rhlB mutant (deficient in mono- and dirhamnolipid production) and an rhlC mutant (deficient in dirhamnolipid production) exhibited altered swarming patterns characterized by irregularly shaped tendrils. In addition, agar supplemented with rhamnolipid-containing spent supernatant inhibited wild-type (WT) swarming, whereas agar supplemented with spent supernatant from mutants that do not make rhamnolipids had no effect on WT P. aeruginosa swarming. Addition of purified rhamnolipids to swarming medium also inhibited swarming motility of the WT strain. We also show that a sadB mutant does not sense and/or respond to other groups of swarming cells and this mutant was capable of swarming on media supplemented with rhamnolipid-containing spent supernatant or purified rhamnolipids. The abilities to produce and respond to rhamnolipids in the context of group behavior are discussed.

Airway biofilms: implications for pathogenesis and therapy of respiratory tract infections

The differentiation of bacterial biofilms in the airway environment, the pathogenesis of airway biofilm, and possible therapeutic methods are discussed. Biofilm diseases that characteristically involve the respiratory system include cystic fibrosis (CF), diffuse panbronchiolitis (DPB), and bronchiectasia with Pseudomonas aeruginosa (P. aeruginosa) infection. There is evidence to suggest that almost all strains of P. aeruginosa have the genetic capacity to synthesize alginate, a main matrix of biofilms, when ecological conditions are unfavorable for their survival. The bacteria inside the mature biofilm show increased resistance to both antibacterials and phagocytic cells, express fewer virulence factors because of their stationary state of growth, and are less stimulatory to the mucosa because of the 'sandwich binding'. These factors facilitate both the colonization of bacteria and their extended survival even under unfavorable conditions. Since the biofilm limits colonization to a latent form, the clinical symptoms in this situation are unremarkable. However, the clinical progression of both CF and DPB proceeds in two characteristic directions. The first is an acute exacerbation caused by planktonic bacteria that have germinated from the biofilm. The second is a slow progression of disease that is induced by harmful immune reactions. The harmful reactions are mediated by alginate, which induces antigen antibody reactions around the airways, as well as formation of circulating immune complexes that are deposited on lung tissue. Furthermore, the highest titer of bacterial permeability increasing anti-neutrophil cytoplasmic autoantibodies (BPI-ANCA) is observed in association with highly impaired pulmonary function in patients with CF and DPB, as well as in patients with a lengthy period of colonization with P. aeruginosa. BPI-ANCA subsequently makes chronic airway infection even more intractable. The long-term use of 14- or 15-ring membered macrolides results in a favorable clinical outcome for patients with DPB and in some patients with CF. In the last 10 years, an increasing number of studies have reported secondary actions of macrolides that include effects on both airway and phagocytic cells, as well as an anti-biofilm activity. The 14- or 15-ring membered macrolides inhibit: (i) the alginate production from P. aeruginosa; (ii) the antibody reaction to alginate, which leads to a decrease in the immune complex formation; and (iii) the activation of the autoinducer 3-O-C12-homoserine lactone and subsequent expression of lasI and rhlI in quorum sensing systems in P. aeruginosa. These anti-biofilm actions of macrolides may represent their basic mechanisms of action on airway biofilm disease.

Production and properties of an inhibitor of the Pseudomonas autoinducer by Pseudomonas aeruginosa

An inhibitor was found in the culture fluid of Pseudomonas aeruginosa PAO1, which could inhibit the activity of the Pseudomonas autoinducer (PAI). The maximal inhibitory activity occurred in stationary phase culture sup ernatant. The PAI inhibitor did not influence the cell growth and the PAI production by P. aeruginosa PAO1 when the PAI inhibitor was added into culture medium. The induced expression of lacZ in the reporter strain Agrobacterium tumefaciens NT1 was suppressed by this PAI inhibitor, whereas inhibition could be relieved by increasing the auto inducer concentration. The quorum sensing of P. aeruginosa was inhibited presumably by inhibiting the inducing activity of Pseudomonas autoinducer but not by inhibiting the production of Pseudomonas autoinducer. It was demonstrated that the structure of the PAI inhibitor was different from that of acyl-homoserine lactones.Key words: quorum sensing, autoinducer, PAI inhibitor, Pseudomonas aeruginosa, N-acylhomoserine lactone.

Regulation of Pseudomonas quinolone signal synthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic lung infections in cystic fibrosis patients and is a major source of nosocomial infections. This bacterium controls many virulence factors by using two quorum-sensing systems, las and rhl. The las system is composed of the LasR regulator protein and its cell-to-cell signal, N-(3-oxododecanoyl) homoserine lactone, and the rhl system is composed of RhlR and the signal N-butyryl homoserine lactone. A third intercellular signal, the Pseudomonas quinolone signal (PQS; 2-heptyl-3-hydroxy-4-quinolone), also regulates numerous virulence factors. PQS synthesis requires the expression of multiple operons, one of which is pqsABCDE. Previous experiments showed that the transcription of this operon, and therefore PQS production, is negatively regulated by the rhl quorum-sensing system and positively regulated by the las quorum-sensing system and PqsR (also known as MvfR), a LysR-type transcriptional regulator protein. With the use of DNA mobility shift assays and beta-galactosidase reporter fusions, we have studied the regulation of pqsR and its relationship to pqsA, lasR, and rhlR. We show that PqsR binds the promoter of pqsA and that this binding increases dramatically in the presence of PQS, implying that PQS acts as a coinducer for PqsR. We have also mapped the transcriptional start site for pqsR and found that the transcription of pqsR is positively regulated by lasR and negatively regulated by rhlR. These results suggest that a regulatory chain occurs where pqsR is under the control of LasR and RhlR and where PqsR in turn controls pqsABCDE, which is required for the production of PQS.

The contribution of MvfR to Pseudomonas aeruginosa pathogenesis and quorum sensing circuitry regulation: multiple quorum sensing-regulated genes are modulated without affecting lasRI, rhlRI or the production of N-acyl-L-homoserine lactones

The transcriptional regulator MvfR is required for full Pseudomonas aeruginosa virulence, the function of multiple quorum sensing (QS)-regulated virulence factors and the synthesis of 4-hydroxy-2-alkylquinolines (HAQs), including the Pseudomonas quinolone signal (PQS). Here we investigate the role of MvfR in the QS circuitry and P. aeruginosa pathogenesis. We demonstrate using a combination of biochemical and molecular approaches, including transcription profiling, that MvfR is involved in the regulation of multiple P. aeruginosa QS-controlled genes without altering the expression of lasRI/rhlRI or the production of N-acyl-L-homoserine lactone (AHL) signals. Dissection of how mvfR is interwoven into the P. aeruginosa QS circuitry reveals that the MvfR system, through the essential contribution of PqsE, positively regulates a subset of genes dependant on both LasR and RhlR. Animal studies show that MvfR contributes to P. aeruginosa virulence by controlling the transcription of genes not under RhlR regulation, and that reduced virulence of a mvfR mutant is caused by the loss of pqsE expression and not only a deficiency in HAQs/PQS production. This study provides novel insights into the unique role of the MvfR system in AHL-mediated QS and further supports its importance in P. aeruginosa pathogenesis.

Sub-inhibitory concentrations of ceftazidime and tobramycin reduce the quorum sensing signals of Pseudomonas aeruginosa

AIM

Concentrations of antimicrobials below minimum inhibitory concentration (subMIC) may reduce the production by Pseudomonas aeruginosa of virulence factors such as elastase. We sought to determine whether the reduction in elastase production may be mediated by a reduction in acyl-homoserine lactones.

METHODS

Pseudomonas aeruginosa in broth was exposed to three conditions for ceftazidime and tobramycin: control, 6% MIC and 25% MIC. Elastase was assayed using elastin congo red. N-(3-Oxododecanoyl)-homoserine lactone (C12-HSL) and N-butyryl-homoserine lactone (C4-HSL) were assayed using biosensor Escherichia coli.

RESULTS

Elastase was unchanged with ceftazidime. Elastase was reduced by 16% at 6% MIC tobramycin and reduced by 70% at 25% MIC tobramycin (P<0.0001). As a percentage of control, C12-HSL was mean 69.4% (SEM 7.3%) at 6% MIC tobramycin, and 31.7% (3.3%) at 25% MIC tobramycin (P=0.0001). C12-HSL was 78.9% (5.3%) at 6% MIC ceftazidime and was 29.7% (1.8%) at 25% MIC ceftazidime (P=0.0001). Both ceftazidime and tobramycin were associated with reduced C4-HSL at 6% MIC and 25% MIC (P<0.03).

CONCLUSIONS

SubMIC tobramycin but not ceftazidime reduced elastase production by P. aeruginosa. In contrast, subMIC concentrations of both antimicrobials reduced C12-HSL and C4-HSL. It is unlikely that reduced HSL is the sole explanation for the reduction in elastase.

Degradation and synthesis kinetics of quorum-sensing autoinducer in Pseudomonas aeruginosa cultivation

The quorum-sensing (las and rhl) systems play critical roles in the pathogenicity of Pseudomonas aeruginosa and its synthesis of the important biosurfactants, rhamnolipids. In this work, P. aeruginosa PAO1 and its rhlI and rhlR null mutants were used to study the degradation and synthesis kinetics of the rhl system's autoinducer PAI2 (N-butanoyl-homoserine lactone). The two mutants, lacking the ability of synthesizing PAI2 or RhlR protein, produced insignificant amounts of rhamnolipids while having similar growth profiles as the wild-type culture. The regulatory RhlR:PAI2 complex is thus essential to rhamnolipid synthesis. In batch culture of the wild-type PAO1, the autoinducer PAI2 concentration increased along cell growth, especially during the transition from exponential-growth phase to stationary phase, and began to decrease after entering the stationary phase. The decrease in the stationary phase resulted from a faster PAI2 degradation than its synthesis. The degradation kinetics was studied using PAI2-containing supernatants (from centrifuged broth of wild-type culture) with and without the rhlI(-) mutant cells incapable of PAI2 synthesis. Being insignificant in the cell-free systems, PAI2 degradation was found predominantly cell-associated and could be described empirically by the first-order, exponential decay kinetics with the best-fit degradation constant (k(d)) of 0.195 h(-1). When similarly modeled with a first-order kinetics, PAI2 synthesis in stationary-phase wild-type culture was derived to have a synthesis constant (k(s)) of 0.189 h(-1). The PAI2 concentration in batch cultivation of the rhlR(-) mutant also showed an increase-then-decrease profile. However, the maximum PAI2 concentration was about one third of that from the wild-type culture. The constitutive rate of PAI2 synthesis was therefore significantly lower than the rate attainable with active auto-induction by RhlR-PAI2 complex.

Solubility and bioactivity of the Pseudomonas quinolone signal are increased by a Pseudomonas aeruginosa-produced surfactant

Pseudomonas aeruginosa is a gram-negative bacterium that causes serious infections in immunocompromised individuals and cystic fibrosis patients. This opportunistic pathogen controls many of its virulence factors and cellular functions through the activity of three cell-to-cell signals, N-(3-oxododecanoyl)-L-homoserine lactone, N-butyryl-L-homoserine lactone, and the Pseudomonas quinolone signal (PQS). The activity of these signals is dependent upon their ability to dissolve in and freely diffuse through the aqueous solution in which P. aeruginosa happens to reside. Despite this, our data indicated that PQS was relatively insoluble in aqueous solutions, which led us to postulate that P. aeruginosa could be producing a PQS-solubilizing factor. In this report, we show that the P. aeruginosa-produced biosurfactant rhamnolipid greatly enhances the solubility of PQS in aqueous solutions. The enhanced solubility of PQS led to an increase in PQS bioactivity, as measured by both a gene induction assay and an apoptosis assay. This is the first demonstration of the importance of a bacterial surfactant in the solubilization and bioactivity of a cell-to-cell signal.

[Quorum sensing: the possibility of the new antibiotic target in bacterial infection]

Expression of many virulence factors in P. aeruginosa is regulated by a cell density dependent mechanism called quorum sensing. Quorum sensing allows P. aeruginosa to sense the density of the surrounding bacterial population and to coordinately regulate transcription of various virulence genes. Pseucomonas aeruginosa is a common pathogen infecting chronic respiratory infections, such as diffuse panbronchiolitis (DPB) patients. Although these patients are typically treated with multiple anti-pseudomonal antibiotics, the infection is rarely eradicated and often results in mortality. In the 1980s it was reported that long-term therapy with low doses of erythromycin improved the clinical symptoms of DPB patients colonized with P. qeruginosa. Recently it has been demonstrated that sub MIC concentrations of macrolides strongly inhibite Pseudomonas quorum sensing system. These data suggested a novel mechanism of quorum sensing regulation of antibiotic sensitivity.

Role of quorum sensing in the pathogenicity of Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of human and animal melioidosis. The role of quorum sensing (QS) in the in vivo pathogenicity of B. pseudomallei via inhalational exposure of BALB/c mice and intraperitoneal challenge of Syrian hamsters has not been reported. This investigation demonstrates that B. pseudomallei encodes a minimum of three luxI and five luxR homologues that are involved in animal pathogenicity. Mass spectrometry analysis of culture supernatants revealed that wild-type B. pseudomallei and the luxI mutants synthesized numerous signalling molecules, including N-octanoyl-homoserine lactone, N-decanoyl-homoserine lactone, N-(3-hydroxyoctanoyl)-l-homoserine lactone, N-(3-hydroxydecanoyl)-l-homoserine lactone and N-(3-oxotetradecanoyl)-l-homoserine lactone, which was further confirmed by heterologous expression of the B. pseudomallei luxI alleles in Escherichia coli. Mutagenesis of the B. pseudomallei QS system increased the time to death and reduced organ colonization of aerosolized BALB/c mice. Further, intraperitoneal challenge of Syrian hamsters with the B. pseudomallei QS mutants resulted in a significant increase in the LD50. Using semi-quantitative plate assays, preliminary analysis suggests that QS does not affect lipase, protease and phospholipase C biosynthesis/secretion in B. pseudomallei. The findings of the investigation demonstrate that B. pseudomallei encodes multiple luxIR genes, and disruption of the QS alleles reduces animal pathogenicity, but does not affect exoproduct secretion.

Transcriptional regulation of Pseudomonas aeruginosa rhlR, encoding a quorum-sensing regulatory protein

The Pseudomonas aeruginosa rhlR gene encodes the transcriptional regulator RhlR which has a central role in the quorum-sensing response. Different gene products involved in bacterial pathogenesis are regulated at the transcriptional level by two quorum-sensing response systems, Las and Rhl. The expression of rhlR has been reported to be under the control of the Las system, but its transcriptional regulation has not been studied in detail. Here, the rhlR promoter region has been characterized and shown to present four different transcription start sites, two of which are included in the upstream gene (rhlB) coding region. It was found that rhlR expression is not only dependent on LasR but also on different regulatory proteins such as Vfr and RhlR itself, and also on the alternative sigma factor sigma(54). It is reported that rhlR expression is partially LasR-independent under certain culture conditions and is strongly influenced by environmental factors.

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

Quorum sensing (QS)-based transcriptional responses in Pseudomonas aeruginosa have been defined on the basis of increases in transcript levels of QS-controlled genes such as lasB and aprA following the hierarchical transcriptional increases of central controllers such as the lasR gene. These increases occur at high bacterial concentrations such as early-stationary-phase growth in vitro. However, the extent to which the increases occur in a variety of clinical and environmental isolates has not been determined nor is there extensive information on allelic variation in lasR genes. An analysis of the sequences of the lasR gene among 66 clinical and environmental isolates showed that 81% have a sequence either identical to that of strain PAO1 or with a silent mutation, 15% have nucleotide changes resulting in amino acid changes, and 5% have an insertion sequence in the lasR gene. Using real-time PCR to quantify transcript levels of lasR, lasB, and aprA in the early log and early stationary phases among 35 isolates from bacteremia and pneumonia cases and the environment, we found most (33 of 35) strains had increases in lasR transcripts in early stationary phase but with a very wide range of final transcript levels per cell. There was a strong correlation (r(2) = 0.84) between early-log- and early-stationary-phase transcript levels in all strains, but this finding remained true only for the 50% of strains above the median level of lasR found in early log phase. There were significant (P < 0.05) but weak-to-modest correlations of lasR transcript levels with aprA (r(2) = 0.2) and lasB (r(2) = 0.5) transcript levels, but again this correlation occurred only in the 50% of P. aeruginosa strains with the highest levels of lasR transcripts in early stationary phase. There were no differences in distribution of lasR alleles among the bacteremia, pneumonia, or environmental isolates. Overall, only about 50% of P. aeruginosa strains from clinical and environmental sources show a lasR-dependent increase in the transcription of aprA and lasB genes, indicating that for about 50% of clinical isolates this regulatory system may not play a significant role in pathogenesis.

Pseudomonas aeruginosa quorum sensing as a potential antimicrobial target

Pseudomonas aeruginosa has two complete quorum-sensing systems. Both of these systems have been shown to be important for Pseudomonas virulence in multiple models of infection. Thus, these systems provide unique targets for novel antimicrobial drugs.

Pseudomonas aeruginosa quorum sensing as a potential antimicrobial target

Pseudomonas aeruginosa has two complete quorum-sensing systems. Both of these systems have been shown to be important for Pseudomonas virulence in multiple models of infection. Thus, these systems provide unique targets for novel antimicrobial drugs.

Dueling quorum sensing systems in Pseudomonas aeruginosa control the production of the Pseudomonas quinolone signal (PQS)

The opportunistic human pathogen Pseudomonas aeruginosa regulates the production of numerous virulence factors via the action of two separate but coordinated quorum sensing systems, las and rhl. These systems control the transcription of genes in response to population density through the intercellular signals N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C(12)-HSL) and N-(butanoyl)-L-homoserine lactone (C(4)-HSL). A third P. aeruginosa signal, 2-heptyl-3-hydroxy-4-quinolone [Pseudomonas quinolone signal (PQS)], also plays a significant role in the transcription of multiple P. aeruginosa virulence genes. PQS is intertwined in the P. aeruginosa quorum sensing hierarchy with its production and bioactivity requiring the las and rhl quorum sensing systems, respectively. This report presents a preliminary transcriptional analysis of pqsA, the first gene of the recently discovered PQS biosynthetic gene cluster. We show that pqsA transcription required pqsR, a transcriptional activator protein encoded within the PQS biosynthetic gene cluster. It was also found that the transcription of pqsA and subsequent production of PQS was induced by the las quorum sensing system and repressed by the rhl quorum sensing system. In addition, PQS production was dependent on the ratio of 3-oxo-C(12)-HSL to C(4)-HSL, suggesting a regulatory balance between quorum sensing systems. These data are an important early step toward understanding the regulation of PQS synthesis and the role of PQS in P. aeruginosa intercellular signaling.

Biosurfactants: evolution and diversity in bacteria

In summary, biosurfactants are an example of a class of microbial natural products that has coevolved among many genera. But whereas the biosurfactants produced in the bacterial and archaeal domains are convergent in function (suggesting that they are very important), they have developed in parallel with respect to genotype and phenotype (the surfactants are not related genetically or in terms of molecular structure). Because of this parallel evolution, currently available molecular screening techniques are of little use for the discovery of new biosurfactants. Development of such techniques will continue to be problematic because there is no relationship between the surfactants produced by different microbial genera and even species. Yet, the potential for application of biosurfactants and other natural products is great due to growing demand for biodegradable and environmentally friendly analogues for synthetic chemicals.

The Pseudomonas aeruginosa quinolone signal molecule overcomes the cell density-dependency of the quorum sensing hierarchy, regulates rhl-dependent genes at the onset of stationary phase and can be produced in the absence of LasR

In Pseudomonas aeruginosa, diverse exoproduct virulence determinants are regulated via N-acylhomoserine lactone-dependent quorum sensing. Here we show that 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) is also an integral component of the quorum sensing circuitry and is required for the production of rhl-dependent exoproducts at the onset of stationary phase. Analysis of spent P. aeruginosa culture supernatants revealed that PQS is produced at the end of exponential phase in the parent strain and in the late stationary phase of a lasR mutant. Mutants defective in both PQS production (pqsR-) and response (pqsE-) produced substantially reduced levels of exoproducts but retained wild-type N-butanoyl homoserine lactone (C4-HSL) levels. In the wild type, provision of exogenous PQS at the time of inoculation significantly increased PA-IL lectin, pyocyanin and elastase production during early stationary phase and promoted biofilm formation. Exogenous PQS but not PQS derivatives lacking the 3-hydroxy group overcame the cell density but not growth phase-dependent production of exoproducts. PQS also overcame the transcriptional and post-transcriptional repression of lecA (which codes for the PA-IL lectin) mediated via the negative regulators MvaT and RsmA respectively. Increased expression of lecA in the presence of exogenous PQS can be explained partially by increases in RhlR, RpoS and C4-HSL levels. A refined model for quorum sensing in P. aeruginosa is presented.

rhlA is required for the production of a novel biosurfactant promoting swarming motility in Pseudomonas aeruginosa: 3-(3-hydroxyalkanoyloxy)alkanoic acids (HAAs), the precursors of rhamnolipids

Pseudomonas aeruginosa produces extracellular glycolipids composed of L-rhamnose and 3-hydroxyalkanoic acid called rhamnolipids. Although these compounds are usually regarded as biosurfactants or haemolysins, their exact physiological function is not well understood. Rhamnolipids are synthesized by a rhamnosyltransferase, encoded by the rhlAB operon, which catalyses the transfer of TDP-L-rhamnose to 3-(3-hydroxyalkanoyloxy)alkanoic acid (HAA) moieties of various lengths. RhlB is the catalytic protein of the rhamnosyltransferase. rhlA is indispensable for rhamnolipid synthesis, but its function is unknown. Using a liquid chromatography/mass spectrometry method, the production of extracellular HAAs by P. aeruginosa was detected previously and it was demonstrated that they are the actual precursors of rhamnolipid biosynthesis. In this report, evidence is presented indicating that rhlA is required for production of HAAs and that these HAAs display potent surface-active properties. P. aeruginosa can colonize surfaces by swarming motility, a form of organized translocation requiring the production of wetting agents. Using rhlA and rhlB mutants it was observed that swarming requires the expression of the rhlA gene but does not necessitate rhamnolipid production, as HAAs act as surfactants. Finally, it was shown that the use of ammonium instead of nitrate as source of nitrogen and an excess of available iron both decrease rhlA expression and swarming motility.