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

Functional quorum sensing systems affect biofilm formation and protein expression in Yersinia pestis

Gram-negative bacteria predominantly use two types of quorum sensing (QS) systems--LuxI-LuxR, responsible for synthesis of N-acylhomoserine lactones (AHL or AI-1 signal molecule), and LuxS, which makes furanones (AI-2 signal molecule). We showed that LuxS and two LuxI-LuxR (YtbIR and YpsIR) systems are functional in Y. pestis. Four different AHL molecules were detected in Y. pestis extracts using TLC bioassays. Our data suggest that YtbIR is responsible for the production of long chain AHLs. Confocal laser scanning microscopy showed that biofilm formation is decreased in an ytbIR ypsIR luxS mutant. Two-dimensional gel electrophoresis revealed altered levels of protein expression in a Y. pestis triple QS mutant at 26 degrees C and 37 degrees C.

Surface-enhanced Raman spectroscopy for in situ measurements of signaling molecules (autoinducers) relevant to bacteria quorum sensing

Autoinducer (AI) molecules are used by quorum sensing (QS) bacteria to communicate information about their environment and are critical to their ability to coordinate certain physiological activities. Studying how these organisms react to environmental stresses could provide insight into methods to control these activities. To this end, we are investigating spectroscopic methods of analysis that allow in situ measurements of these AI molecules under different environmental conditions. We found that for one class of AIs, N-acyl-homoserine lactones (AHLs), surface-enhanced Raman spectroscopy (SERS) is a method capable of performing such measurements in situ. SERS spectra of seven different AHLs with acyl chain lengths from 4 to 12 carbons were collected for the first time using Ag colloidal nanoparticles synthesized via both citrate and borohydride reduction methods. Strong SERS spectra were obtained in as little as 10 seconds for 80 microM solutions of AI that exhibited the strongest SERS response, whereas 20 seconds was typical for most AI SERS spectra collected during this study. Although all spectra were similar, significant differences were detected in the SERS spectra of C4-AHL and 3-oxo-C6-AHL and more subtle differences were noted between all AHLs. Initial results indicate a detection limit of approximately 10(-6)M for C6-AHL, which is within the limits of biologically relevant concentrations of AI molecules (nM-microM). Based on these results, the SERS method shows promise for monitoring AI molecule concentrations in situ, within biofilms containing QS bacteria. This new capability offers the possibility to "listen in" on chemical communications between bacteria in their natural environment as that environment is stressed.

Uptake, degradation and chiral discrimination of N-acyl-D/L-homoserine lactones by barley (Hordeum vulgare) and yam bean (Pachyrhizus erosus) plants

Bacterial intraspecies and interspecies communication in the rhizosphere is mediated by diffusible signal molecules. Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) as autoinducers in the quorum sensing response. While bacterial signalling is well described, the fate of AHLs in contact with plants is much less known. Thus, adsorption, uptake and translocation of N-hexanoyl- (C6-HSL), N-octanoyl- (C8-HSL) and N-decanoyl-homoserine lactone (C10-HSL) were studied in axenic systems with barley (Hordeum vulgare L.) and the legume yam bean (Pachyrhizus erosus (L.) Urban) as model plants using ultra-performance liquid chromatography (UPLC), Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and tritium-labelled AHLs. Decreases in AHL concentration due to abiotic adsorption or degradation were tolerable under the experimental conditions. The presence of plants enhanced AHL decline in media depending on the compounds' lipophilicity, whereby the legume caused stronger AHL decrease than barley. All tested AHLs were traceable in root extracts of both plants. While all AHLs except C10-HSL were detectable in barley shoots, only C6-HSL was found in shoots of yam bean. Furthermore, tritium-labelled AHLs were used to determine short-term uptake kinetics. Chiral separation by GC-MS revealed that both plants discriminated D-AHL stereoisomers to different extents. These results indicate substantial differences in uptake and degradation of different AHLs in the plants tested.

Cross-kingdom signalling: exploitation of bacterial quorum sensing molecules by the green seaweed Ulva

The green seaweed Ulva has been shown to detect signal molecules produced by bacteria. Biofilms that release N-acylhomoserine lactones (AHLs) attract zoospores--the motile reproductive stages of Ulva. The evidence for AHL involvement is based on several independent lines of evidence, including the observation that zoospores are attracted to wild-type bacteria that produce AHLs but are not attracted to mutants that do not produce signal molecules. Synthetic AHL also attracts zoospores and the attraction is lost in the presence of autoinducer inactivation (AiiA) protein. The mechanism of attraction is not chemotactic but involves chemokinesis. When zoospores detect AHLs, the swimming rate is reduced and this results in accumulation of cells at the source of the AHL. It has been demonstrated that the detection of AHLs results in calcium influx into the zoospore. This is the first example of a calcium signalling event in a eukaryote in response to bacterial quorum sensing molecules. The role of AHLs in the ecology of Ulva is discussed. It is probable that AHLs act as cues for the settlement of zoospores, rather than being directly involved as a signalling mechanism.

The hydrolysis of unsubstituted N-acylhomoserine lactones to their homoserine metabolites

Derivatives of N-acylhomoserine lactones (HSLs) with different alkanoyl side chains occur as quorum or diffusion sensing molecules in gram-negative bacteria and their quantitative chemical analysis became important as a possible way to follow regulation processes of their pathogenicity towards plants and animals. The lactone-ring of HSLs is chemically and biologically not stable: the corresponding serines can be formed in alkaline conditions and these may presumably behave inactive for the biological system. A fast and MS compatible liquid chromatographic method applying high pressure (ultra performance liquid chromatography) with diode array detection was optimized for the rapid quantitative determination of HSLs and their corresponding hydrolysis products. The technique was used to follow and model the hydrolysis reactions of HSLs as function of pH under controlled conditions. Moreover, the method could be triggered to allow a confirmation in the assignment of the potential HSLs in real samples by analysis of the real samples before and after hydrolysis. Quantitative performance characteristics and the character of the hydrolysis reaction were studied as well. The optimized method was successfully applied to a bacterial culture supernatant real sample containing HSLs.

Quorum sensing, virulence and secondary metabolite production in plant soft-rotting bacteria

Quorum sensing describes the ability of bacteria to sense their population density and respond by modulating gene expression. In the plant soft-rotting bacteria, such as Erwinia, an arsenal of plant cell wall-degrading enzymes is produced in a cell density-dependent manner, which causes maceration of plant tissue. However, quorum sensing is central not only to controlling the production of such destructive enzymes, but also to the control of a number of other virulence determinants and secondary metabolites. Erwinia synthesizes both N-acylhomoserine lactone (AHL) and autoinducer-2 types of quorum sensing signal, which both play a role in regulating gene expression in the phytopathogen. We review the models for AHL-based regulation of carbapenem antibiotic production in Erwinia. We also discuss the importance of quorum sensing in the production and secretion of virulence determinants by Erwinia, and its interplay with other regulatory systems.

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.

Thermoregulation of N-acyl homoserine lactone-based quorum sensing in the soft rot bacterium Pectobacterium atrosepticum

The psychrotolerant bacterium Pectobacterium atrosepticum produces four N-acyl homoserine lactones under a wide range of temperatures. Their thermoregulation differs from that of the exoenzyme production, described as being under quorum-sensing control. A mechanism involved in this thermoregulation consists of controlling N-acyl homoserine lactones synthase production at a transcriptional level.

Environmental regulation of Pseudomonas aeruginosa PAO1 Las and Rhl quorum-sensing systems

The lasI-lasR and the rhlI-rhlR quorum-sensing systems in Pseudomonas aeruginosa regulate the expression of numerous cellular and secreted virulence factor genes and play important roles in the development of biofilms. The las and rhl systems themselves are known to be directly or indirectly regulated by a number of transcriptional regulators, and consequently, their expression is sensitive to environmental conditions. In this report, the activities of these two quorum-sensing systems have been examined systematically under 46 growth conditions, and the regulation by environmental conditions has been investigated. The relative timing and strength of expression of these two systems varied significantly under different conditions, which contrasts with the notion of a preset hierarchy with these two systems in P. aeruginosa. Depending on the growth conditions, the correlation between each synthase and its cognate transcriptional regulator also varied, suggesting that the transcription of these genes independently allows for further fine tuning of each system. Finally, we observe that the activities of both the lasI-lasR and the rhlI-rhlR quorum-sensing systems were dramatically enhanced in the presence of extracts of sputum samples from cystic fibrosis patients.

Detection and characterization of bacteria from the potato rhizosphere degrading N-acyl-homoserine lactone

Quorum sensing plays a role in the regulation of soft rot diseases caused by the plant pathogenic bacterium Pectobacterium carotovorum subsp. carotovorum. The signal molecules involved in quorum sensing in P. carotovorum subsp. carotovorum belong to the group of N-acyl homoserine lactones (AHLs). In our study, we screened bacteria isolated from the potato rhizosphere for the ability to degrade AHLs produced by P. carotovorum subsp. carotovorum. Six isolates able to degrade AHLs were selected for further studies. According to 16S rDNA sequence analysis and fatty acid methyl ester profiling, the isolates belonged to the genera Ochrobactrum, Rhodococcus, Pseudomonas, Bacillus, and Delftia. For the genera Ochrobactrum and Delftia, for the first time AHL-degrading isolates were found. Data presented in this study revealed for the first time that Ochrobactrum sp. strain A44 showed the capacity to inactivate various synthetic AHL molecules; the substituted AHLs were inactivated with a lower efficiency than the unsubstituted AHLs. Compared with the other isolates, A44 was very effective in the degradation of AHLs produced by P. carotovorum subsp. carotovorum. It was verified by polymerase chain reaction, DNA-DNA hybridization, and a lactone ring reconstruction assay that Ochrobactrum sp. strain A44 did not possess AHL lactonase activity. AHL degradation in Ochrobactrum sp. strain A44 occurred intracellularly; it was not found in the culture supernatant. AHL-degrading activity of A44 was thermo sensitive. Experiments in planta revealed that Ochrobactrum sp. strain A44 significantly inhibited the maceration of potato tuber tissue. Since A44 did not produce antibiotics, the attenuation of the decay might be due to the quenching of quorum- sensing-regulated production of pectinolytic enzymes. The strain can potentially serve to control P. carotovorum subsp. carotovorum in potato.

Biosensing systems for the detection of bacterial quorum signaling molecules

Bacterial quorum sensing (QS) is a cell-to-cell communication phenomenon that allows bacteria to control the expression of certain specialized genes depending on their cell population size. Signaling molecules such N-acylhomoserine lactones (AHLs) mediate the communication, and their concentration reflects the bacterial population density. Quorum sensing regulates several processes including bacterial pathogenicity. We developed a method for the rapid, sensitive, and quantitative detection of AHLs in biological samples such as saliva and stools. The method is based on whole-cell sensing systems that employ QS regulatory systems as recognition elements and the luxCDABE gene cassette as a reporter. The method proved to be reproducible when applied to real samples and was able to detect low analyte concentrations down to 1 x 10(-9) M without requiring extensive sample preparation. We envision that this novel biosensing system could be employed in the diagnosis and management of various bacteria-related disorders, thus supporting the use of quorum sensing molecules as potential biomarkers of disease. Due to cost-effectiveness and high throughput, these biosensing systems could be successfully employed as a new tool for the screening of novel drugs that target quorum sensing mechanisms.

Quorum-sensing-deficient (lasR) mutants emerge at high frequency from a Pseudomonas aeruginosa mutS strain

In Pseudomonas aeruginosa, quorum sensing constitutes a highly complex cell-to-cell communication system that, along with the cognate acylhomoserine lactone signals and regulators LasR and RhlR, modulates the production of virulence factors and a wide range of metabolic functions. In a previous paper, the authors reported that mismatch repair disruption in P. aeruginosa results in the spontaneous and reproducible emergence of defined morphological colony variants after a relatively short period of cultivation in an aerated rich medium, in contrast to the non-mutator parental strain, which does not display any kind of diversification under identical incubation conditions. One of the morphotypical variants, mS2, emerges at a high frequency and displays differences in virulence traits that could be regulated by major quorum-sensing regulators. The present study shows that mutS mS2 variants had defective LasR function due to simple but different point mutations along the lasR gene sequence, indicating that LasR inactivation is the main cause of mS2 phenotypic diversification. Moreover, it was determined that a non-functional LasR would confer a selective advantage in the late stationary phase, since viability was notably higher for mS2. Interestingly, in all mS2 variants analysed, no sequence alterations were found in the gacA and rhlR genes, suggesting that the selective pressures for GacA/RhlR and LasR were not the same and differed from those in other Pseudomonas species, which, when incubated in nutrient-rich liquid stationary-phase cultures, show specific high instability in the gacA-gacS genes.

Characterization of a luxI/luxR-type quorum sensing system and N-acyl-homoserine lactone-dependent regulation of exo-enzyme and antibacterial component production in Serratia plymuthica RVH1

Quorum sensing by means of N-acyl-l-homoserine lactones (AHLs) is widespread in Gram-negative bacteria, where diverse AHLs influence a wide variety of functions, even in a single genus such as Serratia. Here we report the identification and characterization of the quorum sensing system of Serratia plymuthica strain RVH1. This strain isolated from a raw vegetable processing line produces at least three AHLs which were identified as N-butanoyl- (C4-HSL), N-hexanoyl- (C6-HSL) and N-(3-oxo-hexanoyl)-homoserine lactone (3-oxo-C6-HSL). The identified LuxI homolog SplI synthesizes 3-oxo-C6-HSL, and influences the production of C4-HSL and C6-HSL, as splI gene inactivation resulted in loss of 3-oxo-C6-HSL production and smaller amounts of C4-HSL and C6-HSL produced. SplI-dependent quorum sensing controls 2,3-butanediol fermentation (previously reported) and the production of an extracellular chitinase, nuclease, protease and antibacterial compound. The identity of the latter is not yet elucidated, but appears to be different from the known antibacterial compounds produced by Serratia strains. SplR, the homolog of the LuxR regulator, appears to act as a repressor of synthesis of extracellular enzymes and antibacterial compound and to autorepress its own expression, probably by binding to a 21bp lux box sequence.

Quorum sensing regulates dpsA and the oxidative stress response in Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a fatal human tropical disease. The non-specific DNA-binding protein DpsA plays a key role in protecting B. pseudomallei from oxidative stress mediated, for example, by organic hydroperoxides. The regulation of dpsA expression is poorly understood but one possibility is that it is regulated in a cell population density-dependent manner via N-acylhomoserine lactone (AHL)-dependent quorum sensing (QS) since a lux-box motif has been located within the dpsA promoter region. Using liquid chromatography and tandem mass spectrometry, it was first established that B. pseudomallei strain PP844 synthesizes AHLs. These were identified as N-octanoylhomoserine lactone (C8-HSL), N-(3-oxooctanoyl)homoserine lactone (3-oxo-C8-HSL), N-(3-hydroxyoctanoyl)-homoserine lactone (3-hydroxy-C8-HSL), N-decanoylhomoserine lactone (C10-HSL), N-(3-hydroxydecanoyl) homoserine lactone (3-hydroxy-C10-HSL) and N-(3-hydroxydodecanoyl)homoserine lactone (3-hydroxy-C12-HSL). Mutation of the genes encoding the LuxI homologue BpsI or the LuxR homologue BpsR resulted in the loss of C8-HSL and 3-oxo-C8-HSL synthesis, demonstrating that BpsI was responsible for directing the synthesis of these AHLs only and that bpsI expression and hence C8-HSL and 3-oxo-C8-HSL production depends on BpsR. In bpsI, bpsR and bpsIR mutants, dpsA expression was substantially down-regulated. Furthermore, dpsA expression in Escherichia coli required both BpsR and C8-HSL. bpsIR-deficient mutants exhibited hypersensitivity to the organic hydroperoxide tert-butyl hydroperoxide by displaying a reduction in cell viability which was restored by provision of exogenous C8-HSL (bpsI mutant only), by complementation with the bpsIR genes or by overexpression of dpsA. These data indicate that in B. pseudomallei, QS regulates the response to oxidative stress at least in part via the BpsR/C8-HSL-dependent regulation of DpsA.

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.

Synthesis of N-acyl homoserine lactone analogues reveals strong activators of SdiA, the Salmonella enterica serovar Typhimurium LuxR homologue

N-Acyl homoserine lactones (AHLs) are molecules that are synthesized and detected by many gram-negative bacteria to monitor the population density, a phenomenon known as quorum sensing. Salmonella enterica serovar Typhimurium is an exceptional species since it does not synthesize its own AHLs, while it does encode a LuxR homologue, SdiA, which enables this bacterium to detect AHLs that are produced by other species. To obtain more information about the specificity of the ligand binding by SdiA, we synthesized and screened a limited library of AHL analogues. We identified two classes of analogues that are strong activators of SdiA: the N-(3-oxo-acyl)-homocysteine thiolactones (3O-AHTLs) and the N-(3-oxo-acyl)-trans-2-aminocyclohexanols. To our knowledge, this is the first report of compounds (the 3O-AHTLs) that are able to activate a LuxR homologue at concentrations that are lower than the concentrations of the most active AHLs. SdiA responds with greatest sensitivity to AHTLs that have a keto modification at the third carbon atom and an acyl chain that is seven or eight carbon atoms long. The N-(3-oxo-acyl)-trans-2-aminocyclohexanols were found to be less sensitive to deactivation by lactonase and alkaline pH than the 3O-AHTLs and the AHLs are. We also examined the activity of our library with LuxR of Vibrio fischeri and identified three new inhibitors of LuxR. Finally, we performed preliminary binding experiments which suggested that SdiA binds its activators reversibly. These results increase our understanding of the specificity of the SdiA-ligand interaction, which could have uses in the development of anti-quorum-sensing-based antimicrobials.

Occurrence of proteolytic activity and N-acyl-homoserine lactone signals in the spoilage of aerobically chill-stored proteinaceous raw foods

Proteolytic pseudomonads dominate the spoilage flora of aerobically chill-stored proteinaceous raw foods. Proteolysis during spoilage of these food systems affects both food quality and the dynamics of the bacterial community because it increases the availability of nutrients to the community as a whole. Quorum sensing, or cell-cell signaling, is associated closely with ecological interactions among bacteria in mixed communities. The potential role of quorum sensing in proteolytic food spoilage was examined, based on the evaluation of N-acyl-homoserine lactone (AHL) signal molecules. The occurrence of proteolytic activity and AHL signals was studied during spoilage of aerobically chill-stored ground beef, fish, chicken, and raw milk. Pseudomonads dominated the psychrotrophic flora, followed distantly by members of the Enterobacteriaceae. The growth of pseudomonads was correlated with the occurrence of proteolytic activity in all food systems. AHL concentration began increasing significantly only after the onset of proteolytic activity. Widely divergent AHL profiles were revealed by thin-layer chromatography analysis of the different food samples, and these profiles were likely determined by the undefined bacterial flora in these systems and by the characterized pseudomonads and Enterobacteriaceae. Although Hafnia alvei was a major component of the Enterobacteriaceae flora in all foods tested and a strong AHL producer, the signal molecules produced by H. alvei strain EB1 did not influence protease production by Pseudomonas fluorescens strain 395 in vitro. These results do not indicate any clear correlation between the overall detectable AHL signal molecules accumulated in the food samples and proteolytic activity.

Development and application of a method for the analysis of N-acylhomoserine lactones by solid-phase extraction and ultra high pressure liquid chromatography

A robust method based on solid-phase extraction (SPE) followed by ultra high pressure liquid chromatography (with trade name of Ultra Performance Liquid Chromatography: UPLC; Waters, Milford, MA, USA) is proposed for the determination of five derivatives of N-acylhomoserine lactones (AHLs) that play a biological role as signal molecules of several gram-negative bacteria. Different commercial SPE cartridges were tested for sample extraction, clean-up and preconcentration. Since the sample matrix was a complex growth media, careful optimization of the SPE with respect to washing procedure, elution solvent and sample solvent was necessary. No sample loss was observed when up to 100 mL spiked full media was added onto the cartridge. Applying UPLC for the determination of AHLs, the performance characteristics of the method showed good separation efficiency and high speed. In order to demonstrate the applicability of the method, supernatants with the known AHL producer Burkholderia cepacia LA3 grown in different media were investigated. Additionally, the method was successfully used for the degradation/uptake study of AHLs from a liquid matrix in which barley was grown under controlled condition.

Comprehensive profiling of N-acylhomoserine lactones produced by Yersinia pseudotuberculosis using liquid chromatography coupled to hybrid quadrupole–linear ion trap mass spectrometry

A method for the comprehensive profiling of the N-acylhomoserine lactone (AHL) family of bacterial quorum-sensing molecules is presented using liquid chromatography (LC) coupled to hybrid quadrupole-linear ion trap (QqQLIT) mass spectrometry. Information-dependent acquisition (IDA), using triggered combinations of triple-quadrupole and linear ion trap modes in the same LC-MS/MS run, was used to simultaneously screen, quantify and identify multiple AHLs in a single sample. This MS method uses common AHL fragment ions attributed to the homoserine moiety and the 3-oxo-, 3-hydroxy- or unsubstituted acyl side chains, to identify unknown AHLs in cell-free culture supernatants in an unbiased manner. This LC-MS technique was applied to determine the relative molar ratios of AHLs produced by Yersinia pseudotuberculosis and the consequences of inactivating by mutation either or both of the AHL synthase genes (ypsI and ytbI) on AHL profile and concentration. The Y. pseudotuberculosis wild type but not the ypsI ytbI double mutant produced at least 24 different AHLs with acyl chains ranging from C4 to C15 with or without 3-oxo or 3-hydroxy substituents. YtbI, in contrast to YpsI, could direct the synthesis of all of the AHLs identified. The most abundant and hence most biologically relevant Y. pseudotuberculosis AHLs were found to be the 3-oxo-substituted C6, C7 and C8 AHLs and the unsubstituted C6 and C8 compounds. The LC-QqQLIT methodology is broadly applicable to quorum-sensing signal molecule analysis and can provide comprehensive AHL profiles and concentrations from a single sample and simultaneously collect confirmatory spectra for each AHL identified.

Effect of temperature and glucose concentration on the N-butanoyl-l-homoserine lactone production by Aeromonas hydrophila

N-butanoyl-L-homoserine lactone (C4-HSL) production by Aeromonas hydrophila 519 has been established. C4-HSL production at 22 and 30 degrees C was found after 20-24 h of incubation corresponding to a population density of ca. 10(8)-10(9) cfu/ml, respectively. Reduced C4-HSL production was noted after 72 h of incubation at 12 degrees C when the culture reached ca. 10(9) cfu/ml. No C4-HSL production was detected at 37 degrees C, although a dense population was obtained. In LB broth supplemented with 0.1% and 0.5% glucose, C4-HSL production was noted whereas with 1% glucose no C4-HSL was detected although a high colony count was obtained. In the latter culture residual levels of glucose (0.65%) were found after 43 h whereas in the 0.1% and 0.5% supplemented LB, glucose was quickly consumed which may have stimulated C4-HSL production. In conclusion, the present study shows an effect of environmental conditions (temperature, glucose concentration) on the C4-HSL production and warrants further investigation to elucidate the effect of external conditions on production of AHL signal molecules to reveal the relevance of quorum sensing in, e.g. foods.

Quorum sensing in Erwinia species

The term quorum sensing (QS) refers to the ability of bacteria to regulate gene expression according to the accumulation of signalling molecules that are made by every cell in the population. The erwiniae group of bacteria are often phytopathogens and the expression of a number of their important virulence determinants and secondary metabolites is under QS control. The erwiniae utilise two types of QS signalling molecules: N-acyl homoserine lactones and AI-2-type signalling molecules. Here, we review the regulatory networks involving QS in the soft rot erwiniae.

Acyl-homoserine lactones modulate the settlement rate of zoospores of the marine alga Ulva intestinalis via a novel chemokinetic mechanism

Bacteria utilize quorum sensing to regulate the expression of cell density-dependant phenotypes such as biofilm formation and virulence. Zoospores of the marine alga Ulva intestinalis exploit the acyl-homoserine lactone (AHL) quorum sensing system to identify bacterial biofilms for preferential settlement. Here, we demonstrate that AHLs act as strong chemoattractants for Ulva zoospores. Chemoattraction does not involve a chemotactic orientation towards the AHL source. Instead, it occurs through a chemokinesis in which zoospore swimming speed is rapidly decreased in the presence of AHLs. The chemoresponse to AHLs was dependant on the nature of the acyl side chain, with N-(3-oxododecanoyl)-homoserine lactone (30-C12-HSL) being the most effective signal molecule. Mean zoospore swimming speed decreased more rapidly over wild-type biofilms of the marine bacteria Vibrio anguillarum relative to biofilms of the vanM mutant, in which AHL synthesis is disrupted. These data implicate a role for AHL-mediated chemokinesis in the location and preferential settlement of Ulva zoospores on marine bacterial assemblages. Exposure to AHLs did not inhibit the negative phototaxis of Ulva zoospores, indicating that chemoattraction to bacterial biofilms does not preclude the response to a light stimulus in substrate location.

Quorum sensing inhibitors: a bargain of effects

Many opportunistic pathogenic bacteria rely on quorum sensing (QS) circuits as central regulators of virulence expression. In Pseudomonas aeruginosa, QS-regulated gene expression contributes to the formation and maintenance of biofilms and their tolerance to conventional antimicrobials and the host innate immune system. Therefore, QS is an obvious target for a novel class of antimicrobial drugs which would function to efficiently block reception of the cognate QS signals in vivo, and thereby be capable of inducing chemical attenuation of pathogens. As QS is not directly involved in processes essential for growth of the bacteria, inhibition of QS does not impose harsh selective pressure for development of resistance as with antibiotics. Numerous chemical libraries of both natural and synthetic origin have been screened and several QS-inhibitory compounds have been identified. In animal pulmonary infection models, such inhibitors have proven able to significantly improve clearing of the infecting bacteria and reduce mortality. In addition, several enzymes that are able to inactivate the bacterial QS signal molecules have been identified. This inactivation leads to blockage of QS-mediated virulence of plant pathogens in several models.

Long- and short-chain plant-produced bacterial N-acyl-homoserine lactones become components of phyllosphere, rhizosphere, and soil

Two N-acyl-homoserine lactone (acyl-HSL) synthase genes, lasI from Pseudomonas aeruginosa and yenI from Yersinia enterocolitica, were introduced into tobacco, individually and in combination. Liquid chromatograph-tandem mass spectrometry and thin-layer chromatography confirmed products of lasI and yenI activity in single and cotransformants. Cotransformants expressing plastid-localized LasI and YenI synthases produced the major acyl-HSLs for each synthase in all tissues tested. Total acyl-HSL signals accumulated in leaf tissue up to 3 pmol/mg of fresh weight, half as much in stem tissue, and approximately 10-fold less in root tissues. Acyl-HSLs were present in aqueous leaf washes from greenhouse-grown transgenic plants. Transgenic lines grown for 14 days under axenic conditions produced detectable levels of acyl-HSLs in root exudates. Ethyl acetate extractions of rhizosphere and nonrhizosphere soil from transgenically grown plants contained active acyl-HSLs, whereas plant-free soil or rhizosphere and nonrhizosphere soil from wild-type plants lacked detectable amounts of acyl-HSLs. This work shows that bioactive acyl-HSLs are exuded from leaves and roots and accumulate in the phytosphere of plants engineered to produce acyl-HSLs. These data further suggest that plants that are bioengineered to synthesize acyl-HSLs can foster beneficial plant-bacteria communications or deter deleterious interactions. Therefore, it is feasible to use bioengineered plants to supplement soils with specific acyl-HSLs to modulate bacterial phenotypes and plant-associated bacterial community structures.

Quorum-sensing inhibitors as anti-pathogenic drugs

Quorum-sensing (QS) signalling systems of pathogens are central regulators for the expression of virulence factors and represent highly attractive targets for the development of novel therapeutics. In Pseudomonas aeruginosa, QS systems are also involved in elevated antibiotic tolerance of biofilms as well as elevated tolerance to the activity of the innate immune system. Gram-negative bacteria commonly use N-acyl homoserine lactones (AHL) as QS signal molecules. The use of signal molecule based drugs to attenuate bacterial pathogenecity rather than bacterial growth is attractive for several reasons, particularly considering the emergence of increasingly antibiotic-resistant bacteria. Compounds capable of this type of interference have been termed anti-pathogenic drugs. A large variety of synthetic AHL analogues and natural products libraries have been screened and a number of QS inhibitors (QSI) have been identified. Promising QSI compounds have been shown to make biofilms more susceptible to antimicrobial treatments, and are capable of reducing mortality and virulence as well as promoting clearance of bacteria in experimental animal models of infection.

Impact of quorum sensing on fitness of Pseudomonas aeruginosa

In Pseudomonas aeruginosa, cell-cell communication based on N-acyl-homoserine lactone (AHL) signal molecules (termed quorum sensing) is known to control the production of extracellular virulence factors. Hence, in pathogenic interactions with host organisms, the quorum-sensing (QS) machinery can confer a selective advantage on P. aeruginosa. However, as shown by transcriptomic and proteomic studies, many intracellular metabolic functions are also regulated by quorum sensing. Some of these serve to regenerate the AHL precursors methionine and S-adenosyl-methionine and to degrade adenosine via inosine and hypoxanthine. The fact that a significant percentage of clinical and environmental isolates of P. aeruginosa is defective for QS because of mutation in the major QS regulatory gene lasR, raises the question of whether the QS machinery can have a negative impact on the organism's fitness. In vitro, lasR mutants have a higher probability to escape lytic death in stationary phase under alkaline conditions than has the QS-proficient wild type. Similar selective forces might also operate in natural environments.

Quorum-sensing signal synthesis by the Yersinia pestis acyl-homoserine lactone synthase YspI

The acyl-homoserine lactone molecular species (AHLs) produced by the Yersinia pestis AHL synthase YspI were identified by biochemical and physical/chemical techniques. Bioassays of extracts from culture supernatants of the recombinant YspI and wild-type Yersinia pestis showed similar profiles of AHLs. Analysis by liquid chromatography-mass spectrometry revealed that the predominant AHLs were N-3-oxooctanoyl-L-homoserine lactone and N-3-oxo-hexanoyl-L-homoserine lactone.

Specificity of acyl-homoserine lactone synthases examined by mass spectrometry

Many gram-negative bacteria produce a specific set of N-acyl-L-homoserine-lactone (AHL) signaling molecules for the purpose of quorum sensing, which is a means of regulating coordinated gene expression in a cell-density-dependent manner. AHLs are produced from acylated acyl-carrier protein (acyl-ACP) and S-adenosyl-L-methionine by the AHL synthase enzyme. The appearance of specific AHLs is due in large part to the intrinsic specificity of the enzyme for subsets of acyl-ACP substrates. Structural studies of the Pantoea stewartii enzyme EsaI and AHL-sensitive bioassays revealed that threonine 140 in the acyl chain binding pocket directs the enzyme toward production of 3-oxo-homoserine lactones. Mass spectrometry was used to examine the range of AHL molecular species produced by AHL synthases under a variety of conditions. An AHL selective normal-phase chromatographic purification with addition of a deuterated AHL internal standard was followed by reverse-phase liquid chromatography-tandem mass spectrometry in order to obtain estimates of the relative amounts of different AHLs from biological samples. The AHLs produced by wild-type and engineered EsaI and LasI AHL synthases show that intrinsic specificity and different cellular conditions influence the production of AHLs. The threonine at position 140 in EsaI is important for the preference for 3-oxo-acyl-ACPs, but the role of the equivalent threonine in LasI is less clear. In addition, LasI expressed in Escherichia coli produces a high proportion of unusual AHLs with acyl chains consisting of an odd number of carbons. Furthermore, these studies offer additional methods that will be useful for surveying and quantitating AHLs from different sources.

N-acylhomoserine lactones antagonize virulence gene expression and quorum sensing in Staphylococcus aureus

Many gram-negative bacteria employ N-acylhomoserine lactone (AHL)-mediated quorum sensing to control virulence. To determine whether gram-positive bacteria such as Staphylococcus aureus respond to AHLs, we used a growth-dependent lux reporter fusion. Exposure of S. aureus to different AHLs revealed that 3-oxo-substituted AHLs with C10 to C14 acyl chains inhibited light output and growth in a concentration-dependent manner, while short-chain AHLs had no effect. N-(3-Oxododecanoyl)-L-homoserine lactone (3-oxo-C12-HSL) inhibited the production of exotoxins and cell wall fibronectin-binding proteins but enhanced protein A expression. Since these processes are reciprocally regulated via the S. aureus agr quorum-sensing system, which in turn, is regulated via sar, we examined the effect of AHLs on sarA and agr. At sub-growth-inhibitory concentrations of 3-oxo-C12-HSL, both sarA expression and agr expression were inhibited, indicating that the action of 3-oxo-C12-HSL is mediated at least in part through antagonism of quorum sensing in S. aureus. Spent culture supernatants from Pseudomonas aeruginosa, which produces both 3-oxo-C12-HSL and N-butanoyl-homoserine lactone (C4-HSL), also inhibited agr expression, although C4-HSL itself was inactive in this assay. Since quorum sensing in S. aureus depends on the activities of membrane-associated proteins, such as AgrB, AgrC, and AgrD, we investigated whether AHLs perturbed S. aureus membrane functionality by determining their influence on the membrane dipole potential. From the binding curves obtained, a dissociation constant of 7 muM was obtained for 3-oxo-C12-HSL, indicating the presence of a specific saturable receptor, whereas no binding was observed for C4-HSL. These data demonstrate that long-chain 3-oxo-substituted AHLs, such as 3-oxo-C12-HSL, are capable of interacting with the S. aureus cytoplasmic membrane in a saturable, specific manner and at sub-growth-inhibitory concentrations, down-regulating exotoxin production and both sarA and agr expression.

Quorum sensing in Yersinia enterocolitica controls swimming and swarming motility

The Yersinia enterocolitica LuxI homologue YenI directs the synthesis of N-3-(oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL). In a Y. enterocolitica yenI mutant, swimming motility is temporally delayed while swarming motility is abolished. Since both swimming and swarming are flagellum dependent, we purified the flagellin protein from the parent and yenI mutant. Electrophoresis revealed that in contrast to the parent strain, the yenI mutant grown for 17 h at 26 degrees C lacked the 45-kDa flagellin protein FleB. Reverse transcription-PCR indicated that while mutation of yenI had no effect on yenR, flhDC (the motility master regulator) or fliA (the flagellar sigma factor) expression, fleB (the flagellin structural gene) was down-regulated. Since 3-oxo-C6-HSL and C6-HSL did not restore swimming or swarming in the yenI mutant, we reexamined the N-acylhomoserine lactone (AHL) profile of Y. enterocolitica. Using AHL biosensors and mass spectrometry, we identified three additional AHLs synthesized via YenI: N-(3-oxodecanoyl)homoserine lactone, N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL), and N-(3-oxotetradecanoyl)homoserine lactone. However, none of the long-chain AHLs either alone or in combination with the short-chain AHLs restored swarming or swimming in the yenI mutant. By investigating the transport of radiolabeled 3-oxo-C12-HSL and by introducing an AHL biosensor into the yenI mutant we demonstrate that the inability of exogenous AHLs to restore motility to the yenI mutant is not related to a lack of AHL uptake. However, both AHL synthesis and motility were restored by complementation of the yenI mutant with a plasmid-borne copy of yenI.

sinI- and expR-dependent quorum sensing in Sinorhizobium meliloti

Quorum sensing (QS) in Sinorhizobium meliloti, the N-fixing bacterial symbiont of Medicago host plants, involves at least half a dozen different N-acyl homoserine lactone (AHL) signals and perhaps an equal number of AHL receptors. The accumulation of 55 proteins was found to be dependent on SinI, the AHL synthase, and/or on ExpR, one of the AHL receptors. Gas chromatography-mass spectrometry and electrospray ionization tandem mass spectrometry identified 3-oxo-C(14)-homoserine lactone (3-oxo-C(14)-HSL), C(16)-HSL, 3-oxo-C(16)-HSL, C(16:1)-HSL, and 3-oxo-C(16:1)-HSL as the sinI-dependent AHL QS signals accumulated by the 8530 expR(+) strain under the conditions used for proteome analysis. The 8530 expR(+) strain secretes additional, unidentified QS-active compounds. Addition of 200 nM C(14)-HSL or C(16:1)-HSL, two of the known SinI AHLs, affected the levels of 75% of the proteins, confirming that their accumulation is QS regulated. A number of the QS-regulated proteins have functions plausibly related to symbiotic interactions with the host, including ExpE6, IdhA, MocB, Gor, PckA, LeuC, and AglE. Seven of 10 single-crossover beta-glucuronidase (GUS) transcriptional reporters in genes corresponding to QS-regulated proteins showed significantly different activities in the sinI and expR mutant backgrounds and in response to added SinI AHLs. The sinI mutant and several of the single-crossover strains were significantly delayed in the ability to initiate nodules on the primary root of the host plant, Medicago truncatula, indicating that sinI-dependent QS regulation and QS-regulated proteins contribute importantly to the rate or efficiency of nodule initiation. The sinI and expR mutants were also defective in surface swarming motility. The sinI mutant was restored to normal swarming by 5 nM C(16:1)-HSL.

The type III secretion system and cytotoxic enterotoxin alter the virulence of Aeromonas hydrophila

Many gram-negative bacteria use a type III secretion system (TTSS) to deliver effector proteins into host cells. Here we report the characterization of a TTSS chromosomal operon from the diarrheal isolate SSU of Aeromonas hydrophila. We deleted the gene encoding Aeromonas outer membrane protein B (AopB), which is predicted to be involved in the formation of the TTSS translocon, from wild-type (WT) A. hydrophila as well as from a previously characterized cytotoxic enterotoxin gene (act)-minus strain of A. hydrophila, thus generating aopB and act/aopB isogenic mutants. The act gene encodes a type II-secreted cytotoxic enterotoxin (Act) that has hemolytic, cytotoxic, and enterotoxic activities and induces lethality in a mouse model. These isogenic mutants (aopB, act, and act/aopB) were highly attenuated in their ability to induce cytotoxicity in RAW 264.7 murine macrophages and HT-29 human colonic epithelial cells. The act/aopB mutant demonstrated the greatest reduction in cytotoxicity to cultured cells after 4 h of infection, as measured by the release of lactate dehydrogenase enzyme, and was avirulent in mice, with a 90% survival rate compared to that of animals infected with Act and AopB mutants, which caused 50 to 60% of the animals to die at a dose of three 50% lethal doses. In contrast, WT A. hydrophila killed 100% of the mice within 48 h. The effects of these mutations on cytotoxicity could be complemented with the native genes. Our studies further revealed that the production of lactones, which are involved in quorum sensing (QS), was decreased in the act (32%) and aopB (64%) mutants and was minimal (only 8%) in the act/aopB mutant, compared to that of WT A. hydrophila SSU. The effects of act and aopB gene deletions on lactone production could also be complemented with the native genes, indicating specific effects of Act and the TTSS on lactone production. Although recent studies with other bacteria have indicated TTSS regulation by QS, this is the first report describing a correlation between the TTSS and Act of A. hydrophila and the production of lactones.

Diversity of N-acyl homoserine lactone-producing and -degrading bacteria in soil and tobacco rhizosphere

In Gram-negative bacteria, quorum-sensing (QS) communication is mostly mediated by N-acyl homoserine lactones (N-AHSL). The diversity of bacterial populations that produce or inactivate the N-AHSL signal in soil and tobacco rhizosphere was investigated by restriction fragment length polymorphism (RFLP) analysis of amplified 16S DNA and DNA sequencing. Such analysis indicated the occurrence of N-AHSL-producing strains among the alpha-, beta- and gamma-proteobacteria, including genera known to produce N-AHSL (Rhizobium, Sinorhizobium and Pseudomonas) and novel genera with no previously identified N-AHSL-producing isolates (Variovorax, Sphingomonas and Massilia). The diversity of N-AHSL signals was also investigated in relation to the genetic diversity of the isolates. However, N-AHSL-degrading strains isolated from soil samples belonged to the Bacillus genus, while strains isolated from tobacco rhizospheres belonged to both the Bacillus genus and to the alpha subgroup of proteobacteria, suggesting that diversity of N-AHSL-degrading strains may be modulated by the presence of the tobacco plant. Among these rhizospheric isolates, novel N-AHSL-degrading genera have been identified (Sphingomonas and Bosea). As the first simultaneous analysis of both N-AHSL-degrading and -producing bacterial communities in a complex environment, this study revealed the coexistence of bacterial isolates, belonging to the same genus or species that may produce or degrade N-AHSL.

Elevated temperature enhances virulence of Erwinia carotovora subsp. carotovora strain EC153 to plants and stimulates production of the quorum sensing signal, N-acyl homoserine lactone, and extracellular proteins

Erwinia carotovora subsp. atroseptica, E. carotovora subsp. betavasculorum, and E. carotovora subsp. carotovora produce high levels of extracellular enzymes, such as pectate lyase (Pel), polygalacturonase (Peh), cellulase (Cel), and protease (Prt), and the quorum-sensing signal N-acyl-homoserine lactone (AHL) at 28 degrees C. However, the production of these enzymes and AHL by these bacteria is severely inhibited during growth at elevated temperatures (31.2 degrees C for E. carotovora subsp. atroseptica and 34.5 degrees C for E. carotovora subsp. betavasculorum and most E. carotovora subsp. carotovora strains). At elevated temperatures these bacteria produce high levels of RsmA, an RNA binding protein that promotes RNA decay. E. carotovora subsp. carotovora strain EC153 is an exception in that it produces higher levels of Pel, Peh, Cel, and Prt at 34.5 degrees C than at 28 degrees C. EC153 also causes extensive maceration of celery petioles and Chinese cabbage leaves at 34.5 degrees C, which correlates with a higher growth rate and higher levels of rRNA and AHL. The lack of pectinase production by E. carotovora subsp. carotovora strain Ecc71 at 34.5 degrees C limits the growth of this organism in plant tissues and consequently impairs its ability to cause tissue maceration. Comparative studies with ahlI (the gene encoding a putative AHL synthase), pel-1, and peh-1 transcripts documented that at 34.5 degrees C the RNAs are more stable in EC153 than in Ecc71. Our data reveal that overall metabolic activity, AHL levels, and mRNA stability are responsible for the higher levels of extracellular protein production and the enhanced virulence of EC153 at 34.5 degrees C compared to 28 degrees C.

Osmotic stress and phosphate limitation alter production of cell-to-cell signal molecules and rhamnolipid biosurfactant by Pseudomonas aeruginosa

In Pseudomonas aeruginosa, rhamnolipid production is controlled by the quorum-sensing system RhlRI, which itself depends on LasRI. These systems use cell-to-cell signal molecules: N-butyryl-l-homoserine lactone (C4-HSL) and N-(3-oxododecanoyl)-l-homoserine lactone (3OC(12)-HSL), respectively. Whereas both HSLs were produced in M63 medium, rhamnolipid synthesis was not achieved. Phosphate limitation reduced the HSL concentrations, while allowing rhamnolipid production. Hyperosmotic shock applied during the exponential growth phase stopped the accumulation of 3OC(12)-HSL, and prevented C4-HSL and rhamnolipid production. These defects result from lower expression of genes involved in C4-HSL and rhamnolipid syntheses. The osmoprotectant glycine betaine partially restored C4-HSL and rhamnolipid production.

Human and murine paraoxonase 1 are host modulators of Pseudomonas aeruginosa quorum-sensing

The pathogenic bacterium Pseudomonas aeruginosa uses acyl-HSL quorum-sensing signals to regulate genes controlling virulence and biofilm formation. We found that paraoxonase 1 (PON1), a mammalian lactonase with an unknown natural substrate, hydrolyzed the P. aeruginosa acyl-HSL 3OC12-HSL. In in vitro assays, mouse serum-PON1 was required and sufficient to degrade 3OC12-HSL. Furthermore, PON2 and PON3 also degraded 3OC12-HSL effectively. Serum-PON1 prevented P. aeruginosa quorum-sensing and biofilm formation in vitro by inactivating the quorum-sensing signal. Although 3OC12-HSL production by P. aeruginosa was important for virulence in a mouse sepsis model, Pon1-knock-out mice were paradoxically protected. These mice showed increased levels of PON2 and PON3 mRNA in epithelial tissues suggesting a possible compensatory mechanism. Thus, paraoxonase interruption of bacterial communication represents a novel mechanism to modulate quorum-sensing by bacteria. The consequences for host immunity are yet to be determined.

Quorum-sensing-negative (lasR) mutants of Pseudomonas aeruginosa avoid cell lysis and death

In Pseudomonas aeruginosa, N-acylhomoserine lactone signals regulate the expression of several hundreds of genes, via the transcriptional regulator LasR and, in part, also via the subordinate regulator RhlR. This regulatory network termed quorum sensing contributes to the virulence of P. aeruginosa as a pathogen. The fact that two supposed PAO1 wild-type strains from strain collections were found to be defective for LasR function because of independent point mutations in the lasR gene led to the hypothesis that loss of quorum sensing might confer a selective advantage on P. aeruginosa under certain environmental conditions. A convenient plate assay for LasR function was devised, based on the observation that lasR mutants did not grow on adenosine as the sole carbon source because a key degradative enzyme, nucleoside hydrolase (Nuh), is positively controlled by LasR. The wild-type PAO1 and lasR mutants showed similar growth rates when incubated in nutrient yeast broth at pH 6.8 and 37 degrees C with good aeration. However, after termination of growth during 30 to 54 h of incubation, when the pH rose to > or = 9, the lasR mutants were significantly more resistant to cell lysis and death than was the wild type. As a consequence, the lasR mutant-to-wild-type ratio increased about 10-fold in mixed cultures incubated for 54 h. In a PAO1 culture, five consecutive cycles of 48 h of incubation sufficed to enrich for about 10% of spontaneous mutants with a Nuh(-) phenotype, and five of these mutants, which were functionally complemented by lasR(+), had mutations in lasR. The observation that, in buffered nutrient yeast broth, the wild type and lasR mutants exhibited similar low tendencies to undergo cell lysis and death suggests that alkaline stress may be a critical factor providing a selective survival advantage to lasR mutants.

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.

Quorum sensing: the power of cooperation in the world of Pseudomonas

Work over the past few years has provided evidence that quorum sensing is a generic regulatory mechanism that allows bacteria to launch a unified, coordinated response in a population density-dependent manner to accomplish tasks which would be difficult, if not impossible, to achieve for a single bacterial cell. Quorum sensing systems are widespread among pseudomonads and the one of the human opportunistic pathogen Pseudomonas aeruginosa belongs to the most extensively studied cell-to-cell communication systems. In this organism, quorum sensing is highly complex and is made up of two interlinked N-acyl homoserine lactone (AHL)-dependent regulatory circuits, which are further modulated by a non-AHL-related signal molecule and numerous regulators acting both at the transcriptional and post-transcriptional level. This genetic complexity may be one of the key elements responsible for the tremendous environmental versatility of P. aeruginosa. Work of the past few years showed that quorum sensing is essential for the expression of a battery of virulence factors as well as for biofilm formation in P. aeruginosa and thus represents an attractive target for the design of novel drugs for the treatment of P. aeruginosa infections. Furthermore, the cell-to-cell communication ability was also demonstrated in a number of additional pseudomonads.

Identification of acyl-homoserine lactone signal molecules produced by Nitrosomonas europaea strain Schmidt

Nitrosomonas europaea strain Schmidt produces at least three acyl homoserine lactone (AHL) signal molecules: C(6)-homoserine lactone (HSL), C(8)-HSL, and C(10)-HSL. These compounds were identified in extracts of chemostat culture effluent by three independent methods. The concentrations of AHL in effluent were low (0.4 to 2.2 nM) but within the range known to induce AHL-responsive systems. The absence of LuxI and LuxM homologs from the genome of N. europaea strain Schmidt suggested that AHL synthesis occurs by an alternate pathway, possibly mediated by an HdtS homolog. To the best of our knowledge, the present report is the first to document the types and levels of AHLs produced by N. europaea.

Disruption of quorum sensing in seawater abolishes attraction of zoospores of the green alga Ulva to bacterial biofilms

Zoospores of the eukaryotic green seaweed Ulva respond to bacterial N-acylhomoserine lactone (AHL) quorum sensing signal molecules for the selection of surface sites for permanent attachment. In this study we have investigated the production and destruction of AHLs in biofilms of the AHL-producing marine bacterium, Vibrio anguillarum and their stability in seawater. While wild type V. anguillarum NB10 was a strong attractor of zoospores, inactivation of AHL production in this strain by either expressing the recombinant Bacillus lactonase coding gene aiiA, or by mutating the AHL biosynthetic genes, resulted in the abolition of zoospore attraction. In seawater, with a pH of 8.2, the degradation of AHL molecules was temperature-dependent, indicating that the AHLs produced by marine bacterial biofilms have short half-lives. The Ulva zoospores sensed a range of different AHL molecules and in particular more zoospores settled on surfaces releasing AHLs with longer (>six carbons) N-linked acyl chains. However, this finding is likely to be influenced by the differential diffusion rates of AHLs from the experimental surface matrix. Molecules with longer N-acyl chains, such as N-(3-oxodecanoyl)- L-homoserine lactone, diffused more slowly than those with shorter N-acyl chains such as N-(3-hydroxy-hexanoyl)- L-homoserine lactone. Image analysis using GFP-tagged V. anguillarum biofilms revealed that spores settle directly on bacterial cells and in particular on microcolonies which we show are sites of concentrated AHL production.