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Protein & Cell

, Volume 6, Issue 1, pp 26–41 | Cite as

The hierarchy quorum sensing network in Pseudomonas aeruginosa

  • Jasmine Lee
  • Lianhui ZhangEmail author
Open Access
Review

Abstract

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.

Keywords

quorum sensing IQS PQS las rhl Pseudomonas aeruginosa virulence environmental factors 

Abbreviations

AHL

acylhomoserine lactone

CNP

C-type natriuretic peptide

HCN

hydrogen cyanide

IFN-γ

interferon gamma

IQS

integrated quorum sensing system

QS

quorum sensing

QSIs

quorum sensing inhibitors

Introduction

Pseudomonas aeruginosa is a ubiquitous, gram-negative bacterium that thrives in diverse habitats and environments. Usually a commensal on the host body, P. aeruginosa is capable of transforming into an opportunistic pathogen when there is a breach of host tissue barriers or a suppressed immune system (Van Delden and Iglewski, 1998). P. aeruginosa is an important nosocomial pathogen, affecting a wide category of patients convalescing in hospitals. They include patients with cystic fibrosis and other lung diseases, traumatized cornea, burns, Gustilo open fractures, long-term intubated patients, the immune-compromised and elderly individuals. The infections caused by P. aeruginosa are usually resistant to treatment by multiple antibiotics and can lead to severe and persistent infections (Bonomo and Szabo, 2006; Chernish and Aaron, 2003; Doshi et al., 2011; Tan, 2008). This translates into further complications and secondary fungal infections, extension of hospital stay, therapeutic failure, and in some cases, premature death of cystic fibrosis patients (Henry et al., 1992; Kosorok et al., 2001; Rabin et al., 2004; Tan, 2008). Because P. aeruginosa grows and survives in various environmental conditions, it makes acquiring an infection extremely easy and outbreaks of extreme drug-resistant strains are common among hospital wards and intensive care units.

It is believed that understanding the regulatory mechanisms with which P. aeruginosa governs virulence gene expression may hold the key to develop alternative therapeutic interventions to control and prevent the bacterial infections (Fig. 1). The recent research progresses show that a bacterial cell-cell communication mechanism, widely known as quorum sensing (QS), plays a key role in modulating the expression of virulence genes in P. aeruginosa. The term quorum sensing was proposed two-decades ago by three renowned microbiologists based on the bacterial population density-dependent regulatory mechanisms found in several microbial organisms, including Vibrio fischeri, Agrobacterium tumefaciens, P. aeruginosa and Erwinia carotovora (Fuqua et al., 1994). Since then, various QS systems have been found in many bacterial pathogens, which are commonly associated with the regulation of virulence gene expression and biofilm formation (Deng et al., 2011; Ng and Bassler, 2009; Pereira et al., 2013; Whitehead et al., 2001). Typically, quorum sensing bacteria produce and release small chemical signals, and at a high population density, the accumulated signals interact with cognate receptors to induce the transcriptional expression of various target genes including those encoding production of virulence factors. While QS becomes a popular concept, it is worthy to note that opinions arose on whether QS is the most-fitted term for mechanistic explanation of the above-mentioned bacterial group behavior. The point of contention stemmed from the fact that autoinducer concentration, the key determinant of “quorum” as defined by QS, was not simply a function of bacterial cell density, but a combined output of many factors such as diffusion rate and spatial distribution, and hence alternative terms such as “diffusion sensing”, “efficiency sensing” and “combinatorial quorum sensing” were proposed (Hense et al., 2007; Redfield, 2002; Cornforth et al., 2014). Whilst interesting, these alternative opinions await further experimental endorsement and by far QS remains as the most rigorously tested mechanism of bacteria cell-cell communication and collective responses.
Figure 1

Virulence mechanisms employed during P. aeruginosa infections

Given its importance as a human pathogen, P. aeruginosa has been the subject of intensive investigations and become one of the model organisms in QS research. The research progresses in the last two decades have unveiled a sophisticated hierarchy QS network in this pathogen, which consists of a few sets of connected systems, including las, iqs, pqs and rhl. Particularly, recent findings show that the QS network in P. aeruginosa is highly adaptable and capable of responding to external biostress cues, which provides the pathogen flexibility in the control of virulence gene expression. It would not be surprising that other bacterial pathogens may have also evolved similar flexible QS systems which could respond to changed environmental conditions. This is an important factor to consider in the development of quorum sensing inhibitors (QSIs) as therapeutics, since bacteria routinely encounters adverse environmental conditions when infecting host organisms. This review will provide an overview on the QS systems in P. aeruginosa, focusing on a recently discovered integrated quorum sensing system (IQS), and on the interactions between all the four QS systems and how environmental cues could affect the QS hierarchy.

Quorum Sensing Systems in Pseudomonas aeruginosa

History of quorum sensing

The concept of quorum sensing in P. aeruginosa was an extension of the studies based on the prototype luxI-luxR system in Vibrio fischeri, in which luxI encodes the biosynthesis of an acylhomoserine lactone (AHL) signal N-(3-oxohexanoyl)-L-homoserine lactone (OHHL), and luxR encodes an AHL-dependent transcription factor (Eberhard, 1972; Nealson et al., 1970; Stewart and Williams, 1992; Williams et al., 1992). With significant homology to the LuxR protein, LasR in P. aeruginosa was initially identified to be a key regulator in the expression of lasB gene encoding for a metalloprotease elastase (Cook, 1992; Gambello and Iglewski, 1991). Subsequently, LasR was also shown to be required for the transcription of aprA, lasA and toxA, and thus it was thought to be a global regulator of the virulence genes in P. aeruginosa (Gambello and Iglewski, 1991; Gambello et al., 1993; Passador et al., 1993; Toder et al., 1991). LasI, the LuxI equivalent in P. aeruginosa, was proposed to synthesize AHL signals with autoinducing and elastase-regulating properties (Jones et al., 1993). One year later, the actual chemical structure of this Pseudomonas autoinducer (PAI) was characterized as N-(3-oxododecanoyl)-homoserine lactone (OdDHL) (Pearson et al., 1994). PAI is structurally related to the autoinducers discovered in other gram-negative bacteria species (Cao and Meighen, 1993; Eberhard et al., 1981; Zhang et al., 1993).

Shortly after, a second autoinducer, factor 2, was discovered in P. aeruginosa (Pearson et al., 1995). This discovery was made following a puzzling observation that an unusually high concentration of OdDHL was required to activate the lasB promoter (Pearson et al., 1995), suggesting that another factor in PAO1 may be required for lasB activation. The P. aeruginosa factor 2 was structurally identified to be N-butyrylhomoserine lactone (BHL) (Pearson et al., 1995). BHL was not shown to interact with LasR protein directly to activate lasB gene expression, nor does it directly regulate the latter (Pearson et al., 1995), triggering another hunt for its cognate receptor. Within the same year, RhlR, a regulatory protein encoded by the rhamnolipid synthase gene cluster rhlABR, was identified to be the cognate receptor of BHL (Ochsner and Reiser, 1995). The rhlI gene, which encodes the biosynthesis of BHL and sharing significant sequence homologies to luxI and lasI, was found at the downstream of the rhlABR cluster. Expression of RhlI could restore the production of several exoproducts such as elastase, pyocyanin, hemolysin and rhamnolipids, and both RhlI and RhlR are required for the full activation of the rhlABR and lasB promoters (Brint and Ohman, 1995; Ochsner and Reiser, 1995).

The las and rhl quorum sensing systems

These key discoveries in P. aeruginosa QS systems inspired further researches on their functions, regulons and the molecular mechanisms with which the las and rhl circuits activate the expression of QS-responsive genes. The results showed that upon binding with the respective autoinducers OdDHL and BHL, the receptor proteins LasR and RhlR get activated and form complexes. The LasR-OdDHL and RhlR-BHL complexes bind to the conserved las-rhl boxes residing in the promoters of target genes, thereby activating their transcriptional expression (Schuster and Greenberg, 2007; Whiteley and Greenberg, 2001; Whiteley et al., 1999). Transcriptomic studies based on lasI and rhlI mutants revealed that the regulons are on a continuum, with some genes that respond dramatically well to OdDHL (e.g. lasA), some with BHL specificities (e.g. rhlAB), and some equally well to both signals (Schuster and Greenberg, 2006; Schuster et al., 2003). These genes constitute nearly 10% of P. aeruginosa genome, and therefore accounts for a majority of the physiological processes and virulence phenotypes (Schuster and Greenberg, 2006). Some of these key virulence genes are listed for the convenience of discussion (Table 1).
Table 1

Examples of quorum sensing (QS) regulated virulence factors and their effects to the human host

QS regulated gene

Protein or virulence factor

Effects to host during infections

Benefits to P. aeruginosa

References

lasB

Elastase

Degradation of elastin, collagen, and other matrix proteins

Extracellular iron acquisition from host proteins

Wolz et al. (1994); Yanagihara et al. (2003)

lasA

Protease

Disruption of epithelial barrier

Staphylolytic activity, host immune evasion and enhanced colonization

Kessler et al. (1993); Park et al. (2000)

toxA

Exotoxin A

Cell death

Establishment of infection; enhanced colonization

Daddaoua et al. (2012); McEwan et al. (2012)

aprA

Alkaline protease

Degradation of host complement system and cytokines

Immune evasion and persistent colonization

Laarman et al. (2012)

rhlAB

Rhamnosyl-transferases (rhamnolipids)

Necrosis of host macrophage and polymorphonuclear lymphocytes

Immune evasion; biofilm development

Jensen et al. (2006); Lequette and Greenberg (2005)

lecA

Lectin (galactophilic lectin)

Paralysis of airway cilia

Establishment of infection; enhanced colonization

Adam et al. (1997)

hcnABC

Hydrogen cyanide

Cellular respiration arrest; Poorer lung function

Enhanced colonization

Ryall et al. (2008); Solomonson (1981)

phzABCDEFG, phzM

Pyocyanin

Oxidative effects dampen host cellular respiration and causes oxidative stress; Paralysis of airway cilia; Delayed inflammatory response to P. aeruginosa infections through neutrophil damage

Establishment of infection; enhanced colonization; immune evasion

Denning et al. (1998); Jackowski et al. (1991); Lau et al. (2004)

LasR also induces the expression of RsaL, a transcriptional repressor of lasI. Binding of RsaL to the bidirectional rsaL-lasI promoter inhibits the expression of both genes, which generates a negative feedback loop that counteracts the positive signal feedback loop mentioned earlier, thereby balancing the levels of OdDHL (Rampioni et al., 2007). Whilst LasR/OdDHL and RsaL do not compete for the same binding site on the lasI promoter region, the repression by RsaL is stronger than the activation by LasR (Rampioni et al., 2007). RsaL also inhibits the expression of some QS target genes such as biosynthetic genes of pyocyanin and cyanide (Rampioni et al., 2007). A range of positive and negative regulatory proteins were subsequently identified and they control the las and rhl systems in a variety of ways. Noteworthy are the regulatory effects of QscR and VqsR, which are homologues of LuxR. QscR forms heterodimers with LasR/OdDHL and RhlR/BHL and prevents their binding with the promoter DNA of downstream responsive genes, therein dampening the las and rhl QS signalling effects (Ledgham et al., 2003a). QscR also binds to OdDHL and utilize it for activating its own regulon (Chugani et al., 2001; Fuqua, 2006; Schuster and Greenberg, 2006). VqsR is a positive regulator of the las QS system and is itself regulated by the LasR/OdDHL complex (Li et al., 2007). More recently, an anti-activator QslA was identified, which binds to LasR via protein-protein interaction and prevents the interaction of the latter with promoter DNA of the las responsive genes. The inhibitory effect of QslA on LasR is irrespective of OdDHL concentrations. By disrupting the ability of LasR to trigger the expression of downstream genes and cause a QS response, QslA controls the overall QS activation threshold (Seet and Zhang, 2011). There are quite a few other super-regulators of the AHL-based QS systems which are summarized in the table below (Table 2). In addition, quorum quenching enzymes, which degrade AHL signals, the AHL-acylases PvdQ and QuiP, are also involved in balancing the level of AHL signals in P. aeruginosa (Huang et al., 2006; Sio et al., 2006).
Table 2

Super-regulators of QS in P. aeruginosa

Regulator

Mechanism of action

References

AlgR2

Negative transcriptional regulator of lasR and rhlR

Ledgham et al. (2003a); Westblade et al. (2004)

DksA

Negative transcriptional regulator of rhlI

Branny et al. (2001); Jude et al. (2003); van Delden et al. (2001)

GacA/GacS

Positive transcriptional regulator of lasR and rhlR

Parkins et al. (2001); Reimmann et al. (1997)

MvaT

Negative transcriptional regulator (global regulation)

Diggle et al. (2002)

QscR

Negative regulator (anti-activator) of LasR protein

Chugani et al. (2001); Ledgham et al. (2003b)

QslA

Negative regulator (anti-activator) of LasR and PqsR proteins

Seet and Zhang (2011)

QteE

Negative post-translational regulator of LasR and RhlR

Siehnel et al. (2010)

RpoN

Negative transcriptional regulator of lasRI and rhlRI

Heurlier et al. (2003); Thompson et al. (2003)

RpoS

Negative transcriptional regulator of rhlI

Latifi et al. (1996); Schuster et al. (2004); Whiteley et al. (2000)

RsaL

Negative transcriptional regulator of lasI

Bertani and Venturi (2004); de Kievit et al. (1999)

RsmA

Negative transcriptional regulator of lasI

Pessi et al. (2001)

Vfr

Positive transcriptional regulator of lasR and rhlR

Albus et al. (1997)

VpsR

Positive transcriptional regulator of lasI

Juhas et al. (2004)

Quinolone-based intercellular signaling

The third QS signal, PQS, was purified and characterized in 1999 by Pesci and co-workers when they observed that spent culture media from wild type PAO1 causes a dramatic induction of lasB expression in a lasR mutant of P. aeruginosa, which could not be mimicked by OdDHL or BHL (Pesci et al., 1999). PQS is structurally identified as 2-heptyl-3-hydroxy-4-quinolone, and it is chemically unique from the AHL signals of the las and rhl systems. Originally studied as an antibacterial molecule (Cornforth and James, 1956; Lightbown and Jackson, 1956), this is the first instance that a 4-quinolone compound was reported as a signalling molecule in bacteria. The PQS synthesis cluster has been identified to consist of pqsABCD, phnAB and pqsH (Gallagher et al., 2002). Shortly after the identification of PQS signal, the receptor PqsR (then known as MvfR) has been implicated in the regulation of PQS production (Cao et al., 2001). PqsA is an anthranilate-coenzyme A ligase (Coleman et al., 2008; Gallagher et al., 2002), which activates anthranilate to form anthraniloyl-coenzyme A, initiating the first step of the PQS biosynthesis. A pqsA mutant does not produce any akyl-quinolones (AQs) (Deziel et al., 2004). PqsB, PqsC and PqsD are probable 3-oxoacyl-(acyl carrier protein) synthases and they mediate the conversion of anthranilate into 2-heptyl-4-quinolone (HHQ) by incorporation of β-ketodecanoic acid (Deziel et al., 2004; Gallagher et al., 2002). HHQ is the precursor of PQS and can be intercellularly transmitted between P. aeruginosa cells. HHQ is converted into PQS by the action of PqsH, a putative flavin-dependent monooxygenase that purportedly hydroxylates HHQ at the 3-position (Deziel et al., 2004; Dubern and Diggle, 2008; Gallagher et al., 2002; Schertzer et al., 2009). The transcription of pqsH is controlled by LasR, implying that the PQS system is controlled by the las system (Schertzer et al., 2009). PqsL is also predicted to be a monooxygenase and is most likely to be involved in the synthesis of the AQ N-oxides, (e.g. 4-hydroxy-2-heptylquinoline-N-oxide, HQNO) (Lépine et al., 2004). Disruption in PqsL caused an overproduction of PQS (D’Argenio et al., 2002), probably owing to a blocked AQ N-oxide pathway which leads to an accumulation of HHQ (Deziel et al., 2004; Lépine et al., 2004). In certain strains of P. aeruginosa, accumulation of PQS and HHQ leads to autolysis and cell death (D’Argenio et al., 2002; D’Argenio et al., 2007; Whitchurch et al., 2005). The role of PqsE remains largely unknown, which is a probable metallo-β-lactamase. Mutation of pqsE does not affect PQS biosynthesis (Gallagher et al., 2002), but the mutants failed to respond to PQS (Diggle et al., 2003; Farrow et al., 2008; Gallagher et al., 2002), and did not express the PQS-controlled phenotypes such as pyocyanin and PA-IL lectin production. In contrast, overexpression of PqsE alone led to enhanced pyocyanin and rhamnolipid production, which is otherwise dependent on the PQS signaling system (Farrow et al., 2008). These puzzling phenomena need to be further investigated for elucidating the role of PqsE in the bacterial physiology and virulence.

PqsR is a LysR-type transcriptional regulator that binds to the promoter region of pqsABCDE operon and directly controls the expression of the operon (Cao et al., 2001; Gallagher et al., 2002). The expression of pqsR is in turn controlled by LasR/OdDHL (Camilli and Bassler, 2006). PqsR is the cognate receptor of PQS and also its co-inducer, as the activity of PqsR in inducing the expression of pqsABCDE is dramatically increased when PQS is bound by the receptor (Wade et al., 2005; Xiao et al., 2006b). HHQ was also found to be able to bind to and induce the expression of PqsR, though it does so with ~ 100-fold less potency than PQS (Wade et al., 2005; Xiao et al., 2006a). Mutation of pqsR resulted in non-production of any AQs and pyocyanin (Cao et al., 2001; Gallagher et al., 2002; Schertzer et al., 2009; von Bodman et al., 2008), indicating that PqsR is essential for executing PQS signal transduction.

The importance of pqs signaling system in the bacterial infection has been illustrated by a range of studies. Null mutation of the pqs system resulted in reduced biofilm formation and decreased production of virulence factors such as pyocyanin, elastase, PA-IL lectin and rhamnolipids (Cao et al., 2001; Diggle et al., 2003; Rahme et al., 2000; Rahme et al., 1997). PQS is also required for full virulence towards plants (Cao et al., 2001), nematodes (Gallagher et al., 2002) and mice (Cao et al., 2001; Lau et al., 2004). In burn-wound mouse models, the killing abilities of pqsA are attenuated compared to the wild type parental strain (Déziel et al., 2005; Xiao et al., 2006b). Intriguingly, the pqsH mutant did not result in a decrease in virulence in burn-wound mouse model (Xiao et al., 2006b), but displayed a reduced killing on nematodes (Gallagher et al., 2002), hence the importance of PQS in regulation of virulence remains debatable. PQS, its precursor HHQ, and the derivative HQNO (4-hydroxy-2-heptylquinoline-N-oxide), are often found in the sputum, bronchoalveolar fluid and mucopurulent fluid of cystic fibrosis sufferers (Collier et al., 2002). Taken together, this could suggest that the precursors of PQS may play an equally important role as PQS in virulence and infections.

An integrated QS system

Recently, a fourth inter-cellular communication signal has been discovered to be capable of integrating environmental stress cues with the quorum sensing network (Lee et al., 2013). Named as IQS, it belongs to a new class of quorum sensing signal molecules and was structurally established to be 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde. The genes that are involved in IQS synthesis are a non-ribosomal peptide synthase gene cluster ambBCDE. When disrupted, it caused a decrease in the production of PQS and BHL signals, as well as the virulence factors such as pyocyanin, rhamnolipids and elastase. Upon addition of 10 nmol/L IQS to the mutants, these phenotypes could be restored fully, indicating that IQS is a potent inter-cellular communication signal compared with its counterparts (Fig. 2). Further, IQS has been shown to contribute to the full virulence of P. aeruginosa in four different animal host models (mouse, zebrafish, fruitfly and nematode), highlighting the important roles of this new QS system in modulation of bacterial pathogenesis. Importantly, under phosphate depletion stress conditions, IQS was demonstrated to be able to partially take over the functions of the central las system (Lee et al., 2013), providing critical clues in understanding the puzzling phenomenon that the clinical isolates of P. aeruginosa frequently harbour mutated lasI or lasR genes (Ciofu et al., 2010; D’Argenio et al., 2007; Hoffman et al., 2009; Smith et al., 2006).
Figure 2

Structures ofP. aeruginosaquorum sensing (QS) signals. Clockwise from left, N-(3-oxododecanoyl)-homoserine lactone (OdDHL); N-butyrylhomoserine lactone (BHL); 2-heptyl-3-hydroxy-4-quinolone (Pseudomonas Quinolone Signal, PQS); 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde (Integrated Quorum Sensing Signal, IQS)

Interconnection between the four QS systems

The QS circuits in P. aeruginosa are organized in a hierarchical manner. At the top of the signalling hierarchy is the las system. When activated by OdDHL, LasR-OdDHL complex multimerizes and activates the transcription of rhlR, rhlI, lasI (hence a positive feedback loop), and other virulence genes that are part of its regulon (Kiratisin et al., 2002; Latifi et al., 1996; Pesci et al., 1997). The RhlR-BHL complex also dimerizes and similarly activates the expression of its own regulon and rhlI, forming the second positive feedback loop (Ventre et al., 2003; Winson et al., 1995). LasR-OdDHL also positively regulates PqsR, the transcriptional regulator of the HHQ/PQS biosynthesis operon pqsABCD, as well as the expression of pqsH, the gene encoding the final converting enzyme of PQS from HHQ (Deziel et al., 2004; Gallagher et al., 2002; Xiao et al., 2006a). PQS, in turn, was found to be able to enhance the transcription of rhlI, thus influencing BHL production and the overall expression of the rhl QS system, thus indirectly modulating the rhl-dependent phenotypes (McKnight et al., 2000; Pesci et al., 1999). Interestingly, pqsR and pqsABCDE expression is inhibited by RhlR/BHL (Cao et al., 2001), suggesting that the ratio of the concentrations between OdDHL and BHL play a decisive role in the dominance of the pqs signaling system (Cao et al., 2001).

With las governing the expression of both pqs and rhl systems, it was often described as being at the top of the QS hierarchy. The rhl system on the other hand, is under the control of both las and pqs, yet many QS-dependent virulence factors are predominantly activated by RhlR-BHL (Latifi et al., 1995; Schuster and Greenberg, 2007; Schuster et al., 2004; Whiteley et al., 1999; Winzer et al., 2000), thus the rhl system functions like a workhorse for the QS command. Since LasR-OdDHL controls the onset and activation of both the pqs and rhl QS circuits, these systems therefore represent a step-wise activation cascade that will be triggered by attainment of a “quorum” in P. aeruginosa cultures. The recently identified IQS was also found to be tightly controlled by LasRI under rich medium conditions. Disruption of either lasR or lasI completely abrogates the expression of ambBCDE and the production of IQS (Lee et al., 2013) (Fig. 3).
Figure 3

Schematic representation of the four QS signaling networks inP. aeruginosaand their respective regulons. Arrows indicate a stimulatory effect. Perpendicular lines indicate an inhibitory effect

However, exceptions do occur. The lasR mutants were found to have a delayed production of PQS, instead of having an abolished PQS system as previously thought, and PQS could also overcome the dependency on LasR in activating the expression of rhl QS system and production of downstream virulence factors (Diggle et al., 2003). It was subsequently discovered that this could be due to the effects of RhlR, as the lasR and rhlR double mutant had barely any detectable PQS, but when rhlR was overexpressed, the production of PQS, as well as virulence factors such as LasB elastase and LasA protease, are restored (Dekimpe and Deziel, 2009). RhlR was also shown to upregulate the expression of lasI, the most-specific LasR-regulated gene, and OdDHL production was consequently increased (Dekimpe and Deziel, 2009). This indicates that compensation by the rhl QS system could override this hierarchy and maintain the expression of QS-dependent virulence factors in spite of a non-functional central las system. Similarly, the dominance of las on IQS signal production was reversed when P. aeruginosa was subjected to phosphate depletion stress, and the iqs system could up-regulate the expression of pqs and rhl systems and the production of QS-dependent virulence factors in the lasI or lasR mutant (Lee et al., 2013). Low phosphate levels also elevate IQS production in wild type P. aeruginosa (Lee et al., 2013). These findings highlight the importance of environmental factors in modulating the bacterial QS systems and the plasticity of the QS networks in accommodation and exploitation of environmental changes for the benefit of bacterial pathogens. The next section is dedicated to discussion of such examples in details with the aim to shed light on understanding the complicated and sophisticated QS regulatory mechanisms in P. aeruginosa.

Environmental triggers and the QS responses

Evidence is accumulating that environmental stress conditions could exert substantial influence on the QS systems of P. aeruginosa. Starvation, phosphate and iron depletion are known to promote the expression and activity of RhlR in the absence of lasR (Jensen et al., 2006; Van Delden et al., 1998). More recently, it was found that phosphate depletion could induce IQS production even in the absence of functional las system (Lee et al., 2013). This discovery is clinically significant as substantial amount of P. aeruginosa chronic infection isolates bear a loss-of-function las system (Cabrol et al., 2003; Denervaud et al., 2004; Hamood et al., 1996; Schaber et al., 2004; Smith et al., 2006). The roles and the molecular mechanisms with which various environmental cues and host immune factors modulate the QS systems of P. aeruginosa will be discussed separately in the following sections.

Phosphate-depletion stress

Phosphate is essential for all living cells owing to its key roles in signal transduction reactions such as phospho-relay, and as an essential component of the energy molecule ATP, nucleotides, phospholipids and other important biomolecules. Foreseeably, bacterial pathogens may encounter strong competition for free phosphates from host cells during the process of pathogen-host interaction. Therefore, the ability to withstand phosphate starvation and the response mechanisms of harnessing phosphate from external sources is critical for P. aeruginosa survival and establishment of infections. As a result, phosphate-depletion stress has been shown to have far-reaching effects on QS signalling profiles, gene expression, physiology and virulence of bacterial pathogens (Chugani and Greenberg, 2007; Frisk et al., 2004; Jensen et al., 2006; Lee et al., 2013; Zaborin et al., 2009).

When facing with phosphate limitation, P. aeruginosa exhibits increased swarming motility and cytotoxicity towards the human bronchial epithelial cell line 16HBE14o- (Bains et al., 2012), attesting to the strong responses phosphate deprivation could elicit from the pathogen. Additionally, phosphate depletion stress was shown to prompt the up-regulation of iron chelator pyoverdine biosynthesis, which in turn, could result in the inactivation of the phosphate acquisition pathway. When the pyoverdine signalling pathway was interrupted, pyochelin biosynthesis was in turn increased as compensation (Zaborin et al., 2009). This resulted in high amounts of ferric ions to be acquired. Coupled with the dramatic increase in PQS production (part of the phosphate starvation response), the lethal PQS-Fe(III) red coloured complex was formed. When ingested, the red-spotted P. aeruginosa caused rapid mortality in C. elegans, a phenomenon known as “red death” (Zaborin et al., 2009). Such signalling cross-talk demonstrates the interconnectivity between the phosphate and iron acquisition systems in P. aeruginosa, the investment in resources the bacteria makes to maintain their homeostasis, and the deleterious effects on the host when the fine balance is tipped.

The lack of phosphate also dramatically activates the expression of pqsR and the PqsR-regulated pqsABCDE and phnAB genes. Along with the enhanced pqs system, the expression of QS-associated virulence genes responsible for the synthesis of rhamnolipids, phenazines, cyanide, exotoxin A and LasA protease are similarly induced (Bains et al., 2012; Zaborin et al., 2009). This was thought to lead to the acute mortality rate of the host organism Caenorhabditis elegans after being infected by P. aeruginosa that were grown in phosphate starvation medium (Zaborin et al., 2009). These observations correlate and could well be explained by our current knowledge on IQS. With depletion in phosphate, expression of iqs system is induced (Lee et al., 2013), which in turn triggers an up-regulation of the downstream pqs and rhl QS systems, and eventually, an observed boost in QS-associated virulence factors production and killing rates.

It is crucial to note that the two-component sensor-response regulator system PhoBR plays an indispensable role in detection and signal transduction of phosphate stress cues (Anba et al., 1990; Filloux et al., 1988; Hsieh and Wanner, 2010), as disruption of phoB completely abolished the virulence of P. aeruginosa towards C. elegans (Zaborin et al., 2009), and dramatically diminished its swarming motility and cytotoxicity (Bains et al., 2012). PhoB (and the pho regulon) was also shown to participate in the inhibition of biofilm formation, c-di-GMP signal degradation and repression of the type III secretion systems (Haddad et al., 2009), all of which could significantly affect the clinical outcome during P. aeruginosa infections (Abe et al., 2005; Costerton, 2001; Hauser et al., 2002; Hueck, 1998; Roy-Burman et al., 2001). The phoB mutant grows poorly in low phosphate medium and failed to produce the QS-dependent virulence factor pyocyanin (Lee and Zhang, unpublished data). Remarkably, PhoBR is indispensable for coordinating the las-independent, phosphate-dependent IQS signalling activation, wherein the “IQS phenotype” would be abolished in a phoB mutant (Lee et al., 2013). The PhoBR-IQS loop could also explain the observations by Jensen and co-workers, who reported that low phosphate prompted an enhancement of the rhl QS system even when las was functionally absent and this is coordinated by PhoB (Jensen et al., 2006).

Iron and PQS signaling system

Unlike phosphate, the modulatory effect of iron starvation on P. aeruginosa QS networks appears to be less direct. A deficiency in iron does lead to notable increases in the expression of genes involved in iron acquisition (ferric uptake siderophores, pyochelin and pyoverdine; ferrous iron transporters like haem and feo), exoenzymes that could cleave iron-bound host proteins (alkaline protease, lasB elastase) and other redox enzymes and toxins (exotoxin A) (Ochsner et al., 2002). Further, the iron depletion stress response was found to lead to an inhibition of oxygen transfer from the atmosphere to liquid P. aeruginosa cultures, thus protecting bacteria cells from oxidative stress. Production of the virulence factor LasB elastase is also significantly increased in these iron depletion cultures (Kim et al., 2003). Although some of the upregulated virulence factors, like alkaline protease and elastase, are known to be regulated by the QS systems of P. aeruginosa (see Table 1), a direct link between iron deprivation and up-regulation of central QS genes such as lasI, lasR, rhlI or rhlR has yet to be found. In a report by Diggle and co-workers, the PQS molecules were found to function as an iron trap when secreted into the extracellular milieu of P. aeruginosa (Diggle et al., 2007). This was hypothesized to serve the purpose of storing up free ferric ions which could subsequently be internalized into the cells by the siderophores, in order to safeguard against a sudden dip in iron concentration. Iron starvation could also trigger a Fur-dependent de-repression of the small regulatory RNAs prrF1 and prrF2 expression. PrrF1 and PrrF2 bind to and inhibit the expression of antABC genes which encode for the anthranilate degradation enzymes AntABC. Since anthranilate is the precursor of PQS biosynthesis, inhibition of its degradation could lead to accumulation of anthranilate, which consequently elevates the concentration of HHQ and PQS in the bacteria cells. This in turn might boost the PQS-PqsR signaling pathway. PqsR was also found to inhibit antABC expression, albeit in a PrrF1,2-independent manner (Oglesby et al., 2008). Taken together, the above findings seem to suggest that iron depletion stress may modulate bacterial virulence through the pqs system, which awaits further investigations.

ANR and oxygen deprivation

Low oxygen tension is a key factor affecting cyanide biosynthesis (cyanogenesis) in P. aeruginosa (Castric, 1994; Castric, 1983). The final product, hydrogen cyanide (HCN), is a highly potent extracellular virulence factor and contributes to high mortality rates during infection of host organisms (Ryall et al., 2008; Solomonson, 1981). Additionally, increase in P. aeruginosa cell density was also shown to remarkably elevate expression of hcnABC, the synthase genes for HCN, and reaches its optimum levels during the transit from exponential to stationary growth phase of the bacteria (Castric et al., 1979). This may suggest a cooperative link between oxygen deprivation and QS in the regulatory mechanism of cyanogenesis, which was subsequently demonstrated through characterization of ANR, a transcriptional regulator associated with bacterial anaerobic growth.

ANR, which is converted into its active form when oxygen tension is low, is a key regulator controlling the expression of arginine deiminase and nitrate reductase. ANR belongs to the FNR (fumarate and nitrate reductase regulator) family of transcriptional regulators and is the main transcriptional regulator that acts in parallel with the QS systems for the expression of hydrogen cyanide biosynthesis genes (Pessi and Haas, 2000). ANR, together with LasR-OdDHL or RhlR-BHL, bind to the promoter region of the hcnABC cluster, exhibiting a synergistic effect brought upon by oxygen limitation stress. Further, the PRODORIC promoter analysis programme predicted the FNR/ANR binding consensus sequences in up to 25% of the predicted QS-controlled promoters, implying that ANR might be an important co-regulator of the QS-dependent virulence genes in anaerobic environments (Schuster and Greenberg, 2006).

Starvation stress

When exposed to unfavourable environments and nutrient starvation, P. aeruginosa must rapidly cope and elicit a prompt response to modify their metabolic profiles for survival. This process is termed as the stringent response and brings about diverse effects ranging from inhibition of growth processes to cell division arrest (Joseleau-Petit et al., 1999; Svitil et al., 1993) and more importantly, a premature activation of the P. aeruginosa QS systems that is independent of cell-density (van Delden et al., 2001). The QS signals BHL and N-hexanoyl-homoserine lactone (HHL) are prematurely produced and PQS synthesis inhibited (Baysse et al., 2005). The spike in BHL QS signal is likely to result in the concomitant increase in production of downstream virulence factors elastase and rhamnolipids (Schafhauser et al., 2014).

The QS-based response is mediated by the stringent response protein RelA. In face of amino acid shortage, uncharged tRNA triggers the activity of the ribosome-associated RelA, which in turn synthesizes ppGpp (nucleotide guanosine 3’,5’-bisdiphosphate), an intracellular signal that enables the bacteria cell to self-perceive their inability in synthesis of proteins (Gentry and Cashel, 1996). When overexpressed, RelA leads to early transcriptional expression of the lasR and rhlR genes, as well as production of QS signals OdDHL and BHL (van Delden et al., 2001), hence leading to the overproduction of the aforementioned QS-dependent virulence factors. Furthermore, RelA and ppGpp was also shown to coordinate the stress response associated with alterations in membrane phospholipid composition and loss of membrane fluidity. When the phospholipid biosynthesis protein LptA was deleted, an increase in relA expression and ppGpp production was observed, which resulted in a premature activation of BHL and HHL QS signals biosynthesis (Baysse et al., 2005).

In a recent study, Schafhauser and co-workers observed that the synthesis of the starvation signal ppGpp negatively regulates the biosynthesis of HHQ and PQS signals, and is required for full expression of both the las and rhl QS systems (Schafhauser et al., 2014). In the relA and spoT double mutant that is unable to synthesize ppGpp, both the las and rhl QS systems are down-regulated, and the production of QS-dependent virulence factors rhamnolipid and elastase are reduced (Schafhauser et al., 2014). Whilst it has been previously reported that ppGpp increases the expression of LasR and RhlR and the resultant downstream factors (Baysse et al., 2005; van Delden et al., 2001), repression on the pqs system by ppGpp is somewhat unexpected. More experiments are required to investigate on the significance of this selective dampening of the pqs system.

Response to host factors

It has been traditionally thought that opportunistic pathogens such as Pseudomonas aeruginosa invade hosts with a weakened immune system or attenuated epithelial barrier in a passive manner, until an important observation was made by Wu and colleagues that P. aeruginosa major outer-membrane protein OprF is able to recognize and bind to human T cell-based cytokine interferon gamma (IFN-γ). This in turn activates the rhl QS system and substantially enhances the expression of lecA and production of its encoded virulence protein, galactophilic lectin. Pyocyanin, an additional QS-regulated virulence factor, was also found to be up-regulated in the presence of IFN-γ (Wu et al., 2005). Although IFN-γ was the only cytokine found to activate the rhl QS system and it is not known whether and if yes, how the upstream las and pqs networks are affected, this work presents a direct evidence of the interactions between host-derived immune factors and bacterial membrane proteins, which consequently leads to QS-based responses. In another example, dynorphin, an endogenous κ-receptor agonist, was found to penetrate the bacterial membrane and potently induce the expression of pqsR and pqsABCDE, and lead to increased biosynthesis of PQS, HHQ and the related derivative HQNO. The growth advantage against probiotic gut microorganisms Lactobacillus spp. and virulence towards C. elegans is also remarkably enhanced when P. aeruginosa is exposed to dynorphin (Zaborina et al., 2007). This finding is of particular significance to P. aeruginosa caused gut infections as dynorphin is usually in high concentrations in the intestinal mucosa and epithelial cells, attesting to the remarkable mechanisms utilized by the bacteria to enhance virulence by integrating host opioids into its existing QS circuitry.

Further, human hormones, particularly the C-type natriuretic peptide (CNP) that is produced by endothelial cells and used for maintaining body fluid homeostasis and blood pressure control, was demonstrated to have positive effects on P. aeruginosa virulence. Through activation of the P. aeruginosa membrane natriuretic peptides sensor, CNP induces a rise in intracellular cAMP concentration and lead to the activation of the global virulence activator Vfr, which either alone or together with another regulator PtxR, enhances the synthesis of QS signals OdDHL and BHL, and inhibits the production of PQS. Vfr also drives the increased expression of virulence factors hydrogen cyanide and lipopolysaccharide, thereby elevating the mortality rate in C. elegans infected with CNP-treated P. aeruginosa (Blier et al., 2011).

Most recently, the human host defence peptide LL-37, the only cathelicidin class of cationic antimicrobial peptides synthesized by phagocytes, epithelial cells and keratinocytes, was revealed to exert a positive effect on P. aeruginosa QS and virulence profiles. When stimulated by exogenous LL-37 at physiological concentrations, P. aeruginosa exhibits heightened production of virulence factors pyocyanin, hydrogen cyanide, elastase and rhamnolipids. The PQS signal level is also elevated. LL-37 was also found to decrease the susceptibility of the bacteria to gentamicin and ciprofloxacin antibiotics. These phenotypes were suggested to be mediated by the quinolone response protein and virulence regulator PqsE (Strempel et al., 2013).

Summary and perspectives

Pseudomonas aeruginosa is one of the most notorious opportunistic human pathogens as it employs a variety of virulence factors and mechanisms during infection (Fig. 1). The type of virulence pathways activated is often dependent on the environment conditions and stresses the bacteria encounter. Extensive research over the past two decades has documented numerous instances of environmental cues including the biostresses of host origin, which could dramatically influence the virulence phenotypes of P. aeruginosa. The findings from recent research progresses suggest that these effects could largely be through modulation of the bacterial QS network, which comprises at least four QS signaling mechanisms including las, iqs, pqs and rhl. In particular, the most recently identified IQS highlights how a bacterial QS system could integrate environmental cues with bacterial quorum information. These four systems interact closely with one another giving rise to an intricately linked intercellular communication network. Such a complicated and multi-component QS network may enable P. aeruginosa to accommodate various environmental cues and biostresses (Fig. 4).
Figure 4

Schematic representation of how environmental conditions and host factors influence theP. aeruginosaQS signaling hierarchy. For simplicity, the QS systems are represented as a whole unit, namely, las, iqs, pqs and rhl

Previous efforts in the design of anti-QS therapeutics were focused primarily on inhibition of the las system (Borlee et al., 2010; Mattmann and Blackwell, 2010). However, in light of the recent discovery that IQS could replace the functions of las in conditions that closely mimics host infection (Lee et al., 2013), coupled with the high mutation frequencies of lasR typical of P. aeruginosa clinical isolates (Ciofu et al., 2010; D’Argenio et al., 2007; Hoffman et al., 2009; Smith et al., 2006), it becomes clear that the ongoing strategies targeting the las system is insufficient, and that the prevalence of IQS system in clinical isolates should be evaluated to ensure development of potent anti-QS therapeutics. Furthermore, we should also keep in mind that there are many unknowns that require further investigations for clear understanding of how the bacterial QS network could act on various environmental cues in regulation of bacterial virulence and biofilm formation. For example, it is not clear how IQS could regulate the downstream pqs and rhl signaling systems and what is the impact of iqs system on the virulence of clinical isolates. Similarly, much remains to be done in understanding whether and if yes, how environmental cues could modulate the las, pqs and rhl systems. Recognition of how the external stressors change the way the QS network is connected may generate tremendous impact on the perspective from which therapeutic interventions could be developed, especially those environmental cues almost always encountered by P. aeruginosa during infections of the host. For instance, successful establishment of an infection and colonization of the cystic fibrosis lung chambers would require P. aeruginosa strains to sense, withstand and respond to deprivation of iron, phosphate, and attacks by lung macrophage-derived factors (Campodonico et al., 2008; Konings et al., 2013; Krieg et al., 1988). Then, as the pathogen transits into a long-term, chronic infection mode, the stresses of living within a biofilm matrix may include oxygen deprivation and nutrient limitation (Jackson et al., 2013; Sauer et al., 2004). Investigation along this line will further advance our understanding of the complicated and sophisticated QS regulatory mechanisms and may continue to generate unexpected interesting findings.

Notes

Acknowledgements

This work was funded by the Biomedical Research Council, Agency for Science, Technology and Research (A*STAR), Singapore, and by the National Natural Science Foundation of China (Grant No. 31330002). We apologize to the scientists who made contributions to the field, but their works have not been cited due to space limitations.

Compliance with Ethics Guidelines

Jasmine Lee and Lian-Hui Zhang declare that they have no conflict of interest and this article does not contain any studies with human or animal subjects performed by the any of the authors.

References

  1. Abe A, Matsuzawa T, Kuwae A (2005) Type-III effectors: sophisticated bacterial virulence factors. C R Biol 328:413–428Google Scholar
  2. Adam EC, Mitchell BS, Schumacher DU, Grant G, Schumacher U (1997) Pseudomonas aeruginosa II lectin stops human ciliary beating: therapeutic implications of fucose. Am J Respir Crit Care Med 155:2102–2104Google Scholar
  3. Albus AM, Pesci EC, Runyen-Janecky LJ, West SE, Iglewski BH (1997) Vfr controls quorum sensing in Pseudomonas aeruginosa. J Bacteriol 179:3928–3935Google Scholar
  4. Anba J, Bidaud M, Vasil ML, Lazdunski A (1990) Nucleotide sequence of the Pseudomonas aeruginosa phoB gene, the regulatory gene for the phosphate regulon. J Bacteriol 172:4685–4689Google Scholar
  5. Bains M, Fernandez L, Hancock RE (2012) Phosphate starvation promotes swarming motility and cytotoxicity of Pseudomonas aeruginosa. Appl Environ Microbiol 78:6762–6768Google Scholar
  6. Baysse C, Cullinane M, Dénervaud V, Burrowes E, Dow JM, Morrissey JP, Tam L, Trevors JT, O’Gara F (2005) Modulation of quorum sensing in Pseudomonas aeruginosa through alteration of membrane properties. Microbiology 151:2529–2542Google Scholar
  7. Bertani I, Venturi V (2004) Regulation of the N-acyl homoserine lactone-dependent quorum-sensing system in rhizosphere Pseudomonas putida WCS358 and cross-talk with the stationary-phase RpoS sigma factor and the global regulator GacA. Appl Environ Microbiol 70:5493–5502Google Scholar
  8. Blier A-S, Veron W, Bazire A, Gerault E, Taupin L, Vieillard J, Rehel K, Dufour A, Le Derf F, Orange N (2011) C-type natriuretic peptide modulates quorum sensing molecule and toxin production in Pseudomonas aeruginosa. Microbiology 157:1929–1944Google Scholar
  9. Bonomo RA, Szabo D (2006) Mechanisms of multidrug resistance in Acinetobacter species and Pseudomonas aeruginosa. Clinical Infectious Diseases 43(Suppl 2):S49–S56Google Scholar
  10. Borlee BR, Geske GD, Blackwell HE, Handelsman J (2010) Identification of synthetic inducers and inhibitors of the quorum-sensing regulator LasR in Pseudomonas aeruginosa by high-throughput screening. Appl Environ Microbiol 76:8255–8258Google Scholar
  11. Branny P, Pearson JP, Pesci EC, Kohler T, Iglewski BH, Van Delden C (2001) Inhibition of quorum sensing by a Pseudomonas aeruginosa dksA homologue. J Bacteriol 183:1531–1539Google Scholar
  12. Brint JM, Ohman DE (1995) Synthesis of multiple exoproducts in Pseudomonas aeruginosa is under the control of RhlR-RhlI, another set of regulators in strain PAO1 with homology to the autoinducer-responsive LuxR-LuxI family. Journal of Bacteriology 177:7155–7163Google Scholar
  13. Cabrol S, Olliver A, Pier GB, Andremont A, Ruimy R (2003) Transcription of quorum-sensing system genes in clinical and environmental isolates of Pseudomonas aeruginosa. J Bacteriol 185:7222–7230Google Scholar
  14. Camilli A, Bassler BL (2006) Bacterial small-molecule signaling pathways. Science 311:1113–1116Google Scholar
  15. Campodonico VL, Gadjeva M, Paradis-Bleau C, Uluer A, Pier GB (2008) Airway epithelial control of Pseudomonas aeruginosa infection in cystic fibrosis. Trends Mol Med 14:120–133Google Scholar
  16. Cao JG, Meighen EA (1993) Biosynthesis and stereochemistry of the autoinducer controlling luminescence in Vibrio harveyi. J Bacteriol 175:3856–3862Google Scholar
  17. Cao H, Krishnan G, Goumnerov B, Tsongalis J, Tompkins R, Rahme LG (2001) A quorum sensing-associated virulence gene of Pseudomonas aeruginosa encodes a LysR-like transcription regulator with a unique self-regulatory mechanism. Proc Natl Acad Sci USA 98:14613–14618Google Scholar
  18. Castric PA (1983) Hydrogen cyanide production by Pseudomonas aeruginosa at reduced oxygen levels. Can J Microbiol 29:1344–1349Google Scholar
  19. Castric P (1994) Influence of oxygen on the Pseudomonas aeruginosa hydrogen cyanide synthase. Curr Microbiol 29:19–21Google Scholar
  20. Castric PA, Ebert RF, Castric KF (1979) The relationship between growth phase and cyanogenesis in Pseudomonas aeruginosa. Curr Microbiol 2:287–292Google Scholar
  21. Chernish RN, Aaron SD (2003) Approach to resistant gram-negative bacterial pulmonary infections in patients with cystic fibrosis. Curr Opin Pulm Med 9:509–515Google Scholar
  22. Chugani S, Greenberg E (2007) The influence of human respiratory epithelia on Pseudomonas aeruginosa gene expression. Microbial pathogenesis 42:29–35Google Scholar
  23. Chugani SA, Whiteley M, Lee KM, D’Argenio D, Manoil C, Greenberg EP (2001) QscR, a modulator of quorum-sensing signal synthesis and virulence in Pseudomonas aeruginosa. Proc Natl Acad Sci USA 98:2752–2757Google Scholar
  24. Ciofu O, Mandsberg LF, Bjarnsholt T, Wassermann T, Høiby N (2010) Genetic adaptation of Pseudomonas aeruginosa during chronic lung infection of patients with cystic fibrosis: strong and weak mutators with heterogeneous genetic backgrounds emerge in mucA and/or lasR mutants. Microbiology 156:1108–1119Google Scholar
  25. Coleman JP, Hudson LL, McKnight SL, Farrow JM, Calfee MW, Lindsey CA, Pesci EC (2008) Pseudomonas aeruginosa PqsA is an anthranilate-coenzyme A ligase. J Bacteriol 190:1247–1255Google Scholar
  26. Collier DN, Anderson L, McKnight SL, Noah TL, Knowles M, Boucher R, Schwab U, Gilligan P, Pesci EC (2002) A bacterial cell to cell signal in the lungs of cystic fibrosis patients. FEMS Microbiol Lett 215:41–46Google Scholar
  27. Cook JMI, Harragan B (1992) In: Proceedings of the 92nd annual meeting of the american society for microbiology. Paper presented at 92nd annual meeting of the american society for microbiology, New OrleansGoogle Scholar
  28. Cornforth JW, James AT (1956) Structure of a naturally occurring antagonist of dihydrostreptomycin. The Biochemical Journal 63:124–130Google Scholar
  29. Cornforth DM, Popat R, McNally L, Gurney J, Scott-Phillips TC, Ivens A, Diggle SP, Brown SP (2014) Combinatorial quorum sensing allows bacteria to resolve their social and physical environment. Proc Natl Acad Sci 3(4):220–227Google Scholar
  30. Costerton JW (2001) Cystic fibrosis pathogenesis and the role of biofilms in persistent infection. Trends Microbiol 9:50–52Google Scholar
  31. Daddaoua A, Fillet S, Fernández M, Udaondo Z, Krell T, Ramos JL (2012) Genes for carbon metabolism and the ToxA virulence factor in Pseudomonas aeruginosa are regulated through molecular interactions of PtxR and PtxS. PLoS One 7:e39390Google Scholar
  32. D’Argenio DA, Calfee MW, Rainey PB, Pesci EC (2002) Autolysis and autoaggregation in Pseudomonas aeruginosa colony morphology mutants. J Bacteriol 184:6481–6489Google Scholar
  33. D’Argenio DA, Wu M, Hoffman LR, Kulasekara HD, Deziel E, Smith EE, Nguyen H, Ernst RK, Larson Freeman TJ, Spencer DH et al (2007) Growth phenotypes of Pseudomonas aeruginosa lasR mutants adapted to the airways of cystic fibrosis patients. Mol Microbiol 64:512–533Google Scholar
  34. de Kievit T, Seed PC, Nezezon J, Passador L, Iglewski BH (1999) RsaL, a novel repressor of virulence gene expression in Pseudomonas aeruginosa. J Bacteriol 181:2175–2184Google Scholar
  35. Dekimpe V, Deziel E (2009) Revisiting the quorum-sensing hierarchy in Pseudomonas aeruginosa: the transcriptional regulator RhlR regulates LasR-specific factors. Microbiology 155:712–723Google Scholar
  36. Denervaud V, TuQuoc P, Blanc D, Favre-Bonte S, Krishnapillai V, Reimmann C, Haas D, van Delden C (2004) Characterization of cell-to-cell signaling-deficient Pseudomonas aeruginosa strains colonizing intubated patients. J Clin Microbiol 42:554–562Google Scholar
  37. Deng Y, Wu J, Tao F, Zhang LH (2011) Listening to a new language: DSF-based quorum sensing in Gram-negative bacteria. Chem Rev 111:160–173Google Scholar
  38. Denning GM, Wollenweber LA, Railsback MA, Cox CD, Stoll LL, Britigan BE (1998) Pseudomonas pyocyanin increases interleukin-8 expression by human airway epithelial cells. Infect Immun 66:5777–5784Google Scholar
  39. Deziel E, Lepine F, Milot S, He J, Mindrinos MN, Tompkins RG, Rahme LG (2004) Analysis of Pseudomonas aeruginosa 4-hydroxy-2-alkylquinolines (HAQs) reveals a role for 4-hydroxy-2-heptylquinoline in cell-to-cell communication. Proc Natl Acad Sci USA 101:1339–1344Google Scholar
  40. Déziel E, Gopalan S, Tampakaki AP, Lépine F, Padfield KE, Saucier M, Xiao G, Rahme LG (2005) 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. Mol Microbiol 55:998–1014Google Scholar
  41. Diggle SP, Winzer K, Lazdunski A, Williams P, Camara M (2002) Advancing the quorum in Pseudomonas aeruginosa: MvaT and the regulation of N-acylhomoserine lactone production and virulence gene expression. J Bacteriol 184:2576–2586Google Scholar
  42. Diggle SP, Winzer K, Chhabra SR, Worrall KE, Cámara M, Williams P (2003) 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. Mol Microbiol 50:29–43Google Scholar
  43. Diggle SP, Matthijs S, Wright VJ, Fletcher MP, Chhabra SR, Lamont IL, Kong X, Hider RC, Cornelis P, Cámara M (2007) The Pseudomonas aeruginosa 4-quinolone signal molecules HHQ and PQS play multifunctional roles in quorum sensing and iron entrapment. Chem Biol 14:87–96Google Scholar
  44. Doshi HK, Chua K, Kagda F, Tambyah PA (2011) Multi drug resistant pseudomonas infection in open fractures post definitive fixation leading to limb loss: a report of three cases. International Journal of Case Reports and Images (IJCRI) 2:1–6Google Scholar
  45. Dubern JF, Diggle SP (2008) Quorum sensing by 2-alkyl-4-quinolones in Pseudomonas aeruginosa and other bacterial species. Mol Biosyst 4:882–888Google Scholar
  46. Eberhard A (1972) Inhibition and activation of bacterial luciferase synthesis. J Bacteriol 109:1101–1105Google Scholar
  47. Eberhard A, Burlingame AL, Eberhard C, Kenyon GL, Nealson KH, Oppenheimer NJ (1981) Structural identification of autoinducer of Photobacterium fischeri luciferase. Biochemistry 20:2444–2449Google Scholar
  48. Farrow JM 3rd, Sund ZM, Ellison ML, Wade DS, Coleman JP, Pesci EC (2008) PqsE functions independently of PqsR-Pseudomonas quinolone signal and enhances the rhl quorum-sensing system. J Bacteriol 190:7043–7051Google Scholar
  49. Filloux A, Bally M, Soscia C, Murgier M, Lazdunski A (1988) Phosphate regulation in Pseudomonas aeruginosa: cloning of the alkaline phosphatase gene and identification of phoB-and phoR-like genes. Mol Gen Genet 212:510–513Google Scholar
  50. Frisk A, Schurr JR, Wang G, Bertucci DC, Marrero L, Hwang SH, Hassett DJ, Schurr MJ (2004) Transcriptome analysis of Pseudomonas aeruginosa after interaction with human airway epithelial cells. Infect Immun 72:5433–5438Google Scholar
  51. Fuqua C (2006) The QscR quorum-sensing regulon of Pseudomonas aeruginosa: an orphan claims its identity. J Bacteriol 188:3169–3171Google Scholar
  52. Fuqua WC, Winans SC, Greenberg EP (1994) Quorum sensing in bacteria: the LuxR-LuxI family of cell density-responsive transcriptional regulators. J Bacteriol 176:269–275Google Scholar
  53. Gallagher LA, McKnight SL, Kuznetsova MS, Pesci EC, Manoil C (2002) Functions required for extracellular quinolone signaling by Pseudomonas aeruginosa. J Bacteriol 184:6472–6480Google Scholar
  54. Gambello MJ, Iglewski BH (1991) Cloning and characterization of the Pseudomonas aeruginosa lasR gene, a transcriptional activator of elastase expression. J Bacteriol 173:3000–3009Google Scholar
  55. Gambello MJ, Kaye S, Iglewski BH (1993) LasR of Pseudomonas aeruginosa is a transcriptional activator of the alkaline protease gene (apr) and an enhancer of exotoxin A expression. Infect Immun 61:1180–1184Google Scholar
  56. Gentry DR, Cashel M (1996) Mutational analysis of the Escherichia coli spoT gene identifies distinct but overlapping regions involved in ppGpp synthesis and degradation. Mol Microbiol 19:1373–1384Google Scholar
  57. Haddad A, Jensen V, Becker T, Haussler S (2009) The Pho regulon influences biofilm formation and type three secretion in Pseudomonas aeruginosa. Environ Microbiol Rep 1:488–494Google Scholar
  58. Hamood AN, Griswold J, Colmer J (1996) Characterization of elastase-deficient clinical isolates of Pseudomonas aeruginosa. Infect Immun 64:3154–3160Google Scholar
  59. Hauser AR, Cobb E, Bodí M, Mariscal D, Vallés J, Engel JN, Rello J (2002) Type III protein secretion is associated with poor clinical outcomes in patients with ventilator-associated pneumonia caused by Pseudomonas aeruginosa. Crit Care Med 30:521–528Google Scholar
  60. Henry RL, Mellis CM, Petrovic L (1992) Mucoid Pseudomonas aeruginosa is a marker of poor survival in cystic fibrosis. Pediatr Pulmonol 12:158–161Google Scholar
  61. Hense BA, Kuttler C, Muller J, Rothballer M, Hartmann A, Kreft JU (2007) Does efficiency sensing unify diffusion and quorum sensing? Nature Rev Microbiol 5:230–239Google Scholar
  62. Heurlier K, Dénervaud V, Pessi G, Reimmann C, Haas D (2003) Negative control of quorum sensing by RpoN (σ54) in Pseudomonas aeruginosa PAO1. J Bacteriol 185:2227–2235Google Scholar
  63. Hoffman LR, Kulasekara HD, Emerson J, Houston LS, Burns JL, Ramsey BW, Miller SI (2009) Pseudomonas aeruginosa lasR mutants are associated with cystic fibrosis lung disease progression. J Cyst Fibros 8:66–70Google Scholar
  64. Hsieh Y-J, Wanner BL (2010) Global regulation by the seven-component Pi signaling system. Curr Opin Microbiol 13:198–203Google Scholar
  65. Huang JJ, Petersen A, Whiteley M, Leadbetter JR (2006) Identification of QuiP, the product of gene PA1032, as the second acyl-homoserine lactone acylase of Pseudomonas aeruginosa PAO1. Appl Environ Microbiol 72:1190–1197Google Scholar
  66. Hueck CJ (1998) Type III protein secretion systems in bacterial pathogens of animals and plants. Microbiol Mol Biol Rev 62:379–433Google Scholar
  67. Jackowski JT, Szepfalusi Z, Wanner DA, Seybold Z, Sielczak MW, Lauredo IT, Adams T, Abraham WM, Wanner A (1991) Effects of P. aeruginosa-derived bacterial products on tracheal ciliary function: role of O2 radicals. Am J Physiol 260:L61–L67Google Scholar
  68. Jackson AA, Gross MJ, Daniels EF, Hampton TH, Hammond JH, Vallet-Gely I, Dove SL, Stanton BA, Hogan DA (2013) Anr and its activation by PlcH activity in Pseudomonas aeruginosa host colonization and virulence. J Bacteriol 195:3093–3104Google Scholar
  69. Jensen V, Löns D, Zaoui C, Bredenbruch F, Meissner A, Dieterich G, Münch R, Häussler S (2006) RhlR expression in Pseudomonas aeruginosa is modulated by the Pseudomonas quinolone signal via PhoB-dependent and-independent pathways. J Bacteriol 188:8601–8606Google Scholar
  70. Jones S, Yu B, Bainton NJ, Birdsall M, Bycroft BW, Chhabra SR, Cox AJ, Golby P, Reeves PJ, Stephens S et al (1993) The lux autoinducer regulates the production of exoenzyme virulence determinants in Erwinia carotovora and Pseudomonas aeruginosa. EMBO J 12:2477–2482Google Scholar
  71. Joseleau-Petit D, Vinella D, D’Ari R (1999) Metabolic alarms and cell division in Escherichia coli. J Bacteriol 181:9–14Google Scholar
  72. Jude F, Kohler T, Branny P, Perron K, Mayer MP, Comte R, van Delden C (2003) Posttranscriptional control of quorum-sensing-dependent virulence genes by DksA in Pseudomonas aeruginosa. J Bacteriol 185:3558–3566Google Scholar
  73. Juhas M, Wiehlmann L, Huber B, Jordan D, Lauber J, Salunkhe P, Limpert AS, von Gotz F, Steinmetz I, Eberl L et al (2004) Global regulation of quorum sensing and virulence by VqsR in Pseudomonas aeruginosa. Microbiology 150:831–841Google Scholar
  74. Kessler E, Safrin M, Olson JC, Ohman DE (1993) Secreted LasA of Pseudomonas aeruginosa is a staphylolytic protease. J Biol Chem 268:7503–7508Google Scholar
  75. Kim EJ, Sabra W, Zeng AP (2003) Iron deficiency leads to inhibition of oxygen transfer and enhanced formation of virulence factors in cultures of Pseudomonas aeruginosa PAO1. Microbiology 149:2627–2634Google Scholar
  76. Kiratisin P, Tucker KD, Passador L (2002) LasR, a transcriptional activator of Pseudomonas aeruginosa virulence genes, functions as a multimer. J Bacteriol 184:4912–4919Google Scholar
  77. Konings AF, Martin LW, Sharples KJ, Roddam LF, Latham R, Reid DW, Lamont IL (2013) Pseudomonas aeruginosa uses multiple pathways to acquire iron during chronic infection in cystic fibrosis lungs. Infect Immun 81:2697–2704Google Scholar
  78. Kosorok MR, Zeng L, West SE, Rock MJ, Splaingard ML, Laxova A, Green CG, Collins J, Farrell PM (2001) Acceleration of lung disease in children with cystic fibrosis after Pseudomonas aeruginosa acquisition. Pediatr Pulmonol 32:277–287Google Scholar
  79. Krieg DP, Helmke RJ, German VF, Mangos JA (1988) Resistance of mucoid Pseudomonas aeruginosa to nonopsonic phagocytosis by alveolar macrophages in vitro. Infect Immun 56:3173–3179Google Scholar
  80. Laarman AJ, Bardoel BW, Ruyken M, Fernie J, Milder FJ, van Strijp JA, Rooijakkers SH (2012) Pseudomonas aeruginosa alkaline protease blocks complement activation via the classical and lectin pathways. J Immunol 188:386–393Google Scholar
  81. Latifi A, Winson MK, Foglino M, Bycroft BW, Stewart GS, Lazdunski A, Williams P (1995) Multiple homologues of LuxR and LuxI control expression of virulence determinants and secondary metabolites through quorum sensing in Pseudomonas aeruginosa PAO1. Mol Microbiol 17:333–343Google Scholar
  82. Latifi A, Foglino M, Tanaka K, Williams P, Lazdunski A (1996) A hierarchical quorum-sensing cascade in Pseudomonas aeruginosa links the transcriptional activators LasR and RhIR (VsmR) to expression of the stationary-phase sigma factor RpoS. Mol Microbiol 21:1137–1146Google Scholar
  83. Lau GW, Ran H, Kong F, Hassett DJ, Mavrodi D (2004) Pseudomonas aeruginosa pyocyanin is critical for lung infection in mice. Infect Immun 72:4275–4278Google Scholar
  84. Ledgham F, Soscia C, Chakrabarty A, Lazdunski A, Foglino M (2003a) Global regulation in Pseudomonas aeruginosa: the regulatory protein AlgR2 (AlgQ) acts as a modulator of quorum sensing. Res Microbiol 154:207–213Google Scholar
  85. Ledgham F, Ventre I, Soscia C, Foglino M, Sturgis JN, Lazdunski A (2003b) Interactions of the quorum sensing regulator QscR: interaction with itself and the other regulators of Pseudomonas aeruginosa LasR and RhlR. Mol Microbiol 48:199–210Google Scholar
  86. Lee J, Wu J, Deng Y, Wang J, Wang C, Wang J, Chang C, Dong Y, Williams P, Zhang LH (2013) A cell-cell communication signal integrates quorum sensing and stress response. Nature Chem Biol 9:339–343Google Scholar
  87. Lépine F, Milot S, Déziel E, He J, Rahme LG (2004) Electrospray/mass spectrometric identification and analysis of 4-hydroxy-2-alkylquinolines (HAQs) produced by Pseudomonas aeruginosa. J Am Soc Mass Spectrom 15:862–869Google Scholar
  88. Lequette Y, Greenberg EP (2005) Timing and localization of rhamnolipid synthesis gene expression in Pseudomonas aeruginosa biofilms. J Bacteriol 187:37–44Google Scholar
  89. Li LL, Malone JE, Iglewski BH (2007) Regulation of the Pseudomonas aeruginosa quorum-sensing regulator VqsR. J Bacteriol 189:4367–4374Google Scholar
  90. Lightbown JW, Jackson FL (1956) Inhibition of cytochrome systems of heart muscle and certain bacteria by the antagonists of dihydrostreptomycin: 2-alkyl-4-hydroxyquinoline N-oxides. Biochem J 63:130–137Google Scholar
  91. Mattmann ME, Blackwell HE (2010) Small molecules that modulate quorum sensing and control virulence in Pseudomonas aeruginosa. J Org Chem 75:6737–6746Google Scholar
  92. McEwan DL, Kirienko NV, Ausubel FM (2012) Host translational inhibition by Pseudomonas aeruginosa exotoxin A triggers an immune response in Caenorhabditis elegans. Cell Host Microbe 11:364–374Google Scholar
  93. McKnight SL, Iglewski BH, Pesci EC (2000) The Pseudomonas quinolone signal regulates rhl quorum sensing in Pseudomonas aeruginosa. J Bacteriol 182:2702–2708Google Scholar
  94. Nealson KH, Platt T, Hastings JW (1970) Cellular control of the synthesis and activity of the bacterial luminescent system. J Bacteriol 104:313–322Google Scholar
  95. Ng WL, Bassler BL (2009) Bacterial quorum-sensing network architectures. Annu Rev Genet 43:197–222Google Scholar
  96. Ochsner UA, Reiser J (1995) Autoinducer-mediated regulation of rhamnolipid biosurfactant synthesis in Pseudomonas aeruginosa. Proc Natl Acad Sci USA 92:6424–6428Google Scholar
  97. Ochsner UA, Wilderman PJ, Vasil AI, Vasil ML (2002) GeneChip® expression analysis of the iron starvation response in Pseudomonas aeruginosa: identification of novel pyoverdine biosynthesis genes. Mol Microbiol 45:1277–1287Google Scholar
  98. Oglesby AG, Farrow JM, Lee J-H, Tomaras AP, Greenberg E, Pesci EC, Vasil ML (2008) The influence of iron on Pseudomonas aeruginosa Physiology: a regulatory link between iron and quorum sensing. J Biol Chem 283:15558–15567Google Scholar
  99. Park PW, Pier GB, Preston MJ, Goldberger O, Fitzgerald ML, Bernfield M (2000) Syndecan-1 shedding is enhanced by LasA, a secreted virulence factor of Pseudomonas aeruginosa. J Biol Chem 275:3057–3064Google Scholar
  100. Parkins MD, Ceri H, Storey DG (2001) Pseudomonas aeruginosa GacA, a factor in multihost virulence, is also essential for biofilm formation. Mol Microbiol 40:1215–1226Google Scholar
  101. Passador L, Cook JM, Gambello MJ, Rust L, Iglewski BH (1993) Expression of Pseudomonas aeruginosa virulence genes requires cell-to-cell communication. Science 260:1127–1130Google Scholar
  102. Pearson JP, Gray KM, Passador L, Tucker KD, Eberhard A, Iglewski BH, Greenberg EP (1994) Structure of the autoinducer required for expression of Pseudomonas aeruginosa virulence genes. Proc Natl Acad Sci USA 91:197–201Google Scholar
  103. Pearson JP, Passador L, Iglewski BH, Greenberg EP (1995) A second N-acylhomoserine lactone signal produced by Pseudomonas aeruginosa. Proc Natl Acad Sci USA 92:1490–1494Google Scholar
  104. Pereira CS, Thompson JA, Xavier KB (2013) AI-2-mediated signalling in bacteria. FEMS Microbiol Rev 37:156–181Google Scholar
  105. Pesci EC, Pearson JP, Seed PC, Iglewski BH (1997) Regulation of las and rhl quorum sensing in Pseudomonas aeruginosa. J Bacteriol 179:3127–3132Google Scholar
  106. Pesci EC, Milbank JB, Pearson JP, McKnight S, Kende AS, Greenberg EP, Iglewski BH (1999) Quinolone signaling in the cell-to-cell communication system of Pseudomonas aeruginosa. Proc Natl Acad Sci USA 96:11229–11234Google Scholar
  107. Pessi G, Haas D (2000) Transcriptional control of the hydrogen cyanide biosynthetic genes hcnABC by the anaerobic regulator ANR and the quorum-sensing regulators LasR and RhlR in Pseudomonas aeruginosa. J Bacteriol 182:6940–6949Google Scholar
  108. Pessi G, Williams F, Hindle Z, Heurlier K, Holden MT, Cámara M, Haas D, Williams P (2001) The global posttranscriptional regulator RsmA modulates production of virulence determinants and N-acylhomoserine lactones in Pseudomonas aeruginosa. J Bacteriol 183:6676–6683Google Scholar
  109. Rabin HR, Butler SM, Wohl MEB, Geller DE, Colin AA, Schidlow DV, Johnson CA, Konstan MW, Regelmann WE (2004) Pulmonary exacerbations in cystic fibrosis. Pediatr Pulmonol 37:400–406Google Scholar
  110. Rahme LG, Tan M-W, Le L, Wong SM, Tompkins RG, Calderwood SB, Ausubel FM (1997) Use of model plant hosts to identify Pseudomonas aeruginosa virulence factors. Proc Natl Acad Sci USA 94:13245–13250Google Scholar
  111. Rahme LG, Ausubel FM, Cao H, Drenkard E, Goumnerov BC, Lau GW, Mahajan-Miklos S, Plotnikova J, Tan MW, Tsongalis J et al (2000) Plants and animals share functionally common bacterial virulence factors. Proc Natl Acad Sci USA 97:8815–8821Google Scholar
  112. Rampioni G, Schuster M, Greenberg EP, Bertani I, Grasso M, Venturi V, Zennaro E, Leoni L (2007) RsaL provides quorum sensing homeostasis and functions as a global regulator of gene expression in Pseudomonas aeruginosa. Mol Microbiol 66:1557–1565Google Scholar
  113. Redfield RJ (2002) Is quorum sensing a side effect of diffusion sensing? Trends Microbiol 10:365–370Google Scholar
  114. Reimmann C, Beyeler M, Latifi A, Winteler H, Foglino M, Lazdunski A, Haas D (1997) The global activator GacA of Pseudomonas aeruginosa PAO positively controls the production of the autoinducer N-butyryl-homoserine lactone and the formation of the virulence factors pyocyanin, cyanide, and lipase. Mol Microbiol 24:309–319Google Scholar
  115. Roy-Burman A, Savel RH, Racine S, Swanson BL, Revadigar NS, Fujimoto J, Sawa T, Frank DW, Wiener-Kronish JP (2001) Type III protein secretion is associated with death in lower respiratory and systemic Pseudomonas aeruginosa infections. J Infect Dis 183:1767–1774Google Scholar
  116. Ryall B, Davies JC, Wilson R, Shoemark A, Williams HD (2008) Pseudomonas aeruginosa, cyanide accumulation and lung function in CF and non-CF bronchiectasis patients. Eur Respir J 32:740–747Google Scholar
  117. Sauer K, Cullen MC, Rickard AH, Zeef LA, Davies DG, Gilbert P (2004) Characterization of nutrient-induced dispersion in Pseudomonas aeruginosa PAO1 biofilm. J Bacteriol 186:7312–7326Google Scholar
  118. Schaber JA, Carty NL, McDonald NA, Graham ED, Cheluvappa R, Griswold JA, Hamood AN (2004) Analysis of quorum sensing-deficient clinical isolates of Pseudomonas aeruginosa. J Med Microbiol 53:841–853Google Scholar
  119. Schafhauser J, Lepine F, McKay G, Ahlgren HG, Khakimova M, Nguyen D (2014) The stringent response modulates 4-hydroxy-2-alkylquinoline biosynthesis and quorum-sensing hierarchy in Pseudomonas aeruginosa. J Bacteriol 196:1641–1650Google Scholar
  120. Schertzer JW, Boulette ML, Whiteley M (2009) More than a signal: non-signaling properties of quorum sensing molecules. Trends Microbiol 17:189–195Google Scholar
  121. Schuster M, Greenberg EP (2006) A network of networks: quorum-sensing gene regulation in Pseudomonas aeruginosa. Int J Med Microbiol 296:73–81Google Scholar
  122. Schuster M, Greenberg EP (2007) Early activation of quorum sensing in Pseudomonas aeruginosa reveals the architecture of a complex regulon. BMC Genomics 8:287Google Scholar
  123. Schuster M, Lostroh CP, Ogi T, Greenberg EP (2003) Identification, timing, and signal specificity of Pseudomonas aeruginosa quorum-controlled genes: a transcriptome analysis. J Bacteriol 185:2066–2079Google Scholar
  124. Schuster M, Urbanowski ML, Greenberg EP (2004) Promoter specificity in Pseudomonas aeruginosa quorum sensing revealed by DNA binding of purified LasR. Proc Natl Acad Sci USA 101:15833–15839Google Scholar
  125. Seet Q, Zhang LH (2011) Anti-activator QslA defines the quorum sensing threshold and response in Pseudomonas aeruginosa. Mol Microbiol 80:951–965Google Scholar
  126. Siehnel R, Traxler B, An DD, Parsek MR, Schaefer AL, Singh PK (2010) A unique regulator controls the activation threshold of quorum-regulated genes in Pseudomonas aeruginosa. Proc Natl Acad Sci USA 107:7916–7921Google Scholar
  127. Sio CF, Otten LG, Cool RH, Diggle SP, Braun PG, Bos R, Daykin M, Camara M, Williams P, Quax WJ (2006) Quorum quenching by an N-acyl-homoserine lactone acylase from Pseudomonas aeruginosa PAO1. Infect Immun 74:1673–1682Google Scholar
  128. Smith EE, Buckley DG, Wu Z, Saenphimmachak C, Hoffman LR, D’Argenio DA, Miller SI, Ramsey BW, Speert DP, Moskowitz SM et al (2006) Genetic adaptation by Pseudomonas aeruginosa to the airways of cystic fibrosis patients. Proc Natl Acad Sci USA 103:8487–8492Google Scholar
  129. Solomonson LP (1981) Cyanide as a metabolic inhibitor. In: Vennesland EECB, Knowles CJ, Westley J, Wissing F (eds) Cyanide in biology. Academic Press, London, pp 11–28Google Scholar
  130. Stewart GS, Williams P (1992) lux genes and the applications of bacterial bioluminescence. Journal of general microbiology 138:1289–1300Google Scholar
  131. Strempel N, Neidig A, Nusser M, Geffers R, Vieillard J, Lesouhaitier O, Brenner-Weiss G, Overhage J (2013) Human host defense peptide LL-37 stimulates virulence factor production and adaptive resistance in Pseudomonas aeruginosa. PLoS One 8:e82240Google Scholar
  132. Svitil AL, Cashel M, Zyskind JW (1993) Guanosine tetraphosphate inhibits protein synthesis in vivo. A possible protective mechanism for starvation stress in Escherichia coli. J Biol Chem 268:2307–2311Google Scholar
  133. Tan TT (2008) “Future” threat of gram-negative resistance in Singapore. Ann Acad Med Singap 37:884–890Google Scholar
  134. Thompson LS, Webb JS, Rice SA, Kjelleberg S (2003) The alternative sigma factor RpoN regulates the quorum sensing gene rhlI in Pseudomonas aeruginosa. FEMS Microbiol Lett 220:187–195Google Scholar
  135. Toder DS, Gambello MJ, Iglewski BH (1991) Pseudomonas aeruginosa LasA: a second elastase under the transcriptional control of lasR. Mol Microbiol 5:2003–2010Google Scholar
  136. Van Delden C, Iglewski BH (1998) Cell-to-cell signaling and Pseudomonas aeruginosa infections. Emerg Infect Dis 4:551–560Google Scholar
  137. Van Delden C, Pesci EC, Pearson JP, Iglewski BH (1998) Starvation selection restores elastase and rhamnolipid production in a Pseudomonas aeruginosa quorum-sensing mutant. Infect Immun 66:4499–4502Google Scholar
  138. van Delden C, Comte R, Bally AM (2001) Stringent response activates quorum sensing and modulates cell density-dependent gene expression in Pseudomonas aeruginosa. J Bacteriol 183:5376–5384Google Scholar
  139. Ventre I, Ledgham F, Prima V, Lazdunski A, Foglino M, Sturgis JN (2003) Dimerization of the quorum sensing regulator RhlR: development of a method using EGFP fluorescence anisotropy. Mol Microbiol 48:187–198Google Scholar
  140. von Bodman SB, Willey JM, Diggle SP (2008) Cell–cell communication in bacteria: united we stand. J Bacteriol 190:4377–4391Google Scholar
  141. Wade DS, Calfee MW, Rocha ER, Ling EA, Engstrom E, Coleman JP, Pesci EC (2005) Regulation of Pseudomonas quinolone signal synthesis in Pseudomonas aeruginosa. J Bacteriol 187:4372–4380Google Scholar
  142. Westblade LF, Ilag LL, Powell AK, Kolb A, Robinson CV, Busby SJ (2004) Studies of the Escherichia coli Rsd-sigma70 complex. J Mol Biol 335:685–692Google Scholar
  143. Whitchurch CB, Beatson SA, Comolli JC, Jakobsen T, Sargent JL, Bertrand JJ, West J, Klausen M, Waite LL, Kang PJ et al (2005) Pseudomonas aeruginosa fimL regulates multiple virulence functions by intersecting with Vfr-modulated pathways. Mol Microbiol 55:1357–1378Google Scholar
  144. Whitehead NA, Barnard AM, Slater H, Simpson NJ, Salmond GP (2001) Quorum-sensing in Gram-negative bacteria. FEMS Microbiol Rev 25:365–404Google Scholar
  145. Whiteley M, Greenberg EP (2001) Promoter specificity elements in Pseudomonas aeruginosa quorum-sensing-controlled genes. J Bacteriol 183:5529–5534Google Scholar
  146. Whiteley M, Lee KM, Greenberg EP (1999) Identification of genes controlled by quorum sensing in Pseudomonas aeruginosa. Proc Natl Acad Sci USA 96:13904–13909Google Scholar
  147. Whiteley M, Parsek MR, Greenberg EP (2000) Regulation of quorum sensing by RpoS in Pseudomonas aeruginosa. J Bacteriol 182:4356–4360Google Scholar
  148. Williams P, Bainton NJ, Swift S, Chhabra SR, Winson MK, Stewart GS, Salmond GP, Bycroft BW (1992) Small molecule-mediated density-dependent control of gene expression in prokaryotes: bioluminescence and the biosynthesis of carbapenem antibiotics. FEMS Microbiol Lett 100:161–167Google Scholar
  149. Winson MK, Camara M, Latifi A, Foglino M, Chhabra SR, Daykin M, Bally M, Chapon V, Salmond GP, Bycroft BW et al (1995) Multiple N-acyl-l-homoserine lactone signal molecules regulate production of virulence determinants and secondary metabolites in Pseudomonas aeruginosa. Proc Natl Acad Sci USA 92:9427–9431Google Scholar
  150. Winzer K, Falconer C, Garber NC, Diggle SP, Camara M, Williams P (2000) The Pseudomonas aeruginosa lectins PA-IL and PA-IIL are controlled by quorum sensing and by RpoS. J Bacteriol 182:6401–6411Google Scholar
  151. Wolz C, Hohloch K, Ocaktan A, Poole K, Evans RW, Rochel N, Albrecht-Gary AM, Abdallah MA, Doring G (1994) Iron release from transferrin by pyoverdin and elastase from Pseudomonas aeruginosa. Infect Immun 62:4021–4027Google Scholar
  152. Wu L, Estrada O, Zaborina O, Bains M, Shen L, Kohler JE, Patel N, Musch MW, Chang EB, Fu YX et al (2005) Recognition of host immune activation by Pseudomonas aeruginosa. Science 309:774–777Google Scholar
  153. Xiao G, Deziel E, He J, Lepine F, Lesic B, Castonguay MH, Milot S, Tampakaki AP, Stachel SE, Rahme LG (2006a) MvfR, a key Pseudomonas aeruginosa pathogenicity LTTR-class regulatory protein, has dual ligands. Mol Microbiol 62:1689–1699Google Scholar
  154. Xiao G, He J, Rahme LG (2006b) Mutation analysis of the Pseudomonas aeruginosa mvfR and pqsABCDE gene promoters demonstrates complex quorum-sensing circuitry. Microbiology 152:1679–1686Google Scholar
  155. Yanagihara K, Tomono K, Kaneko Y, Miyazaki Y, Tsukamoto K, Hirakata Y, Mukae H, Kadota J, Murata I, Kohno S (2003) Role of elastase in a mouse model of chronic respiratory Pseudomonas aeruginosa infection that mimics diffuse panbronchiolitis. Journal of medical microbiology 52:531–535Google Scholar
  156. Zaborin A, Romanowski K, Gerdes S, Holbrook C, Lepine F, Long J, Poroyko V, Diggle SP, Wilke A, Righetti K et al (2009) Red death in Caenorhabditis elegans caused by Pseudomonas aeruginosa PAO1. Proc Natl Acad Sci USA 106:6327–6332Google Scholar
  157. Zaborina O, Lepine F, Xiao G, Valuckaite V, Chen Y, Li T, Ciancio M, Zaborin A, Petrof EO, Turner JR et al (2007) Dynorphin activates quorum sensing quinolone signaling in Pseudomonas aeruginosa. PLoS Pathog 3:e35Google Scholar
  158. Zhang L, Murphy PJ, Kerr A, Tate ME (1993) Agrobacterium conjugation and gene regulation by N-acyl-l-homoserine lactones. Nature 362:446–448Google Scholar

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Authors and Affiliations

  1. 1.Institute of Molecular and Cell Biology, Agency for Science, Technology and ResearchSingaporeSingapore
  2. 2.Guangdong Province Key Laboratory of Microbial Signals and Disease Control, College of Natural Resources and Environmental SciencesSouth China Agricultural UniversityGuangzhouChina

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