In vitro biosynthesis of the Pseudomonas aeruginosa quorum-sensing signal molecule N-butanoyl-L-homoserine lactone

Bifunctional nanomaterials for simultaneously improving cell adhesion and affecting bacterial biofilm formation on silicon-based surfaces

In the biomedical field, silicon-based materials are widely used as implants, biomedical devices, and drug delivery systems. Although these materials show promise for implant technologies and clinical applications, many of them fail to simultaneously possess key properties, such as mechanical stability, biostability, stretchability, cell adhesiveness, biofilm inhibition, and drug delivery ability. Therefore, there is considerable need for the development and improvement of new biomaterials with improved properties. In this context, we describe the synthesis of a new hybrid nanocomposite material that is prepared by incorporating bifunctional nanomaterials onto glass and polydimethylsiloxane surfaces. The results show that our hybrid nanocomposite material is elastic, stretchable, injectable, biostable, has pH-controlled drug delivery ability, and display improved cell adhesion and proliferation and, at the same time, impacted bacterial biofilm formation on the respective surfaces.

Quorum Quenching Mediated Bacteria Interruption as a Probable Strategy for Drinking Water Treatment against Bacterial Pollution

Pseudomonas aeruginosa in water lines may cause bacteria pollution indrinking fountains that could affect the quality of potable water, thus posing a risk to public health. A clean and efficient strategy is required for drinking water treatment for food safety. In this study, an AiiA-homologous lactonase was cloned from a deep-sea probiotics Bacillus velezensis (DH82 strain), and was heterologously expressed so that the capacity of the enzyme on the N-acyl-L-homoserine lactone (AHL)-degrading, effect of bacterial proliferation, biofilm formation and toxic factors release, and membrane pollution from P. aeruginosa could each be investigated to analyze the effect of the enzyme on water treatment. The enzyme effectively degraded the signal molecules of P. aeruginosa (C6-HSL and C12-HSL), inhibited early proliferation and biofilm formation, significantly reduced toxic products (pyocyanin and rhamnolipid), and inhibited bacterial fouling on the filter membrane, which prevented the secondary contamination of P. aeruginosa in drinking water. The findings demonstrated that the quorum quenching enzyme from probiotics could prevent bacteria pollution and improve potable water quality, and that the enzyme treatment could be used as a probable strategy for drinking water treatment.

The fitness burden imposed by synthesising quorum sensing signals

It is now well established that bacterial populations utilize cell-to-cell signaling (quorum-sensing, QS) to control the production of public goods and other co-operative behaviours. Evolutionary theory predicts that both the cost of signal production and the response to signals should incur fitness costs for producing cells. Although costs imposed by the downstream consequences of QS have been shown, the cost of QS signal molecule (QSSM) production and its impact on fitness has not been examined. We measured the fitness cost to cells of synthesising QSSMs by quantifying metabolite levels in the presence of QSSM synthases. We found that: (i) bacteria making certain QSSMs have a growth defect that exerts an evolutionary cost, (ii) production of QSSMs negatively correlates with intracellular concentrations of QSSM precursors, (iii) the production of heterologous QSSMs negatively impacts the production of a native QSSM that shares common substrates, and (iv) supplementation with exogenously added metabolites partially rescued growth defects imposed by QSSM synthesis. These data identify the sources of the fitness costs incurred by QSSM producer cells, and indicate that there may be metabolic trade-offs associated with QS signaling that could exert selection on how signaling evolves.

Detection and quantitation of bacterial acylhomoserine lactone quorum sensing molecules via liquid chromatography-isotope dilution tandem mass spectrometry

A range of acylhomoserine lactones (AHLs) are used as intraspecies quorum sensing signals by Gram-negative bacteria, and the detection and quantitation of these molecules is of interest. This manuscript reports a liquid chromatographic-isotope dilution tandem mass spectrometric method for the quantitation of these molecules. A divergent solid-phase synthesis of stable-isotope-labeled AHLs suitable for use as an internal standard is reported. This route relies on the biomimetic conversion of a dideuterated methionine equivalent, N-Fmoc-(4,4-(2)H(2))methionine, to the desired labeled AHL, and a representative series of eight of these molecules was produced in >95% purity and yields up to ~50%. The representative AHL internal standards were then used to develop an optimized liquid chromatography-tandem mass spectrometric (LC-MS/MS) separation and detection protocol for these molecules, which relies on a high-efficiency C18 core-shell column to minimize the time necessary for separation. The addition of internal standards at different steps during sampling was also found to affect the analysis for hydrophobic AHLs with addition prior to cell removal giving the most accurate results. Taken together, the use of the internal standards and separation method reported herein provides a rapid and quantitative method for the study of AHL production in bacteria.

Quorum sensing and Bacterial Pathogenicity: From Molecules to Disease

Quorum sensing in prokaryotic biology refers to the ability of a bacterium to sense information from other cells in the population when they reach a critical concentration (i.e. a Quorum) and communicate with them. The “language” used for this intercellular communication is based on small, self-generated signal molecules called as autoinducers. Quorum sensing is thought to afford pathogenic bacteriaa mechanism to minimize host immune responses by delaying theproduction of tissue-damaging virulence factors until sufficientbacteria have amassed and are prepared to overwhelm host defensemechanisms and establish infection. Quorum sensing systems are studied in a large number of gram-negative bacterial species belonging to α, β, and γ subclasses of proteobacteria. Among the pathogenic bacteria, Pseudomonas aeruginosa is perhaps the best understood in terms of the virulence factors regulated and the role the Quorum sensing plays in pathogenicity. Presently, Quorum sensing is considered as a potential novel target for antimicrobial therapy to control multi/all drug-resistant infections. This paper reviews Quorum sensing in gram positive and gram negative bacteria and its role in biofilm formation.

Mechanism of Vibrio cholerae autoinducer-1 biosynthesis

Vibrio cholerae, the causative agent of the disease cholera, uses a cell to cell communication process called quorum sensing to control biofilm formation and virulence factor production. The major V. cholerae quorum-sensing signal CAI-1 has been identified as (S)-3-hydroxytridecan-4-one, and the CqsA protein is required for CAI-1 production. However, the biosynthetic route to CAI-1 remains unclear. Here we report that (S)-adenosylmethionine (SAM) is one of the two biosynthetic substrates for CqsA. CqsA couples SAM and decanoyl-coenzyme A to produce a previously unknown but potent quorum-sensing molecule, 3-aminotridec-2-en-4-one (Ea-CAI-1). The CqsA mechanism is unique; it combines two enzymatic transformations, a β,γ-elimination of SAM and an acyltransferase reaction into a single PLP-dependent catalytic process. Ea-CAI-1 is subsequently converted to CAI-1, presumably through the intermediate tridecane-3,4-dione (DK-CAI-1). We propose that the Ea-CAI-1 to DK-CAI-1 conversion occurs spontaneously, and we identify the enzyme responsible for the subsequent step: conversion of DK-CAI-1 into CAI-1. SAM is the substrate for the synthesis of at least three different classes of quorum-sensing signal molecules, indicating that bacteria have evolved a strategy to leverage an abundant substrate for multiple signaling purposes.

Inactivation of AHLs by Ochrobactrum sp. A44 depends on the activity of a novel class of AHL acylase

The soil isolate Ochrobactrum sp. A44 inactivates N-acyl homoserine lactone (AHL) quorum sensing signal molecules and is capable of quenching the AHL-dependent virulence of Pectobacterium carotovorum in planta. To characterize this AHL inactivating activity, Ochrobactrum cell extracts were prepared and their capacity to degrade a broad range of AHLs was determined. AHLs with acyl chains ranging from C4 to C14 with or without 3-oxo or 3-hydroxy substituents were all inactivated to varying extents; long chain AHLs were generally more susceptible than short chain compounds irrespective of the three position substituent. HPLC and LC-tandem mass spectrometry of the AHL degradation products revealed that the AHL inactivating activity present in the Ochrobactrum cell extract cleaved the AHL amide bond. To identify the gene(s) responsible for AHL degradation, Ochrobactrum sp. A44 was subjected to random transposon (Tn) mutagenesis and the resulting mutants screened for the loss of AHL acylase activity. The Tn insertion in mutant A6731 was mapped to a gene termed aiiO, the translated product of which belongs to the α/β hydrolase superfamily which constitutes a novel type of AHL acylase.

Contribution of quorum-sensing system to hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1

AIMS

To investigate roles of quorum-sensing (QS) system in Acinetobacter sp. strain DR1 and rifampicin-resistant variant (hereinafter DR1R).

METHODS AND RESULTS

The DR1 strain generated three putative acyl homoserine lactones (AHLs), while the DR1R produced only one signal and QS signal production was abrogated in the aqsI (LuxI homolog) mutant. The hexadecane-degradation and biofilm-formation capabilities of DR1, DR1R, and aqsI mutants were compared, along with their proteomic data. Proteomics analysis revealed that the AHL lactonase responsible for degrading QS signal was highly upregulated in both DR1R and aqsI mutant, also showed that several proteins, including ppGpp synthase, histidine kinase sensors, might be under the control of QS signalling. Interestingly, biofilm-formation and hexadecane-biodegradation abilities were reduced more profoundly in the aqsI mutant. These altered phenotypes of the aqsI mutant were restored via the addition of free wild-type cell supernatant and exogenous C(12) -AHL.

CONCLUSIONS

The QS system in strain DR1 contributes to hexadecane degradation and biofilm formation.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report to demonstrate that a specific QS signal appears to be a critical factor for hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1.

Quorum sensing and social networking in the microbial world

For many years, bacterial cells were considered primarily as selfish individuals, but, in recent years, it has become evident that, far from operating in isolation, they coordinate collective behaviour in response to environmental challenges using sophisticated intercellular communication networks. Cell-to-cell communication between bacteria is mediated by small diffusible signal molecules that trigger changes in gene expression in response to fluctuations in population density. This process, generally referred to as quorum sensing (QS), controls diverse phenotypes in numerous Gram-positive and Gram-negative bacteria. Recent advances have revealed that bacteria are not limited to communication within their own species but are capable of 'listening in' and 'broadcasting to' unrelated species to intercept messages and coerce cohabitants into behavioural modifications, either for the good of the population or for the benefit of one species over another. It is also evident that QS is not limited to the bacterial kingdom. The study of two-way intercellular signalling networks between bacteria and both uni- and multicellular eukaryotes as well as between eukaryotes is just beginning to unveil a rich diversity of communication pathways.

Altering the substrate specificity of RhlI by directed evolution

REDUCING VIRULENCE: RhlI catalyzes the synthesis of N-butanoyl homoserine lactone (BHL), with a minor product N-hexanoyl homoserine lactone (HHL). By using directed evolution and a genetic screen, RhlI has been engineered for enhanced production of both BHL and HHL at a similar level. Quorum sensing regulates biofilm formation and virulence factor production in the human opportunistic pathogen Pseudomonas aeruginosa. We used directed evolution to engineer RhlI, an enzyme in the RhlI-RhlR quorum-sensing system of P. aeruginosa, to alter its substrate specificity and gain insight into the molecular mechanisms of quorum sensing. By using a genetic screen, we identified a mutant with improved production of RhlI's two signaling molecules, N-butanoyl- and N-hexanoyl-homoserine lactone (BHL and HHL). In particular, production of BHL has been enhanced by more than two-fold, and the synthesis of HHL has been improved from an undetectable level to a level similar to BHL; this change indicates a significant change in substrate specificity. No significant change in the gene expression level was observed. Sequence alignments suggest that the mutations are most likely to facilitate interactions between the enzyme and the two acylated ACP substrates. This work also demonstrates that the genetic screen/selection should be useful in engineering additional quorum-sensing components.

Identification of hydroxyl radical oxidation products of N-hexanoyl-homoserine lactone by reversed-phase high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A reversed-phase high-performance liquid chromatography/electrospray tandem mass spectrometry method was developed for the characterization of hydroxyl radical oxidation products of N-hexanoyl-homoserine lactone (C6-HSL), a member of the N-acylhomoserine lactone (AHL) class of microbial quorum-sensing signaling molecules identified in many Gram-negative strains of bacteria. Six products were identified: four with molecular weight (MW) of 213 and two with MW of 260. The characteristic product ions formed through collision-induced dissociation (CID) provided diagnostic structural information. One of the photolysis products was determined to be N-(3-oxohexanoyl)homoserine lactone (3OC6-HSL), a highly active quorum-sensing signal, by comparison with a reference standard. Three structural isomers with the same mass as 3OC6-HSL were identified as acyl side chain oxidized C6-HSL (keto/enol functionalized) by accurate mass measurement and the structures of these products were proposed from CID spectral interpretation. Two structural isomers formed from concurrent oxidation and nitration of C6-HSL were also observed and their structures were postulated based on CID spectra. In addition to the six hydroxyl radical oxidation products formed from the C6-HSL precursor, five additional compounds generated from combined oxidation and lactonolysis of C6-HSL were identified and structures were postulated.

Turnover of quorum sensing signal molecules modulates cross-kingdom signalling

N-acylhomoserine lactone (AHL) quorum-sensing molecules modulate the swimming behaviour of zoospores of the macroalga Ulva to facilitate the location of bacterial biofilms. Here we show that the intertidal surfaces colonized by Ulva are dominated by Alphaproteobacteria, particularly the Rhodobacteraceae family, and the Bacteroidetes family Flavobacteriaceae, and that this diverse assemblage both produces and degrades AHLs. N-acylhomoserine lactones could also be extracted from the surfaces of pebbles recovered from intertidal rock-pools. Bacteria representative of this assemblage were isolated and tested for the production and degradation of AHLs, and for their ability to modulate zoospore settlement at different biofilm densities. Of particular interest was a Shewanella sp. This strain produced three major AHLs (OC4, OC10 and OC12) in the late exponential phase, but the longer-chain AHLs were rapidly degraded in the stationary phase. Degradation occurred via both lactonase and amidase activity. A close relationship was found between AHL synthesis and Ulva zoospore settlement. The Shewanella isolate also interfered with AHL production by a Sulfitobacter isolate and its ability to enhance zoospore settlement in a polymicrobial biofilm. This influence on the attachment of Ulva zoospores suggests that AHL-degrading strains can affect bacterial community behaviour by interfering with quorum sensing between neighbouring bacteria. More importantly, these interactions may exert wider ecological effects across different kingdoms.

Look who's talking: communication and quorum sensing in the bacterial world

For many years bacteria were considered primarily as autonomous unicellular organisms with little capacity for collective behaviour. However, we now appreciate that bacterial cells are in fact, highly communicative. The generic term 'quorum sensing' has been adopted to describe the bacterial cell-to-cell communication mechanisms which co-ordinate gene expression usually, but not always, when the population has reached a high cell density. Quorum sensing depends on the synthesis of small molecules (often referred to as pheromones or autoinducers) that diffuse in and out of bacterial cells. As the bacterial population density increases, so does the synthesis of quorum sensing signal molecules, and consequently, their concentration in the external environment rises. Once a critical threshold concentration has been reached, a target sensor kinase or response regulator is activated (or repressed) so facilitating the expression of quorum sensing-dependent genes. Quorum sensing enables a bacterial population to mount a co-operative response that improves access to nutrients or specific environmental niches, promotes collective defence against other competitor prokaryotes or eukaryotic defence mechanisms and facilitates survival through differentiation into morphological forms better able to combat environmental threats. Quorum sensing also crosses the prokaryotic-eukaryotic boundary since quorum sensing-dependent signalling can be exploited or inactivated by both plants and mammals.

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

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

Identification of bacterial N-acylhomoserine lactones (AHLs) with a combination of ultra-performance liquid chromatography (UPLC), ultra-high-resolution mass spectrometry, and in-situ biosensors

N-Acylated homoserine lactones (AHLs) are produced by Gram-negative bacteria as communication signals and are frequently studied as mediators of the "quorum sensing" response of bacterial communities. Several reports have recently been published on the identification of AHLs from different species and attempts have been made to study their role in natural habitats, for example the surface of plant roots in the rhizosphere. In this article, different analytical methods, including bacterial biosensors and chromatographic techniques, are reviewed. A concept for assignment of the structures of AHLs is also presented. The retention behaviour of derivatives of AHLs containing beta-keto or hydroxyl groups and/or double bonds has been evaluated in relation to the separation behaviour of AHLs with saturated and unsubstituted alkanoyl chains. Samples have also been analysed by high resolution mass spectrometry (Fourier-transform ion-cyclotron-resonance mass spectrometry, FTICR-MS), nano liquid chromatography-electrospray ionization ion trap mass spectrometry (nano-LC-MS) and by the aid of a biosensor. The results obtained from ultra performance liquid chromatography (UPLC), FTICR-MS, nano-LC-MS, and bioassays have been compared to attempt structural characterisation of AHL without chemical synthesis of analytical standards. The method was used to identify the major AHL compound produced by the rhizosphere bacterium Acidovorax sp. N35 as N-(3-hydroxydecanoyl)homoserine lactone.

Identification of genes involved in swarming motility using a Pseudomonas aeruginosa PAO1 mini-Tn5-lux mutant library

During a screening of a mini-Tn5-luxCDABE transposon mutant library of Pseudomonas aeruginosa PAO1 for alterations in swarming motility, 36 mutants were identified with Tn5 insertions in genes for the synthesis or function of flagellin and type IV pilus, in genes for the Xcp-related type II secretion system, and in regulatory, metabolic, chemosensory, and hypothetical genes with unknown functions. These mutants were differentially affected in swimming and twitching motility but in most cases had only a minor additional motility defect. Our data provide evidence that swarming is a more complex type of motility, since it is influenced by a large number of different genes in P. aeruginosa. Conversely, many of the swarming-negative mutants also showed an impairment in biofilm formation, indicating a strong relationship between these types of growth states.

N-acyl homoserine lactones are degraded via an amidolytic activity in Comamonas sp. strain D1

Comamonas strain D1 enzymatically inactivates quorum-sensing (QS) signal molecules of the N-acyl homoserine lactone (N-AHSL) family, and exhibits the broadest inactivation range of known bacteria. It degrades N-AHSL with acyl-side chains ranging from 4 to 16 carbons, with or without 3-oxo or 3-hydroxy substitutions. N-AHSL degradation yields HSL but not N-acyl homoserine: strain D1 therefore harbors an amidohydrolase activity. Strain D1 is the fifth bacterium species in which an N-AHSL amidohydrolase is described. Consistent with its N-AHSL degradation ability, strain D1 efficiently quenches various QS-dependent functions in other bacteria, such as violacein production by Chromobacterium violaceum and pathogenicity and antibiotic production in Pectobacterium.

Regulatory roles of spnT, a novel gene located within transposon TnTIR

The transposon TnTIR contains spnIR quorum-sensing system regulating sliding motility and the production of nuclease, biosurfactant, and prodigiosin in Serratia marcescens. Within TnTIR, a gene named spnT is upstream of and co-transcribed with spnI. SpnT is a cytoplasmic protein and its level peaks during early stationary phase. spnT over-expression resulted in inhibition of sliding motility and synthesis of prodigiosin, and biosurfactant similar to spnR. spnT but not spnR over-expression induced cell elongation and aberrant DNA replication in S. marcescens and Escherichia coli strains. In comparison with wild-type E. coli strain, over-expression of spnT in an E. coli priA and dnaC double-mutant strain did not lead to the aberrant cell morphology phenotypes, suggesting SpnT may act through the recombination-dependent DNA replication system. As spnT over-expression inhibited swarming but not swimming motility, SpnT may indirectly function as a negative regulator of surface-dependent migration and secondary metabolite production.

VqsM, a novel AraC-type global regulator of quorum-sensing signalling and virulence in Pseudomonas aeruginosa

Human pathogen Pseudomonas aeruginosa uses quorum-sensing (QS) signalling systems to synchronize the production of virulence factors. There are two interrelated QS systems, las and rhl, in P. aeruginosa. In addition to this complexity, a number of transcriptional regulators were shown to have complicated interplays with las and rhl central QS components. Here, we describe a novel virulence and QS modulator (VqsM) that positively regulates the QS systems in P. aeruginosa. Mutation in vqsM resulted in much reduced production of N-acylhomoserine lactones (AHLs) and extracellular enzymes. Sequence analysis revealed that vqsM encodes a transcriptional regulator with an AraC-type helix-turn-helix DNA binding domain at the C-terminal of the peptide. Global gene expression profile analysis showed at least a total of 302 genes to be influenced, directly or indirectly, by VqsM. Among the 203 VqsM-promoted genes, 52.2% were known to be QS upregulated. Several genes encoding the key regulators implicated in QS, such as rhlR, rsaL, vqsR, mvfR, pprB and rpoS, and two AHL synthesis genes, lasI and rhlI, were suppressed in the vqsM mutant. Similar to the 'AHL-blind' phenotype of vqsR and pprB mutants, vqsM mutant did not respond to external addition of N-3-oxo-dodecanoyl-homoserine lactone signals. Moreover, overexpression of vqsR in vqsM mutant more or less restored the production of both AHL and virulence factors. The results demonstrate that VqsM, largely through modulation of vqsR expression, plays a vital role in regulation of QS signalling in P. aeruginosa.

N-Acylhomoserine lactone quorum-sensing molecules are modified and degraded by Rhodococcus erythropolis W2 by both amidolytic and novel oxidoreductase activities

The Rhodococcus erythropolis strain W2 has been shown previously to degrade the N-acylhomoserine lactone (AHL) quorum-sensing signal molecule N-hexanoyl-l-homoserine lactone, produced by other bacteria. Data presented here indicate that this Gram-positive bacterium is also capable of using various AHLs as the sole carbon and energy source. The enzymic activities responsible for AHL inactivation were investigated in R. erythropolis cell extracts and in whole cells. R. erythropolis cells rapidly degraded AHLs with 3-oxo substituents but exhibited relatively poor activity against the corresponding unsubstituted AHLs. Investigation of the mechanism(s) by which R. erythropolis cells degraded AHLs revealed that 3-oxo compounds with N-acyl side chains ranging from C8 to C14 were initially converted to their corresponding 3-hydroxy derivatives. This oxidoreductase activity was not specific to 3-oxo-AHLs but also allowed the reduction of compounds such as N-(3-oxo-6-phenylhexanoyl)homoserine lactone (which contains an aromatic acyl chain substituent) and 3-oxododecanamide (which lacks the homoserine lactone ring). It also reduced both the d- and l-isomers of n-(3-oxododecanoyl)-l-homoserine lactone. A second AHL-degrading activity was observed when R. erythropolis cell extracts were incubated with N-(3-oxodecanoyl)-l-homoserine lactone (3O,C10-HSL). This activity was both temperature- and pH-dependent and was characterized as an amidolytic activity by HPLC analysis of the reaction mixture treated with dansyl chloride. This revealed the accumulation of dansylated homoserine lactone, indicating that the 3O,C10-HSL amide had been cleaved to yield homoserine lactone. R. erythropolis is therefore capable of modifying and degrading AHL signal molecules through both oxidoreductase and amidolytic activities.

Nonbioluminescent strains of Photobacterium phosphoreum produce the cell-to-cell communication signal N-(3-Hydroxyoctanoyl)homoserine lactone

Bioluminescence is a common phenotype in marine bacteria, such as Vibrio and Photobacterium species, and can be quorum regulated by N-acylated homoserine lactones (AHLs). We extracted a molecule that induced a bacterial AHL monitor (Agrobacterium tumefaciens NT1 [pZLR4]) from packed cod fillets, which spoil due to growth of Photobacterium phosphoreum. Interestingly, AHLs were produced by 13 nonbioluminescent strains of P. phosphoreum isolated from the product. Of 177 strains of P. phosphoreum (including 18 isolates from this study), none of 74 bioluminescent strains elicited a reaction in the AHL monitor, whereas 48 of 103 nonbioluminescent strains did produce AHLs. AHLs were also detected in Aeromonas spp., but not in Shewanella strains. Thin-layer chromatographic profiles of cod extracts and P. phosphoreum culture supernatants identified a molecule similar in relative mobility (Rf value) and shape to N-(3-hydroxyoctanoyl)homoserine lactone, and the presence of this molecule in culture supernatants from a nonbioluminescent strain of P. phosphoreum was confirmed by high-performance liquid chromatography-positive electrospray high-resolution mass spectrometry. Bioluminescence (in a non-AHL-producing strain of P. phosphoreum) was strongly up-regulated during growth, whereas AHL production in a nonbioluminescent strain of P. phosphoreum appeared constitutive. AHLs apparently did not influence bioluminescence, as the addition of neither synthetic AHLs nor supernatants delayed or reduced this phenotype in luminescent strains of P. phosphoreum. The phenotypes of nonbioluminescent P. phosphoreum strains regulated by AHLs remains to be elucidated.

Making 'sense' of metabolism: autoinducer-2, LUXS and pathogenic bacteria

Bacteria exploit many mechanisms to communicate with each other and their surroundings. Mechanisms using small diffusible signals to coordinate behaviour with cell density (quorum sensing) frequently contribute to pathogenicity. However, pathogens must also be able to acquire nutrients and replicate to successfully invade their host. One quorum-sensing system, based on the possession of LuxS, bears the unique feature of contributing directly to metabolism, and therefore has the potential to influence both gene regulation and bacterial fitness. Here, we discuss the influence that LuxS and its product, autoinducer-2, have on virulence, relating the current evidence to the preferred niche of the pathogen and the underlying mechanisms involved.

Anaerobic growth does not support biofilm formation in Escherichia coli K-12

Association with a surface in a structure known as biofilm is the prevailing microbial lifestyle. Here we show the kinetics of biofilm formation of Escherichia coli W3110 in static cultures growing under aerobic or anaerobic conditions. Aerobically growing cells in LB medium started to produce detectable amounts of biofilm after 4 to 8 h, displaying maximal accumulation of formed biofilm at 24 h, corresponding to the onset of stationary phase. Then an abrupt reduction in the biomass of the biofilm was observed. This decrease was not prevented by external addition of fresh nutrients and coincided with the depletion of oxygen as measured by the enzymatic activity of the AdhE protein. No biofilm formation was detected in cultures grown anaerobically in LB or LB supplemented with nitrate, nitrite, DMSO or fumarate, even after 72 h of incubation, well inside the stationary phase, suggesting that under anaerobic growth conditions E. coli cannot form biofilms.

Positive control of swarming, rhamnolipid synthesis, and lipase production by the posttranscriptional RsmA/RsmZ system in Pseudomonas aeruginosa PAO1

In Pseudomonas aeruginosa, the small RNA-binding, regulatory protein RsmA is a negative control element in the formation of several extracellular products (e.g., pyocyanin, hydrogen cyanide, PA-IL lectin) as well as in the production of N-acylhomoserine lactone quorum-sensing signal molecules. RsmA was found to control positively the ability to swarm and to produce extracellular rhamnolipids and lipase, i.e., functions contributing to niche colonization by P. aeruginosa. An rsmA null mutant was entirely devoid of swarming but produced detectable amounts of rhamnolipids, suggesting that factors in addition to rhamnolipids influence the swarming ability of P. aeruginosa. A small regulatory RNA, rsmZ, which antagonized the effects of RsmA, was identified in P. aeruginosa. Expression of the rsmZ gene was dependent on both the global regulator GacA and RsmA, increased with cell density, and was subject to negative autoregulation. Overexpression of rsmZ and a null mutation in rsmA resulted in quantitatively similar, negative or positive effects on target genes, in agreement with a model that postulates titration of RsmA protein by RsmZ RNA.

Identification and characterization of a GDSL esterase gene located proximal to the swr quorum-sensing system of Serratia liquefaciens MG1

Serratia liquefaciens MG1 employs the swr quorum-sensing system to control various functions, including production of extracellular enzymes and swarming motility. Here we report the sequencing of the swr flanking DNA regions. We identified a gene upstream of swrR and transcribed in the same direction, designated estA, which encodes an esterase that belongs to family II of lipolytic enzymes. EstA was heterologously expressed in Escherichia coli, and the substrate specificity of the enzyme was determined in crude extracts. With the aid of zymograms visualizing EstA on polyacrylamide gels and by the analysis of a transcriptional fusion of the estA promoter to the promoterless luxAB genes, we showed that expression of the esterase is not regulated by the swr quorum-sensing system. An estA mutant was generated and was found to exhibit growth defects on minimal medium containing Tween 20 or Tween 80 as the sole carbon source. Moreover, we show that the mutant produces greatly reduced amounts of N-acyl-homoserine lactone (AHL) signal molecules on Tween-containing medium compared with the wild type, suggesting that under certain growth conditions EstA may be important for providing the cell with precursors required for AHL biosynthesis.

Negative control of quorum sensing by RpoN (sigma54) in Pseudomonas aeruginosa PAO1

In Pseudomonas aeruginosa PAO1, the expression of several virulence factors such as elastase, rhamnolipids, and hydrogen cyanide depends on quorum-sensing regulation, which involves the lasRI and rhlRI systems controlled by N-(3-oxododecanoyl)-L-homoserine lactone and N-butyryl-L-homoserine lactone, respectively, as signal molecules. In rpoN mutants lacking the transcription factor sigma(54), the expression of the lasR and lasI genes was elevated at low cell densities, whereas expression of the rhlR and rhlI genes was markedly enhanced throughout growth by comparison with the wild type and the complemented mutant strains. As a consequence, the rpoN mutants had elevated levels of both signal molecules and overexpressed the biosynthetic genes for elastase, rhamnolipids, and hydrogen cyanide. The quorum-sensing regulatory protein QscR was not involved in the negative control exerted by RpoN. By contrast, in an rpoN mutant, the expression of the gacA global regulatory gene was significantly increased during the entire growth cycle, whereas another global regulatory gene, vfr, was downregulated at high cell densities. In conclusion, it appears that GacA levels play an important role, probably indirectly, in the RpoN-dependent modulation of the quorum-sensing machinery of P. aeruginosa.

Controlling infection by tuning in and turning down the volume of bacterial small-talk

As the prevalence of bacterial resistance to multiple antibiotics increases it is becoming progressively more difficult to treat infections and, in many cases, the available therapeutic options are severely limited. Hence, there is a growing urgency to the search for novel targets and the development of new antimicrobials. To infect a host and cause disease bacteria produce an array of virulence determinants that contribute to pathogenesis. It is now known that many different Gram-positive and Gram-negative pathogens communicate via the production and sensing of small, diffusible signal molecules, to coordinate virulence determinant production. As a consequence, this event, now termed quorum sensing, represents a novel therapeutic target offering the opportunity to attenuate virulence, and thus control infection, by blocking cell-to-cell communication.

N-acylhomoserine lactones undergo lactonolysis in a pH-, temperature-, and acyl chain length-dependent manner during growth of Yersinia pseudotuberculosis and Pseudomonas aeruginosa

In gram-negative bacterial pathogens, such as Pseudomonas aeruginosa and Yersinia pseudotuberculosis, cell-to-cell communication via the N-acylhomoserine lactone (AHL) signal molecules is involved in the cell population density-dependent control of genes associated with virulence. This phenomenon, termed quorum sensing, relies upon the accumulation of AHLs to a threshold concentration at which target structural genes are activated. By using biosensors capable of detecting a range of AHLs we observed that, in cultures of Y. pseudotuberculosis and P. aeruginosa, AHLs accumulate during the exponential phase but largely disappear during the stationary phase. When added to late-stationary-phase, cell-free culture supernatants of the respective pathogen, the major P. aeruginosa [N-butanoylhomoserine lactone (C4-HSL) and N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL)] and Y. pseudotuberculosis [N-(3-oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL)] AHLs were inactivated. Short-acyl-chain compounds (e.g., C4-HSL) were turned over more extensively than long-chain molecules (e.g., 3-oxo-C12-HSL). Little AHL inactivation occurred with cell extracts, and no evidence for inactivation by specific enzymes was apparent. This AHL turnover was discovered to be due to pH-dependent lactonolysis. By acidifying the growth media to pH 2.0, lactonolysis could be reversed. By using carbon-13 nuclear magnetic resonance spectroscopy, we found that the ring opening of homoserine lactone (HSL), N-propionyl HSL (C3-HSL), and C4-HSL increased as pH increased but diminished as the N-acyl chain was lengthened. At low pH levels, the lactone rings closed but not via a simple reversal of the ring opening reaction mechanism. Ring opening of C4-HSL, C6-HSL, 3-oxo-C6-HSL, and N-octanoylhomoserine lactone (C8-HSL), as determined by the reduction of pH in aqueous solutions with time, was also less rapid for AHLs with more electron-donating longer side chains. Raising the temperature from 22 to 37 degrees C increased the rate of ring opening. Taken together, these data show that (i) to be functional under physiological conditions in mammalian tissue fluids, AHLs require an N-acyl side chain of at least four carbons in length and (ii) that the longer the acyl side chain the more stable the AHL signal molecule.

Quorum sensing: an emerging target for antibacterial chemotherapy?

The emergence of bacterial strains exhibiting resistance to multiple antibiotic classes poses a major threat to medicine and public health. This has been compounded over the last few decades by the failure of drug discovery programmes to provide new broad spectrum antibacterials with novel modes of action. As a consequence, there is renewed interest in antibacterial targets which disrupt the capacity of pathogenic bacteria to cause infection by attenuating virulence. In this respect, one crucial feature of almost all bacterial infections is that the pathogen must attain a critical cell population density sufficient to overwhelm the host defences. Many pathogens are now known to regulate diverse physiological processes, including virulence, in a cell density dependent manner through cell-cell communication. This phenomenon, which relies upon the interaction of a diffusible signal molecule with a sensor kinase or response regulator, has become known as 'quorum sensing'. This review considers the molecular basis of quorum sensing and whether it constitutes a potential therapeutic target for the design of small molecule antagonists capable of controlling infection by attenuating adaptation to the host environment.

Identification of a quorum-sensing signal molecule in the facultative intracellular pathogen Brucella melitensis

Brucella melitensis is a gram-negative alpha2-proteobacterium responsible for abortion in goats and for Malta fever in humans. This facultative intracellular pathogen invades and survives within both professional and nonprofessional phagocytes. A dichloromethane extract of spent culture supernatant from B. melitensis induces bioluminescence in an Escherichia coli acyl-homoserine lactone (acyl-HSL) biosensor strain based upon the activity of the LasR protein of Pseudomonas aeruginosa. HPLC fractionation of the extract, followed by mass spectrometry, identified the major active molecule as N-dodecanoylhomoserine lactone (C12-HSL). This is the first report of the production of an acyl-HSL by an intracellular pathogen. The addition of synthetic C12-HSL to an early log phase culture of either B. melitensis or Brucella suis 1330 reduces the transcription of the virB operon, which contains virulence genes known to be required for intracellular survival. This mimics events seen during the stationary phase of growth and suggests that quorum sensing may play a role in the control of virulence in Brucella.

LuxS: its role in central metabolism and the in vitro synthesis of 4-hydroxy-5-methyl-3(2H)-furanone

Many bacteria produce extracellular molecules which function in cell-to-cell communication. One of these molecules, autoinducer 2 (AI-2), was first described as an extracellular signal produced by Vibrio harveyi to control luciferase expression. Subsequently, a number of bacteria have been shown to possess AI-2 activity in their culture supernatants, and bear the luxS gene product, which is required for AI-2 synthesis. In Porphyromonas gingivalis, luxS and pfs, encoding a 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase (MTA/SAH'ase), form an operon, suggesting that S-adenosylhomocysteine (SAH) or 5'-methylthioadenosine (MTA) serves as a substrate for AI-2 production. Cell-free extracts of Escherichia coli MG1655, but not DH5alpha (which carries a luxS frame-shift mutation) were capable of generating AI-2 activity upon addition of SAH, but not MTA. S-Ribosyl-homocysteine (RH) derived from SAH also served as a substrate in E. coli MG1655 extracts. RH-supplemented cell-free extracts of Pseudomonas aeruginosa, a bacterium that lacks luxS, only generated AI-2 activity following the introduction of a plasmid containing the Por. gingivalis pfs-luxS operon. In addition, defined in vitro systems consisting of the purified LuxS proteins from Por. gingivalis, E. coli, Neisseria meningitidis or Staphylococcus aureus converted RH to homocysteine and a compound that exhibits AI-2 activity.4-Hydroxy-5-methyl-3(2H)-furanone was identified by mass spectrometry analysis as a major product formed in this in vitro reaction. In E. coli MG1655, expression of T3SH [the bacteriophage T3 S-adenosylmethionine (SAM) hydrolase] significantly reduced AI-2 activity in culture supernatants, suggesting that AI-2 production is limited by the amount of SAH produced in SAM-dependent transmethylase reactions. The authors suggest that the LuxS protein has an important metabolic function in the recycling of SAH. They also show that Ps. aeruginosa is capable of removing AI-2 activity, implying that this molecule may act as a nutrient. In many bacteria AI-2 may in fact represent not a signal molecule but a metabolite which is released early and metabolized in the later stages of growth.

Quorum sensing as a population-density-dependent determinant of bacterial physiology

The discovery that bacterial cells can communicate with each other has led to the realization that bacteria are capable of exhibiting much more complex patterns of co-operative behaviour than would be expected for simple unicellular microorganisms. Now generically termed 'quorum sensing', bacterial cell-to-cell communication enables a bacterial population to mount a unified response that is advantageous to its survival by improving access to complex nutrients or environmental niches, collective defence against other competitive microorganisms or eukaryotic host defence mechanisms and optimization of population survival by differentiation into morphological forms better adapted to combating environmental threats. The principle of quorum sensing encompasses the production and release of signal molecules by bacterial cells within a population. Such molecules are released into the environment and, as cell numbers increase, so does the extracellular level of signal molecule, until the bacteria sense that a threshold has been reached and gene activation, or in some cases depression or repression, occurs via the activity of sensor-regulator systems. In this review, we will describe the biochemistry and molecular biology of a number of well-characterized N-acylhomoserine lactone quorum sensing systems to illustrate how bacteria employ cell-to-cell signalling to adjust their physiology in accordance with the prevailing high-population-density environment.

The global posttranscriptional regulator RsmA modulates production of virulence determinants and N-acylhomoserine lactones in Pseudomonas aeruginosa

Posttranscriptional control is known to contribute to the regulation of secondary metabolism and virulence determinants in certain gram-negative bacteria. Here we report the isolation of a Pseudomonas aeruginosa gene which encodes a global translational regulatory protein, RsmA (regulator of secondary metabolites). Overexpression of rsmA resulted in a substantial reduction in the levels of extracellular products, including protease, elastase, and staphylolytic (LasA protease) activity as well as the PA-IL lectin, hydrogen cyanide (HCN), and the phenazine pigment pyocyanin. While inactivation of rsmA in P. aeruginosa had only minor effects on the extracellular enzymes and the PA-IL lectin, the production of HCN and pyocyanin was enhanced during the exponential phase. The influence of RsmA on N-acylhomoserine lactone-mediated quorum sensing was determined by assaying the levels of N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL) and N-butanoylhomoserine lactone (C4-HSL) produced by the rsmA mutant and the rsmA-overexpressing strain. RsmA exerted a negative effect on the synthesis of both 3-oxo-C12-HSL and C4-HSL, which was confirmed by using lasI and rhlI translational fusions. These data also highlighted the temporal expression control of the lasI gene, which was induced much earlier and to a higher level during the exponential growth phase in an rsmA mutant. To investigate whether RsmA modulates HCN production solely via quorum-sensing control, hcn translational fusions were employed to monitor the regulation of the cyanide biosynthesis genes (hcnABC). RsmA was shown to exert an additional negative effect on cyanogenesis posttranscriptionally by acting on a region surrounding the hcnA ribosome-binding site. This suggests that, in P. aeruginosa, RsmA functions as a pleiotropic posttranscriptional regulator of secondary metabolites directly and also indirectly by modulating the quorum-sensing circuitry.

SdiA of Salmonella enterica is a LuxR homolog that detects mixed microbial communities

Proteins of the LuxR family detect the presence of N-acylhomoserine lactones (AHLs) and regulate transcription accordingly. When AHLs are synthesized by the same species that detects them, the system allows a bacterium to measure the population density of its own species, a phenomenon known as quorum sensing. The sdiA genes of Escherichia coli and Salmonella enterica serovar Typhimurium are predicted to encode LuxR homologs. However, these species do not appear to synthesize AHLs or any other molecule detected by SdiA. It has previously been demonstrated that overexpression of sdiA results in the activation of the ftsQAZ locus in E. coli and four other loci in Salmonella serovar Typhimurium. Here we report that transcriptional fusions to these five loci fall into two classes. The first class requires overexpression of sdiA for activation. The second class responds to sdiA expressed from its natural position in the chromosome if the appropriate AHLs are added to the culture. The only member of the second class is a series of Prck-luxCDABE fusions in Salmonella serovar Typhimurium. SdiA responds with highest sensitivity to AHLs that have a keto modification at the third carbon and an acyl chain length of 6 or 8 (half-maximal response between 1 and 5 nM). Growth of Salmonella in proximity to species known to synthesize these AHLs results in sdiA-dependent activation of the Prck-luxCDABE fusions. SdiA appears to be the first AHL receptor discovered that detects signals emanating exclusively from other species.

Transgenic plants producing the bacterial pheromone N-acyl-homoserine lactone exhibit enhanced resistance to the bacterial phytopathogen Erwinia carotovora

Bacterial pheromones, mainly different homoserine lactones, are central to a number of bacterial signaling processes, including those involved in plant pathogenicity. We previously demonstrated that N-oxoacyl-homoserine lactone (OHL) is essential for quorum sensing in the soft-rot phytopathogen Erwinia carotovora. In this pathogen, OHL controls the coordinate activation of genes encoding the main virulence determinants, extracellular plant cell wall degrading enzymes (PCWDEs), in a cell density-dependent manner. We suggest that E. carotovora employ quorum sensing to avoid the premature production of PCWDEs and subsequent activation of plant defense responses. To test whether modulating this sensory system would affect the outcome of a plant-pathogen interaction, we generated transgenic tobacco, producing OHL. This was accomplished by ectopic expression in tobacco of the E. carotovora gene expI, which is responsible for OHL biosynthesis. We show that expI-positive transgenic tobacco lines produced the active pheromone and partially complemented the avirulent phenotype of expI mutants. The OHL-producing tobacco lines exhibited enhanced resistance to infection by wild-type E. carotovora. The results were confirmed by exogenous addition of OHL to wild-type plants, which also resulted in increased resistance to E. carotovora.

Quorum-sensing in Gram-negative bacteria

It has become increasingly and widely recognised that bacteria do not exist as solitary cells, but are colonial organisms that exploit elaborate systems of intercellular communication to facilitate their adaptation to changing environmental conditions. The languages by which bacteria communicate take the form of chemical signals, excreted from the cells, which can elicit profound physiological changes. Many types of signalling molecules, which regulate diverse phenotypes across distant genera, have been described. The most common signalling molecules found in Gram-negative bacteria are N-acyl derivatives of homoserine lactone (acyl HSLs). Modulation of the physiological processes controlled by acyl HSLs (and, indeed, many of the non-acyl HSL-mediated systems) occurs in a cell density- and growth phase-dependent manner. Therefore, the term 'quorum-sensing' has been coined to describe this ability of bacteria to monitor cell density before expressing a phenotype. In this paper, we review the current state of research concerning acyl HSL-mediated quorum-sensing. We also describe two non-acyl HSL-based systems utilised by the phytopathogens Ralstonia solanacearum and Xanthomonas campestris.

Acylated homoserine lactone detection in Pseudomonas aeruginosa biofilms by radiolabel assay

We describe the development of a new radioactive assay for acyl-HSL production by bacterial cultures. The assay is based on the uptake of radiolabeled methionine and conversion of the radiolabel into SAM. The radiolabeled SAM is then incorporated into acyl-HSL by an acyl-HSL synthase. This assay is faster than previously used bioassays and shows no bias for the detection of acyl-HSLs of a particular length or side chain substitution. Acyl-HSL production can be monitored over a wide range of growth conditions in liquid culture. This assay can also be used in conjunction with a tube biofilm reactor to monitor acyl-HSL production by biofilm cultures. Ultimately this assay will allow comparison of acyl-HSL production by cells subjected to a variety of physiological conditions.

The LuxM homologue VanM from Vibrio anguillarum directs the synthesis of N-(3-hydroxyhexanoyl)homoserine lactone and N-hexanoylhomoserine lactone

Vibrio anguillarum, which causes terminal hemorrhagic septicemia in fish, was previously shown to possess a LuxRI-type quorum-sensing system (vanRI) and to produce N-(3-oxodecanoyl)homoserine lactone (3-oxo-C10-HSL). However, a vanI null mutant still activated N-acylhomoserine lactone (AHL) biosensors, indicating the presence of an additional quorum-sensing circuit in V. anguillarum. In this study, we have characterized this second system. Using high-pressure liquid chromatography in conjunction with mass spectrometry and chemical analysis, we identified two additional AHLs as N-hexanoylhomoserine lactone (C6-HSL) and N-(3-hydroxyhexanoyl)homoserine lactone (3-hydroxy-C6-HSL). Quantification of each AHL present in stationary-phase V. anguillarum spent culture supernatants indicated that 3-oxo-C10-HSL, 3-hydroxy-C6-HSL, and C6-HSL are present at approximately 8.5, 9.5, and 0.3 nM, respectively. Furthermore, vanM, the gene responsible for the synthesis of these AHLs, was characterized and shown to be homologous to the luxL and luxM genes, which are required for the production of N-(3-hydroxybutanoyl)homoserine lactone in Vibrio harveyi. However, resequencing of the V. harveyi luxL/luxM junction revealed a sequencing error present in the published sequence, which when corrected resulted in a single open reading frame (termed luxM). Downstream of vanM, we identified a homologue of luxN (vanN) that encodes a hybrid sensor kinase which forms part of a phosphorelay cascade involved in the regulation of bioluminescence in V. harveyi. A mutation in vanM abolished the production of C6-HSL and 3-hydroxy-C6-HSL. In addition, production of 3-oxo-C10-HSL was abolished in the vanM mutant, suggesting that 3-hydroxy-C6-HSL and C6-HSL regulate the production of 3-oxo-C10-HSL via vanRI. However, a vanN mutant displayed a wild-type AHL profile. Neither mutation affected either the production of proteases or virulence in a fish infection model. These data indicate that V. anguillarum possesses a hierarchical quorum sensing system consisting of regulatory elements homologous to those found in both V. fischeri (the LuxRI homologues VanRI) and V. harveyi (the LuxMN homologues, VanMN).

Quorum sensing and the regulation of virulence gene expression in pathogenic bacteria

For many pathogens, the outcome of the interaction between host and bacterium is strongly affected by the bacterial population size. Coupling the production of virulence factors with cell population density ensures that the mammalian host lacks sufficient time to mount an effective defence against consolidated attack. Such a strategy depends on the ability of an individual bacterial cell to sense other members of the same species and in response, differentially express specific sets of genes. Such cell-cell communication is called "quorum sensing" and involves the direct or indirect activation of a response regulator by a small diffusible signal molecule. A number of chemically distinct quorum-sensing signal molecules have been described including the N-acyl-L-homoserine lactones (AHLs) in Gram-negative bacteria and post-translationally modified peptides in Gram-positive bacteria. For example, the human pathogens Pseudomonas aeruginosa and Staphylococcus aureus employ AHLs and peptides, respectively, to control the expression of multiple virulence genes in concert with cell population density. Apart from their role in signal transduction, certain quorum-sensing signal molecules, notably N-(3-oxododecanoyl)homoserine lactone, possess intrinsic pharmacological and immunomodulatory activities such that they may function as virulence determinants per se. While quorum-sensing signal molecules have been detected in tissues in experimental animal model and human infections, the mutation of genes involved in either quorum-sensing signal generation or signal transduction frequently results in the attenuation of virulence. Thus, interference with quorum sensing represents a promising strategy for the therapeutic or prophylactic control of infection.

Quorum sensing as an integral component of gene regulatory networks in Gram-negative bacteria

Bacterial cell-to-cell communication (quorum sensing) relies upon the interaction of a small diffusible signal molecule with a sensor or transcriptional activator to couple gene expression with cell population density. In Gram-negative bacteria, it is now clear that N-acylhomoserine lactones bind directly to LuxR homologues and can be synthesized via one of three unrelated bacterial protein families and by transgenic plants. New chemical classes of signal molecules have been identified, some of which exhibit crosstalk with N-acylhomoserine-lactone-mediated quorum sensing. As the determinant of cell population density, quorum sensing is emerging as an integral component of bacterial global gene regulatory networks responsible for facilitating bacterial adaptation to environmental stress. N-acylhomoserine lactones are produced during experimental animal and human infections, and a function beyond quorum sensing has been suggested by their intrinsic immunomodulatory and pharmacological activities.

The biocontrol strain Pseudomonas fluorescens F113 produces the Rhizobium small bacteriocin, N-(3-hydroxy-7-cis-tetradecenoyl)homoserine lactone, via HdtS, a putative novel N-acylhomoserine lactone synthase

Several different species of Pseudomonas: produce N:-acylhomoserine lactones (AHLs), quorum-sensing signal molecules which are involved in the cell-density-dependent control of secondary metabolite and virulence gene expression. When Pseudomonas fluorescens F113 was cross-streaked against AHL biosensors capable of sensitively detecting either short (C(4)-C(8)) or long (C(10)-C(14)) acyl chain AHLs, no activity was detectable. However, by extracting cell-free stationary-phase culture supernatants with dichloromethane followed by reverse-phase HPLC, three distinct fractions were obtained capable of activating the AHL biosensors. Three AHLs were subsequently characterized using high-resolution MS and chemical synthesis. These were (i) N:-(3-hydroxy-7-cis-tetradecenoyl)homoserine lactone (3OH, C(14:1)-HSL), a molecule previously known as the Rhizobium leguminosarum small bacteriocin as a consequence of its growth inhibitory properties, (ii) N:-decanoylhomoserine lactone (C(10)-HSL) and (iii) N:-hexanoylhomoserine lactone (C(6)-HSL). A gene (hdtS) capable of directing synthesis of all three P. fluorescens AHLs in Escherichia coli was cloned and sequenced. In vitro transcription/translation of hdtS yielded a protein of approximately 33 kDa capable of directing the synthesis of 3OH, C(14:1)-HSL, C(10)-HSL and C(6)-HSL in E. coli. HdtS does not belong to either of the known AHL synthase families (LuxI or LuxM) and is related to the lysophosphatidic acid acyltransferase family. HdtS may therefore constitute a member of a third protein family capable of AHL biosynthesis.