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SGM Special Lecture |
Institute of Infection, Immunity and Inflammation, Centre for Biomolecular Sciences, University of Nottingham, Nottingham NG7 2RD, UK
Correspondence
Paul Williams
paul.williams{at}nottingham.ac.uk
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ABSTRACT |
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 TOP ABSTRACT IntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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Introduction |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Winzer et al., 2002; Hense et al., 2007). QS also continues to attract significant interest from the perspective of social evolution, fitness and the benefits associated with costly co-operative behaviours (Diggle et al., 2007; West et al., 2006). QS signal molecules are chemically diverse and many bacteria possess several interacting QS gene regulatory ‘modules’ (consisting of the genes which code for the QS signal synthase and QS signal transduction machinery) employing multiple signal molecules from the same or different chemical classes and which constitute regulatory hierarchies. In general QS systems facilitate the co-ordination of population behaviour to enhance access to nutrients or specific environmental niches, collective defence against other competitor organisms or community escape where survival of the population is threatened. Although QS has primarily been studied in the context of single species, the expression of QS systems may be manipulated by the activities of other bacteria within complex microbial consortia which employ different QS signals and by higher organisms. QS signal molecules also exhibit biological properties far beyond their role in co-ordinating gene expression in the producer organism. Consequently both QS systems and QS signal molecules have attracted considerable interest from the biotechnology, pharmaceutical and agricultural industries, particularly with respect to QS as a target for novel antibacterials.
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QS signal molecules are chemically diverse |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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) (Lazazzera, 2001; Chhabra et al., 2005; Williams et al., 2007;). Gram-negative bacteria, for example, employ N-acylhomoserine lactones (AHLs), 2-alkyl-4-quinolones (AQs), long-chain fatty acids and fatty acid methyl esters as well as autoinducer-2 (AI-2), a collective term for a group of interconvertible furanones derived from dihydroxypentanedione (DPD) (Fig. 1). AI-2 is also produced by some Gram-positive bacteria, although generally these organisms prefer linear, modified or cyclic peptides such as the autoinducing peptides (AIPs) made by the staphylococci (Fig. 1). The streptomycetes, however, synthesize 
-butyrolactones such as A-factor (Fig. 1), which are structurally related to the AHLs as both compound classes belong to the butanolides. QS signal molecules can also be further subdivided according to whether they interact with receptors at the cell surface (e.g. the staphylococcal AIPs) or are internalized (e.g. the AHLs, AQs, the Phr peptides of Bacillus subtilis and the mating pheromones of Enterococcus faecalis).
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AHL production is widespread among gram-negative bacteria |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Winson et al., 1998; Andersen et al., 2001; Shaw et al., 1997) or pigment induction (e.g. violacein in Chromobacterium violaceum: McClean et al., 1997). These assays have revealed that bacteria belonging to genera which occupy a wide variety of environmental niches – from marine and freshwater environments to soil, plants and animals, including many pathogens, symbionts, extremophiles and plant-growth-promoting bacteria – produce AHLs. Examples include species of Acidithiobacillus, Acinetobacter, Aeromonas, Agrobacterium, Brucella, Burkholderia, Erwinia, Enterobacter, Chromobacterium, Hafnia, Mesorhizobium, Methylobacter, Paracoccus, Pseudomonas, Ralstonia, Rhodobacter, Rhizobium, Rhanella, Serratia, Sinorhizobium, Vibrio and Yersinia (Williams et al., 2007). Many of these bacteria produce multiple AHLs and possess more than one AHL synthase. As yet no AHL-producing Escherichia coli or Salmonella strains have been identified, although both organisms possess an AHL receptor (SdiA) of the LuxR protein class and respond to AHLs produced by other bacteria (Ahmer, 2004). Table 1 provides a summary of the known AHL-dependent QS systems and the phenotypes controlled.
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AHL structural variation and biosynthesis |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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-positions which is N-acylated with a fatty acyl group at the 
-position (Chhabra et al., 2005) (Fig. 2). Naturally occurring AHLs exhibit variable acyl chain lengths, saturation levels and oxidation states and are exemplified by compounds with 4–18-carbon acyl chains which belong to either the N-acyl, N-(3-oxoacyl) or N-(3-hydroxyacyl) classes (Fig. 2). Some AHLs also have unsaturation in the 5 and 7 positions in a chain of 12 or 14 carbons. Stereochemistry at the -centre of the homoserine lactone (HSL) ring has been unequivocally established to be L for the N-(3-oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) produced by Erwinia carotovora (Bainton et al., 1992b) and by analogy it is extrapolated that all other natural AHLs have the same configuration. In some cases D-isomers have been synthesized and shown to lack activity (Chhabra et al., 1993, 2005). The AHL lactone ring is readily hydrolysed under alkaline conditions to form the corresponding N-acylhomoserine compound (Fig. 2), which is inactive as a QS signal molecule (Yates et al., 2002). The lactone ring of the HSL itself is completely hydrolysed at pHs above 2 whereas 70 % of N-propionylhomoserine lactone (C3-HSL) is hydrolysed at pH 6 (Yates et al., 2002). Consequently, a C4 acyl chain is the minimum chain length likely to be sufficiently stable to function as a signal molecule over the pH range encountered by most bacteria. Indeed, the shortest naturally occurring AHLs so far identified are C4-HSL and 3-hydroxy-C4-HSL, which are produced by Pseudomonas aeruginosa (Winson et al., 1995) and Vibrio harveyi (Cao & Meighen, 1989) respectively. Apart from base-catalysed lactonolysis, 3-oxo-AHLs can also undergo an alkali-driven rearrangement reaction to form the corresponding tetramic acids, compounds with iron chelating and antibacterial activities (Fig. 2; Kaufmann et al., 2005).
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; Salmond et al., 1995; Urbanowski et al., 2004; Zhang et al., 2002). The genes coding for LuxR and LuxI orthologues can be found on chromosomes or on mobile genetic elements (plasmids and transposons) and are often located adjacent to each other as tandem, convergent or divergent pairs (Salmond et al., 1995; Wei et al., 2006). Currently, the Gram-negative bacterial genome databases contain more than 100 examples of LuxI family members. In vitro studies using purified LuxI family proteins have revealed that AHLs are synthesized from precursors derived from fatty acid and amino acid metabolism (Moré et al., 1996; Jiang et al., 1998; Parsek et al., 1999). They catalyse AHL synthesis using the appropriately charged acyl carrier protein, as the main acyl chain donor (acyl-CoA derivatives can also be substrates), and S-adenosylmethionine as the source for the homoserine lactone moiety (Fig. 4). Since LuxI proteins are not particularly closely related it has been impossible to predict the nature of the AHL produced by a given LuxI protein from bioinformatic comparisons alone. This is further complicated by the large number of AHLs which can be synthesized via a given LuxI protein. The Yersinia pseudotuberculosis AHL synthase YtbI, for example, directs the synthesis of at least 24 different AHLs, although four compounds predominate (3-oxo-C6-HSL, 3-oxo-C7-HSL, 3-oxo-C8-HSL and C8-HSL) (Ortori et al., 2006). Interestingly, an N-terminally truncated LuxI homologue (TslI) encoded on a plasmid in Methylobacterium extorquens which appears to be inactive as an AHL synthase is involved in the transcriptional control of AHL synthesis via two other LuxI-type proteins, MsaI and MlaI (Nieto Penalver et al., 2006). The molecular mechanism through which TslI operates has yet to be elucidated.
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; Gould et al., 2004). From the EsaI structural analysis, threonine at position 140 was shown to contribute to the specificity of the enzyme for 3-oxo-acyl-Acyl carrier proteins but not to be of such importance for LasI (Watson et al., 2002; Gould et al., 2004). Interestingly, the AHL substrate chain length specificity of the Er. carotovora synthase, ExpI, can be modified from a C6 to a C8 acyl chain by site-specific mutagenesis of M127T and F69L, with the former modification being critical for the change in substrate specificity and the latter for maintaining wild-type levels of enzyme activity (Brader et al., 2005).
AHL biosynthesis is not exclusively dependent on LuxI homologues but can be directed by members of the LuxM protein family, which has so far only been found in the genus Vibrio (Milton et al., 2001). LuxM proteins employ the same substrates as LuxI proteins and both AHL synthase types co-exist in both Vibrio fischeri and Vibrio anguillarum (Hanzelka et al., 1999; Milton et al., 2001). A third potential AHL synthase (HdtS) from Pseudomonas fluorescens, which does not belong to either the LuxI or LuxM families, was identified by Laue et al. (2000). When expressed in E. coli, HdtS directed the synthesis of the same AHLs [N-(3-hydroxy-7-cis-tetradecenoyl)homoserine lactone (3-hydroxy-C14 : 1HSL), N-decanoylhomoserine lactone (C10-HSL) and N-hexanoylhomoserine lactone (C6-HSL)] produced by P. fluorescens. HdtS is related to the lysophosphatidic acid acyltransferase protein family. An hdtS orthologue (termed act) has also been identified in the extreme acidophile Acidithiobacillus ferrooxidans which, when introduced into E. coli, also directs the synthesis of N-tetradecanoylhomoserine lactone (C14-HSL) together with small amounts of shorter-chain AHLs (Rivas et al., 2007) Although an in vitro demonstration of the AHL synthase activity of HdtS and Act has yet to be undertaken, it is possible that these proteins are involved in both phospholipid and AHL biosynthesis. HdtS clearly has lysophosphatidic acid acyltransferase activity and the hdtS gene complements the growth defect of an E. coli plsC mutant (Cullinane et al., 2005). However, although the production of 3-hydroxy-C14 : 1-HSL production was not reported, a P. fluorescens hdtS mutant still produced parental levels of C6-HSL (Cullinane et al., 2005).
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AHL perception |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Salmond et al., 1995). This is usually a member of the LuxR family of transcriptional regulators although in some cases it may be a histidine kinase sensor protein, the activation of which induces a phosphorelay cascade leading to the transcriptional activation of the target genes (Milton, 2006). Although LuxR proteins are mostly highly AHL specific, many can be activated to different extents by closely related compounds, which explains their utility as components of AHL biosensors based on a given LuxR protein and the promoter of a target gene fused to a reporter such as lux (Bainton et al., 1992a; Winson et al., 1998). LuxR-type proteins consist of two functional domains, an N-terminal AHL-binding and a C-terminal DNA-binding domain. They reversibly bind their cognate AHLs and activate or repress gene expression by binding to specific DNA motifs containing dyad symmetry at their target promoters (Schuster et al., 2004; Urbanowski et al., 2004). Depending on the organism, these related motifs have been called lux boxes (V. fischeri), las boxes (P. aeruginosa) or tra boxes (Agrobacterium tumefaciens) (Schuster et al., 2004; Urbanowski et al., 2004; Zhang et al., 2002). X-ray structural studies have revealed that for LuxR proteins such as the dimeric TraR, each monomer binds to one half of a tra box via the helix–turn–helix DNA-binding motif (Zhang et al., 2002). The crystal structures of the TraR (Zhang et al., 2002) and LasR (Bottomley et al., 2007) dimers bound to their cognate AHL [N-(3-oxooctanoyl)homoserine lactone (3-oxo-C8-HSL) and N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL) respectively] revealed that each monomer contained one deeply buried ligand and that the AHL-binding pocket of LasR is larger than that of TraR since it has to accommodate a longer acyl side chain (C12 instead of C8). For both TraR and LasR, models have been proposed in which AHL binding to the nascent protein facilitates protein folding and dimerization, enabling DNA binding and transcriptional activation of the target QS genes (Zhang et al., 2002; Bottomley et al., 2007). |
Plant–microbe interactions, Erwinia carotovora and AHL-dependent QS |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Cao & Meighen, 1989) but it was not until 1992 that the first paper reporting that many other bacteria including Erwinia, Serratia, Citrobacter, Enterobacter and Pseudomonas produced AHLs was published (Bainton et al., 1992a). In the soft-rot plant pathogen Erwinia carotovora subsp. carotovora ATCC 39048, N-(3-oxo-hexanoyl)homoserine lactone (3-oxo-C6-HSL) was initially found to control the synthesis of a β-lactam antibiotic, 1-carbapen-2-em-3-carboxylic acid. 3-Oxo-C6-HSL synthesis in Er. carotovora is directed via CarI (Bainton et al., 1992a, b). The gene coding for this LuxI homologue, carI (Swift et al., 1993), is not linked to the carbapenem biosynthetic gene cluster (carABCDEFGJH) but both carI and the car operon are linked to adjacent genes coding for LuxR homologues (ExpR and CarR respectively) (McGowan et al., 1995, 1996, 1997). In the biosynthetic cluster, CarA, B and C are absolutely required for antibiotic production, CarD and E probably supply precursors but are not essential while CarF and G confer protection against the endogenously synthesized carbapenem. The function of CarH is unknown but it is also not essential for antibiotic production (McGowan et al., 1996, 1997). Intriguingly, many Erwinia strains are carbapenem-negative but retain the car operon; antibiotic biosynthesis can be restored by introducing a plasmid-borne copy of carR as these strains appear to contain an inactive carR gene (Holden et al., 1998).
Er. carotovora causes the post-harvest soft rotting of many vegetable crops including potato, carrot and green pepper (Barnard et al., 2007). The virulence of this plant pathogen depends on the synthesis and secretion of a number of exoenzymes including pectinases, cellulases and proteases. Expression of the genes coding for these plant cell wall degrading enzymes is controlled by QS. Er. carotovora carI mutants are not only carbapenem-negative but are also downregulated for exoenzyme production and their virulence is severely attenuated in planta (Jones et al., 1993). Both virulence and antibiotic production can be restored by the exogenous provision of 3-oxo-C6-HSL. In contrast, mutation of carR has no impact on exoenzyme synthesis, indicating that an additional LuxR-type protein must be involved (Barnard et al., 2007). Although carI (also called expI) is located adjacent to and overlaps a luxR gene (ExpR), mutation of expR exerts little effect on exoenzyme levels. Instead, a third luxR gene, virR, is required (Burr et al., 2006). Unlike CarR, VirR functions as a repressor which negatively regulates exoenzyme production such that a virR mutation in an Er. carotovora AHL synthase mutant restores virulence to wild-type levels (Burr et al., 2006).
The coupling of antibiotic and plant cell wall degrading enzymes via AHL-dependent QS implies that high cell population densities of Erwinia, having generated a substantial food resource through plant cell wall degradation, protect their investment from other bacteria by producing carbapenem antibiotic. Furthermore it has been suggested that the production of exoenzymes only at high cell population densities has contributed to the success of Erwinia as a plant pathogen since premature exoenzyme synthesis at low cell population densities would trigger a plant defence response and impede further development of infection (Salmond et al., 1995). Such resistance to Erwinia infections can be achieved by inducing plant host defence responses with salicyclic acid (Palva et al., 1994). Consequently, transgenic plants engineered to produce AHLs would be predicted to resist infection more readily by triggering Erwinia exoenzyme synthesis prematurely. Experimental exploration of this hypothesis required the availability of the AHL precursors, S-adenosylmethionine and the appropriately charged acyl-acyl carrier protein, and their accessibility to a recombinant AHL synthase expressed in planta. Such transgenic plants also offer opportunities for manipulating the behaviour of plant-beneficial as well as plant-pathogenic bacteria and for studying the impact of AHLs on the ecology of the rhizosphere.
To determine whether AHLs could be made in planta, Fray et al. (1999) introduced the Yersinia enterocolitica AHL synthase gene yenI into tobacco. In Yersinia, YenI directs the synthesis of a 1 : 1 mixture of 3-oxo-C6-HSL and C6-HSL (Throup et al., 1995). Transgenic tobacco plants synthesizing the same two AHLs in a similar ratio were obtained, provided that the YenI protein was directed to the chloroplasts, presumably because the AHL precursors are most abundantly present in these organelles. Similarly, the P. aeruginosa AHL synthase gene lasI (responsible for the synthesis of 3-oxo-C12-HSL) has been introduced into tobacco plants alone and in combination with yenI (Scott et al., 2006). Both transgenic plant lines produced physiologically significant levels of 3-oxo-C12-HSL and the double transformant produced both the long- and short-chain AHLs. 3-Oxo-C6-HSL, C6-HSL and 3-oxo-C12-HSL were extracted from leaf, stem and root tissues and also detected on the surfaces of the leaves, indicating that the AHLs could freely diffuse across the plant cell plastid and plasma membranes (Fig. 5). AHLs were also detected in root exudates and in both rhizosphere and non-rhizosphere soil from transgenically grown plants, indicating that bioactive AHLs accumulate in the phytosphere and so should have an impact on communities of both plant-beneficial and plant-pathogenic bacteria (Scott et al., 2006).
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). Since tobacco is not a natural host for Erwinia and high inocula are required for infection, YenI has also been introduced into potato (Toth et al., 2004). In common with tobacco, the transgenic potatoes also produced both 3-oxo-C6-HSL and C6-HSL, with the stems appearing to contain higher AHL levels than the tubers. Contrary to the results anticipated, Erwinia strains proved to be more, rather than less, virulent on the transgenic AHL-producing potato plants, with both severity and onset of disease being greater at lower bacterial inocula than required for the control plants (Fig. 6; Toth et al., 2004). Consequently, QS-dependent control of exoenzyme synthesis cannot simply be a means of avoiding plant defences but may rather be a mechanism for increasing nutrient availability at later stages of infection.
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QS and bacterial cross-talk |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Eberl et al., 1996; Swift et al., 1997) while bacteria such as C. violaceum respond to other short-chain AHL-producing bacteria by producing the purple pigment violacein (McClean et al., 1997). QS in bacteria such as C. violaceum and A. hydrophila is antagonized by long-chain AHLs, i.e. the latter inhibit violacein and exoenzyme production respectively (McClean et al., 1997; Swift et al., 1999). P. aeruginosa in contrast produces both long (3-oxo-C12-HSL) and short (e.g. C4-HSL) chain AHLs; however, 3-oxo-C12-HSL does not antagonize the C4-HSL-dependent activation of RhlR in P. aeruginosa, although RhlR is inhibited by 3-oxo-C12-HSL when it is expressed in E. coli (Winzer et al., 2000), indicating that there must be some compartmentalization of the hierarchical las and rhl QS systems. In mixed P. aeruginosa/Burkholderia cepacia biofilm cultures in vitro and in vivo in a mouse chronic lung infection model, B. cepacia was shown to perceive the AHLs produced by P. aeruginosa but not vice versa (Riedel et al., 2001). That this signalling was unidirectional was confirmed by quantifying the levels of exoproteases produced by each organism incubated with or without supernatant extracts prepared from the other, from either the wild-type or the isogenic AHL-deficient mutant (Riedel et al., 2001). Interestingly, a subset of P. aeruginosa virulence genes has also been reported to be upregulated by AI-2, which is not produced by P. aeruginosa (which lacks the AI-2 synthase, LuxS) but is made by the oropharyngeal flora and is present in sputum samples from cystic fibrosis patients infected with P. aeruginosa (Duan et al., 2003).
Although no AHL-producing Gram-positive bacteria have so far been identified, Qazi et al. (2006) examined the growth response of Staphylococcus aureus to a range of AHLs differing in acyl chain length and the presence or absence of a 3-oxo substituent. From the data obtained, the most inhibitory compound was 3-oxo-C12-HSL, the AHL produced via LasI in P. aeruginosa. A reduction in acyl chain or removal of the 3-oxo moiety resulted in the reduction or complete loss of antibacterial activity. For example, C4-HSL which is produced via RhlI in P. aeruginosa, was inactive. When added to S. aureus cultures, 3-oxo-C12-HSL abolished the production of both - and 
-haemolysins and cell wall fibronectin-binding proteins but enhanced protein A levels in a concentration-dependent manner (Qazi et al., 2006). These data suggested that 3-oxo-C12-HSL may be inhibiting the agr QS system of S. aureus.
The agr system regulates the expression of diverse cell surface proteins and exotoxins in concert with cell population density via an AIP signal molecule. As S. aureus reaches stationary phase, agr represses genes coding for cell surface colonization proteins such as protein A and the fibronectin-binding proteins and activates expression of the genes for secreted exotoxins and tissue-degrading exoenzymes (Chan et al., 2004). The agr locus consists of two divergent operons, controlled by the P2 and P3 promoters respectively (Chan et al., 2004). The P2 operon consists of four genes, agrBDCA, which are all required for the activation of transcription from the P2 and P3 promoters while the P3 transcript, RNAIII, is itself the effector for the agr response (Chan et al., 2004). AgrA and AgrC constitute a two-component system in which AgrC is the sensor kinase and AgrA is the response regulator. The system is activated through the interaction of an AIP with AgrC (Chan et al., 2004). In several different experimental animal models of S. aureus infection, agr mutants exhibit significantly reduced virulence, highlighting the key role of this regulatory locus in staphylococcal pathogenicity (Chan et al., 2004).
When the expression of the agrP3 promoter and also sarA (which codes for a regulatory protein which, in common with agr, positively regulates the agr promoters) was examined in S. aureus exposed to 3-oxo-C12-HSL, the expression of both promoters was inhibited in a concentration-dependent manner. Using an agrP3 : : blaZ reporter gene fusion, an IC50 of 2 µM was calculated for 3-oxo-C12-HSL, which is within the concentration range produced by P. aeruginosa (Qazi et al., 2006). Furthermore, 3-oxo-C12-HSL binds to S. aureus membranes with high affinity and may therefore exert its activity either by perturbing the processing of AgrD by AgrB to generate the AIP or by interfering with the capacity of AgrC to sense the AIP signal molecule (Qazi et al., 2006). Although these hypotheses remain to be experimentally confirmed, it is also conceivable that 3-oxo-C12-HSL antagonizes the functions of other two-component signal transduction systems which modulate virulence factor production in S. aureus directly or indirectly via agr and sarA.
3-Oxo-AHLs such as 3-oxo-C12-HSL are readily inactivated as QS signal molecules by exposure to alkaline pHs, which generate the corresponding 3-oxo-fatty amine derivative [e.g. N-(3-oxo-dodecanoyl)homoserine in the case of 3-oxo-C12-HSL] (Yates et al., 2002). While the ring-opened compound has no growth- or agr-inhibiting activity towards S. aureus (J. Cottam, S. R. Chhabra, S. Clarke & P. Williams, unpublished), alkali-mediated hydrolysis can also generate the tetramic acid derivative, 3-(1-hydroxydecylidene)-5-(2-hydroxyethyl)pyrrolidine-2,4-dione (Fig. 2), which has more potent growth-inhibitory properties than 3-oxo-C12-HSL (Kaufmann et al., 2005).
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QS and prokaryote–eukyarote interactions |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Gardiner et al., 2001; Bauer & Mathesius, 2004; Dudler & Eberl, 2006; Schuhegger et al., 2006). At high concentrations (200 µM), 3-oxo-C12-HSL suppresses filamentation in Candida albicans (Hogan et al., 2004) and in common with AQs such as PQS (2-alkyl-3-hydroxy-4-quinolone) it exhibits potent immunomodulatory activity and elicits both pro- and anti-inflammatory responses depending on the concentration and experimental model used (Telford et al., 1998; Smith et al., 2002; Chhabra et al., 2003; Hooi et al., 2004; Pritchard et al., 2005). 3-Oxo-C12-HSL inhibits both mitogen-induced leukocyte proliferation and lipopolysaccharide-mediated secretion of tumour necrosis factor (Telford et al., 1998). Structure–activity studies have revealed that AHLs with an 11–13 carbon side chain and a 3-oxo- or 3-hydroxy- substituent are the optimal structures for immunosuppressive activity. Analogues lacking the homoserine lactone ring or an L-configuration at the chiral centre, or those with polar substituents, were devoid of activity (Chhabra et al., 2003). These data indicated that 3-oxo-C12-HSL may have therapeutic utility in autoimmune diseases such as type 1 diabetes. In the non-obese diabetic (NOD) mouse model of type 1 insulin-dependent diabetes mellitus, destruction of the pancreatic insulin-producing β-cells and the onset of diabetes could be prevented by 3-oxo-C12-HSL (Fig. 7; Pritchard et al., 2005). The mechanism of action in this context is not understood but may involve 3-oxo-C12-HSL directing the immune response away from a T-helper-cell 1 (Th1)- to a Th2-based response (Pritchard et al., 2005).
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; Kravchenko et al., 2006; Shiner et al., 2006). 3-Oxo-C12-HSL signalling in immune cells does not appear to depend on the pathogen-associated molecular recognition receptor pathways (e.g. Toll and Nod receptors) (Kravchenko et al., 2006). At high concentrations, 3-oxo-C12-HSL induces apoptosis in various cell types via a mechanism that can be blocked by inhibitors of calcium signalling (Shiner et al., 2006). Since these do not inhibit immunomodulation, it is likely that multiple 3-oxo-C12-HSL receptors exist. 3-Oxo-C12-HSL also influences smooth muscle contraction in blood vessels (Lawrence et al., 1999) and produces a marked bradycardia in live conscious rats (Gardiner et al., 2001). It is also capable of disrupting epithelial barrier integrity, a process that can partially be prevented by inhibiting p38 and p42/44 MAPK kinases (Vikstrom et al., 2006). Consequently, 3-oxo-C12-HSL can be considered as a virulence factor in its own right, both functioning as a QS signal molecule controlling expression of key virulence genes and conferring on P. aeruginosa a competitive survival advantage in the presence of other prokaryotes and eukaryotes.
As well as promoting pathogen success, certain QS signal molecules exert beneficial effects on the host. The mechanism(s) by which probiotic bacteria such as certain bacilli and lactobacilli exert their protective effects in the gastro-intestinal tract is not well understood but is likely to involve pathogen control or exclusion as well as protection of host tissues against inflammatory responses. Interestingly, the mechanism by which a probiotic Bacillus subtilis strain exerts a protective effect involves a pentapeptide QS signal molecule, the competence and sporulating factor, CSF (also called PhrC). In B. subtilis, CSF, ComX and other Phr peptides regulate multiple processes including the initiation of genetic competence, sporulation, antibiotic and exopolysaccharide synthesis as well as the production of degradative enzymes (Lazazzera, 2001; Auchtung et al., 2006). In the human colonic epithelial cell line Caco2 and in ligated mouse intestinal loops, CSF induces the expression of heat-shock-inducible protein 27 (Hsp27) (Fujiya et al., 2007). Hsps confer protection against a wide variety of stresses and, when overexpressed, can protect intestinal epithelial cells from oxidative injury and so contribute to the maintenance of intestinal homeostasis (Fujiya et al., 2007). This activity of CSF is within the concentration range required for QS in B. subtilis (10–100 nM), an important consideration given that some of observed activities of QS molecules on mammalian cells are only apparent at high, non-physiological concentrations (Pritchard, 2006). In B. subtilis, CSF and the other Phr peptides act intracellularly following internalization via an oligopeptide permease (Opp) by inhibiting the activity of the intracellular Rap receptor proteins (Lazazzera, 2001; Auchtung et al., 2006). Fujiya et al. (2007) also identified a mammalian apical membrane oligopeptide transporter (OCTN2) which is required for CSF uptake. CSF and OCTN2-mediated CSF transport are both required to protect Caco2 cells against oxidant-mediated injury and loss of epithelial barrier function.
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Eukaryote-mediated quorum activation and quenching |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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; Dudler & Eberl, 2006). Plants and animals as well as other bacteria also express enzymes capable of chemically modifying and so inactivating QS signal molecules. For example, the AHL- and AQ-dependent QS systems of P. aeruginosa are induced by -interferon and opioids respectively (Wu et al., 2005; Zaborina et al., 2007) while the plant defence signal molecule salicylic acid upregulates the expression of an Agrobacterium tumefaciens AHL-degrading enzyme (Yuan et al., 2007). Red algae produce brominated furanones which inhibit AHL-dependent QS while plant root exudates contain compounds which stimulate both AHL and AI-2 biosensors (Bauer & Mathesius, 2004; Dudler & Eberl, 2006).
Staphylococcus aureus produces four AIP QS signal molecules which differ in primary amino acid sequence, although AIP-1 and AIP-4 differ by only one amino acid residue (McDowell et al., 2001; Chan et al., 2004). Both of these AIPs incorporate a C-terminal methionine which is readily oxidized during growth to form the corresponding methionine sulphoxide and inactivated (McDowell et al., 2001). In a mouse skin infection model, the phagocyte NADPH oxidase, myeloperoxidase, and the inducible nitric oxide synthase both play critical roles in protecting against AIP-1-producing S. aureus but not against the isogenic agr-negative mutant (Rothfork et al., 2004). Inactivation of AIP-1 in vitro and in vivo by reactive oxygen and nitrogen intermediates was shown to occur, prompting the authors to conclude that oxidant-mediated inactivation of staphylococcal AIPs can be considered as an important component of the innate immune defences against S. aureus infections (Rothfork et al., 2004). Presumably the evolution of the hypervariable region of the agr locus (Dufour et al., 2002), which includes agrD (encoding the AIP precursor protein), is in part driven by the need to evade oxidant-mediated inactivation.
An alternative biochemical strategy used for inactivating QS signal molecules such as the AHLs is enzyme-mediated inactivation (Dong et al., 2007). Many bacteria possess lactonases and acylases which hydrolyse the lactone ring and cleave the amide bond respectively. Some bacteria, notably Rhodococcus, reduce the 3-oxo moiety of 3-oxo-AHLs to form 3-hydroxy-AHLs and so decrease the efficacy of the signal molecule for its receptor (Uroz et al., 2005). The true physiological function of most of these enzymes is not known but they are found in both Gram-negative AHL producers and non-AHL producers as well as in Gram-positive bacteria. In mammalian cells and tissues, AHL inactivation has been associated with the paraoxonase (PON) enzymes which are present in serum and airway epithelia (Dong et al., 2007). The PON enzymes exhibit a wide range of hydrolytic activities; PON1 for example possesses lactonase, arylesterase and organophosphatase activities. In airway epithelia, PON2 appears to be the most efficient degrader of 3-oxo-C12-HSL and consequently is considered likely to contribute to host defences against bacteria such as P. aeruginosa (Stoltz et al., 2007). Germinating seedlings of the leguminous plant Lotus corniculatus have also been shown to be capable of enzymically inactivating a wide range of AHLs, although the mechanism involved has yet to be established (Delalande et al., 2005).
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AHL sensing by zoospores of the green alga Ulva |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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). The selection of a surface by a zoospore involves the sensing of the surface followed by temporary adhesion and detachment if the surface does not offer a sufficiently favourable environment (Joint et al., 2007). The presence of a bacterial biofilm is one important factor which influences zoospore attachment, and bacteria which either stimulate or inhibit zoospore settlement have been isolated from marine surfaces colonized by Ulva (Patel et al., 2003). With respect to bacteria which encourage zoospore settlement, a positive correlation has been reported to exist between zoospore settlement and bacterial biofilm density (Joint et al., 2000). This work raised the question of whether the Ulva zoospores were responding to the topological features and physico-chemical properties of the biofilm or to any diffusible signals released by the biofilm bacteria, or both. To explore the possibility that the zoospores were sensing QS signal molecules, Joint et al. (2002) used Vibrio anguillarum as a model marine bacterium. V. anguillarum possesses a sophisticated QS regulatory system (Milton, 2006) employing both a LuxRI-type system (VanRI) and a LuxMN-type system (VanMN) and produces mainly N-(3-oxodecanoyl)-L-homoserine lactone (3-oxo-C10-HSL) via VanI and N-(3-hydroxyhexanoyl)-L-homoserine lactone (3-hydroxy-C6-HSL) and C6-HSL via VanM (Milton et al., 1997, 2001). The VanRI and VanMN systems form a regulatory hierarchy where the expression of vanR and vanI are dependent on a functional vanM, and consequently both vanIM and vanM mutants do not produce AHLs while a vanI mutant continues to produce 3-hydroxy-C6-HSL and C6-HSL (Milton et al., 2001).
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). However, the zoospores were not attracted to biofilms of either the V. anguillarum vanM or vanIM mutants or to the wild-type strain rendered AHL-negative through the in situ expression of an AHL-inactivating lactonase (Joint et al., 2002; Tait et al., 2005). Further confirmatory evidence for zoospore AHL sensing was obtained by expressing vanI and vanM respectively in E. coli and by evaluating the attraction of zoospores to agarose films containing synthetic AHLs which diffuse out and generate an AHL gradient (Joint et al., 2002; Tait et al., 2005). While the zoospores responded to a wide range of AHLs with differing acyl chain lengths and C-3 position substitutents, they failed to settle on films containing the corresponding ring-open N-acylhomoserine compounds. Although these experiments suggest that zoospores preferentially respond to long-chain AHLs with 3-oxo or 3-hydroxy substituents, the differential solubility, diffusibility and degradation rates of the different AHLs in the experimental system used make it difficult to draw any firm conclusions about AHL structural preference (Tait et al., 2005). However, zoospore settlement onto AHL-producing biofilms can be reduced by adding synthetic AHLs to destroy the AHL gradient. In such experiments, the zoospores were attracted to all of the AHLs tested except the shortest acyl chain compound tested (C4-HSL) (Joint et al., 2002; Tait et al., 2005).
Image analysis using GFP-tagged V. anguillarum biofilms revealed that zoospores settle directly onto the bacterial cells and in particular on microcolonies, which are sites of concentrated AHL biosynthesis (Fig. 9; Tait et al., 2005). Furthermore, surface topography does not appear to play a major role in zoospore settlement on V. anguillarum biofilms since treatment of the biofilm with chloramphenicol or by exposure to UV light to kill the biofilm bacteria significantly reduced settlement without changing the physical integrity of the biofilm (Tait et al., 2005).
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The mechanism of zoospore attraction does not seem to involve chemotactic orientation towards AHLs but instead a chemokinesis in which spore swimming speed rapidly decreases in the presence of AHLs such that the zoospores accumulate at the AHL source (Wheeler et al., 2006). Zoospore swimming speed for example decreases more rapidly over wild-type V. anguillarum biofilms when compared with those of a vanM mutant (Wheeler et al., 2006). AHL detection by zoospores causes an influx of calcium and the reduction in swimming speed may arise via calcium-dependent modulation of the flagellar beat pattern (Wheeler et al., 2006). As indicated above, Ulva zoospores sense a wide range of AHLs (Tait et al., 2005), although the chemoresponse was found to be most marked towards 3-oxo-C12-HSL (Wheeler et al., 2006), an interesting finding given the broad biological activity of this AHL molecule and its capacity to modulate mammalian host cell responses, in part through calcium signalling (Shiner et al., 2006).
The reason(s) why Ulva zoospores target bacterial biofilms as a preferred site for attachment remain(s) unclear but bacterial biofilms are clearly an important factor in biofouling. Many studies have shown that microbial biofilms influence the settlement of marine invertebrates (Joint et al., 2007). Grazing organisms are assumed to exploit bacterial biofilms as food sources but this is unlikely to be a factor in algal attraction. Consequently, the presence of a bacterial biofilm may signal that the environment is benign for Ulva zoospores. However, given that the interaction is very specific, with zoospores settling directly on bacterial cells in the biofilm, rather than merely attaching in the vicinity, there are likely to be other reasons why zoospores prefer attaching to bacterial biofilms. For example, many green algae do not develop normal morphology when grown under axenic conditions (Provasoli & Pintner, 1980). Recently, Matsuo et al. (2003) have shown that differentiation in the green alga Monostroma oxyspermum depends on the presence of specific bacterial strains; i.e. normal morphology depends on particular bacteria and not on bacteria in general. Consequently the preference of Ulva zoospores for settlement on AHL-producing bacterial biofilms may facilitate a close association between the developing thallus and certain essential bacteria (Tait et al., 2005).
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Concluding remarks |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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). Much of the work to date has focused on the function of specific QS signals in a single organism, although there is growing interest in the impact and exploitation of QS in complex microbial communities. In this respect, it is interesting that the addition of AHLs to a phenol-degrading activated sludge community used in wastewater treatment results in significant shifts in community composition and phenol degradation rates (Valle et al., 2004). Eukaryotes clearly respond to bacterial QS signal molecules and in some instances this work has pointed towards their potential future therapeutic exploitation (Gardiner et al., 2001; Pritchard et al., 2005). In pathogenic bacteria, QS represents an exciting target for novel antibacterials: the inhibition of QS in bacteria such as P. aeruginosa attenuates virulence and renders biofilms more susceptible to antimicrobial agents and host defences (Williams, 2002; Bjarnsholt & Givskov, 2007). The development of potent, safe QS inhibitors offers considerable scope in the battle against multi-antibiotic-resistant bacteria and chronic biofilm-centred infections.
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ACKNOWLEDGEMENTS |
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Colworth Prize Lecture 2007
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REFERENCES |
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TOPABSTRACTIntroductionQS signal molecules are...AHL production is widespread...AHL structural variation and...AHL perceptionPlant-microbe interactions,...QS and bacterial cross-talkQS and prokaryote-eukyarote...Eukaryote-mediated quorum...AHL sensing by zoospores...Concluding remarksREFERENCES |
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