Construction and analysis of luxCDABE-based plasmid sensors for investigating N-acyl homoserine lactone-mediated quorum sensing

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.

The ability of Proteus mirabilis to sense surfaces and regulate virulence gene expression involves FliL, a flagellar basal body protein

Proteus mirabilis is a urinary tract pathogen that differentiates from a short swimmer cell to an elongated, highly flagellated swarmer cell. Swarmer cell differentiation parallels an increased expression of several virulence factors, suggesting that both processes are controlled by the same signal. The molecular nature of this signal is not known but is hypothesized to involve the inhibition of flagellar rotation. In this study, data are presented supporting the idea that conditions inhibiting flagellar rotation induce swarmer cell differentiation and implicating a rotating flagellar filament as critical to the sensing mechanism. Mutations in three genes, fliL, fliF, and fliG, encoding components of the flagellar basal body, result in the inappropriate development of swarmer cells in noninducing liquid media or hyperelongated swarmer cells on agar media. The fliL mutation was studied in detail. FliL- mutants are nonmotile and fail to synthesize flagellin, while complementation of fliL restores wild-type cell elongation but not motility. Overexpression of fliL+ in wild-type cells prevents swarmer cell differentiation and motility, a result also observed when P. mirabilis fliL+ was expressed in Escherichia coli. These results suggest that FliL plays a role in swarmer cell differentiation and implicate FliL as critical to transduction of the signal inducing swarmer cell differentiation and virulence gene expression. In concert with this idea, defects in fliL up-regulate the expression of two virulence genes, zapA and hpmB. These results support the hypothesis that P. mirabilis ascertains its location in the environment or host by assessing the status of its flagellar motors, which in turn control swarmer cell gene expression.

Use of a whole-cell biosensor and flow cytometry to detect AHL production by an indigenous soil community during decomposition of litter

Quorum sensing, mediated by acylated homoserine lactones (AHLs), is well described for pure culture bacteria, but few studies report detection of AHL compounds in natural bacterial habitats. In this study, we detect AHL production during a degradation process in soil by use of whole-cell biosensor technology and flow cytometry analysis. An indigenous soil bacterium, belonging to the family of Enterobacteriaceae, was isolated and transformed with a low-copy plasmid harboring a gene encoding an unstable variant of the green fluorescent protein (gfpASV) fused to the AHL-regulated P(luxI) promoter originating from Vibrio fischeri. This resulted in a whole-cell biosensor, responding to the presence of AHL compounds. The biosensor was introduced to compost soil microcosms amended with nettle leaves. After 3 days of incubation, cells were extracted and analyzed by flow cytometry. All microcosms contained induced biosensors. From these microcosms, AHL producers were isolated and further identified as species previously shown to produce AHLs. The results demonstrate that AHL compounds are produced during degradation of litter in soil, indicating the presence of AHL-mediated quorum sensing in this environment.

Monitoring age-related susceptibility of young mice to oral Salmonella enterica serovar Typhimurium infection using an in vivo murine model

Neonates and young children are acutely susceptible to infections by gastrointestinal bacterial pathogens, such as Salmonella enterica serovar Typhimurium (S. typhimurium). To reveal age-related differences in susceptibility to this pathogen, we used in vivo bioluminescence imaging (BLI) to monitor the progression of infection in neonatal (1-wk-old), suckling (2-wk-old), juvenile (4-wk-old), and adult (6-wk-old) BALB/c mice. Mice were orally infected with various doses of a bioluminescent-labeled wild-type or mutant S. typhimurium strain, and progression of infection was monitored by BLI for 2 wks. We found that neonatal and suckling mice were more susceptible to the wild-type strain at inoculum sizes 4 and 2 log(10)'s lower for neonatal and suckling mice, respectively, than those for adult mice. At the lower inocula, newborn mice showed disseminated systemic infection as indicated by the pattern of photon emission assessed by BLI, whereas no bioluminescent signals were detectable in adult mice. In addition, an orgA(-) mutant strain of S. typhimurium with reduced virulence in adult mice produced systemic infection in newborn, suckling, and juvenile mice. Furthermore, as low as 3 log(10) CFU could be detected by BLI in tissue. The present study demonstrates that susceptibility to S. typhimurium infection decreases with age. Also, we established that BLI can be used to monitor the progression of infection in mice. Thus, this model of age-related susceptibility to S. typhimurium using BLI can be used to advance our understanding of the mechanisms involved in newborn susceptibility to infection.

The RssAB two-component signal transduction system in Serratia marcescens regulates swarming motility and cell envelope architecture in response to exogenous saturated fatty acids

Serratia marcescens swarms at 30 degrees C but not at 37 degrees C on a nutrient-rich (LB) agar surface. Mini-Tn5 mutagenesis of S. marcescens CH-1 yielded a mutant (WC100) that swarms not only vigorously at 37 degrees C but also earlier and faster than the parent strain swarms at 30 degrees C. Analysis of this mutant revealed that the transposon was inserted into a gene (rssA) predicted to encode a bacterial two-component signal transduction sensor kinase, upstream of which a potential response regulator gene (rssB) was located. rssA and rssB insertion-deletion mutants were constructed through homologous recombination, and the two mutants exhibited similar swarming phenotypes on LB swarming agar, in which swarming not only occurred at 37 degrees C but also initiated at a lower cell density, on a surface with a higher agar concentration, and more rapidly than the swarming of the parent strain at 30 degrees C. Both mutants also exhibited increased hemolysin activity and altered cell surface topologies compared with the parent CH-1 strain. Temperature and certain saturated fatty acids (SFAs) were found to negatively regulate S. marcescens swarming via the action of RssA-RssB. Analysis of the fatty acid profiles of the parent and the rssA and rssB mutants grown at 30 degrees C or 37 degrees C and under different nutrition conditions revealed a relationship between cellular fatty acid composition and swarming phenotypes. The cellular fatty acid profile was also observed to be affected by RssA and RssB. SFA-dependent inhibition of swarming was also observed in Proteus mirabilis, suggesting that either SFAs per se or the modulation of cellular fatty acid composition and hence homeostasis of membrane fluidity may be a conserved mechanism for regulating swarming motility in gram-negative bacteria.

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Carbapenem antibiotic biosynthesis in Erwinia carotovora is regulated by physiological and genetic factors modulating the quorum sensing-dependent control pathway

Erwinia carotovora produces the beta-lactam antibiotic, carbapenem, in response to a quorum sensing signalling molecule, N-(3-oxohexanoyl)-L-homoserine lactone (OHHL). We have mapped the OHHL-dependent promoter upstream of the first of the biosynthetic genes, carA. We have also analysed the effect on this promoter of the known genetic regulators of carbapenem expression, carR, carI (encoding homologues of LuxR and LuxI respectively) and hor (encoding a SlyA/MarR-like transcriptional regulator). We describe a previously unknown promoter located within the carA-H operon. This promoter does not respond to CarR and is required for quorum sensing-independent expression of the carbapenem resistance determinants encoded by the carFG genes. We have mapped the carR, carI and hor transcription start points, shown that CarR is positively autoregulated in the presence of OHHL, and have demonstrated negative feedback affecting transcription of carI. In addition, various environmental and physiological factors were shown to impinge on the transcription of the car biosynthetic genes. The nature of the carbon source and the temperature of growth influence carbapenem production by modulating the level of the OHHL signalling molecule, and thereby physiologically fine-tune the quorum sensing regulatory system.

The MexGHI-OpmD multidrug efflux pump controls growth, antibiotic susceptibility and virulence in Pseudomonas aeruginosa via 4-quinolone-dependent cell-to-cell communication

In Pseudomonas aeruginosa the production of multiple virulence factors depends on cell-to-cell communication through the integration of N-acylhomoserine lactone (AHL)- and 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS)- dependent signalling. Mutation of genes encoding the efflux protein MexI and the porin OpmD from the MexGHI-OpmD pump resulted in the inability to produce N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-c12-hsl) and pqs and a marked reduction in n-butanoyl-L-homoserine lactone levels. Both pump mutants were impaired in growth and exhibited enhanced rather than reduced antibiotic resistance. Provision of exogenous PQS improved growth and restored AHL and virulence factor production as well as antibiotic susceptibility, indicating that the pump mutants retained their capacity to respond to PQS. RT-PCR analysis indicated that expression of the PQS biosynthetic genes, phnA and pqsA, was inhibited when the mutants reached stationary phase, suggesting that the pleiotropic phenotype observed may be due to intracellular accumulation of a toxic PQS precursor. To explore this hypothesis, double mexI phnA (unable to produce anthranilate, the precursor of PQS) and mexI pqsA mutants were constructed; the improved growth of the former suggested that the toxic compound is likely to be anthranilate or a metabolite of it. Mutations in mexI and opmD also resulted in the attenuation of virulence in rat and plant infection models. In plants, addition of PQS restored the virulence of mexI and opmD mutants. Collectively, these results demonstrate an essential function for the MexGHI-OpmD pump in facilitating cell-to-cell communication, antibiotic susceptibility and promoting virulence and growth in P. aeruginosa.

Development of a chinchilla model to allow direct, continuous, biophotonic imaging of bioluminescent nontypeable Haemophilus influenzae during experimental otitis media

We transformed a nontypeable Haemophilus influenzae clinical isolate with a plasmid containing the luxCDABE operon driven by the H. influenzae outer membrane protein P2 promoter. Herein, we demonstrate the ability to detect bioluminescence and to monitor infection within the nasopharynges, eustachian tubes, and middle ears of chinchillas after intranasal and transbullar challenges.

The plant pathogenErwinia amylovoraproduces acyl-homoserine lactone signal molecules in vitro and in planta

We report for the first time the production of acyl homoserine lactones (AHLs) by Erwina amylovora, an important quarantine bacterial pathogen that causes fire blight in plants. E. amylovora produces one N-acyl homoserine lactone [a N-(3-oxo-hexanoyl)-homoserine lactone or a N-(3-hydroxy-hexanoyl)-homoserine lactone] quorum sensing signal molecule both in vitro and in planta (pear plant). Given the involvement of AHLs in plant pathogenesis, we speculate that AHL-dependent quorum sensing could play an important role in the regulation of E. amylovora virulence.

Regulation of the N-acyl homoserine lactone-dependent quorum-sensing system in rhizosphere Pseudomonas putida WCS358 and cross-talk with the stationary-phase RpoS sigma factor and the global regulator GacA

Quorum sensing is a cell population-density dependent regulatory system which in gram-negative bacteria often involves the production and detection of N-acyl homoserine lactones (AHLs). Some Pseudomonas putida strains have been reported to produce AHLs, and one quorum-sensing locus has been identified. However, it appears that the majority of strains do not produce AHLs. In this study we report the identification and regulation of the AHL-dependent system of rhizosphere P. putida WCS358. This system is identical to the recently identified system of P. putida strain IsoF and very similar to the las system of Pseudomonas aeruginosa. It is composed of three genes, the luxI family member ppuI, the putative repressor rsaL, and the luxR family member ppuR. A genomic ppuR::Tn5 mutant of strain WCS358 was identified by its inability to produce AHLs when it was cross-streaked in close proximity to an AHL biosensor, whereas an rsaL::Tn5 genomic mutant was identified by its ability to overproduce AHL molecules. Using transcriptional promoter fusions, we studied expression profiles of the rsaL, ppuI, and ppuR promoters in various genetic backgrounds. At the onset of the stationary phase, the autoinducer synthase ppuI gene expression is under positive regulation by PpuR-AHL and under negative regulation by RsaL, indicating that the molecules could be in competition for binding at the ppuI promoter. In genomic rsaL::Tn5 mutants ppuI expression and production of AHL levels increased dramatically; however, both processes were still under growth phase regulation, indicating that RsaL is not involved in repressing AHL production at low cell densities. The roles of the global response regulator GacA and the stationary-phase sigma factor RpoS in the regulation of the AHL system at the onset of the stationary phase were also investigated. The P. putida WCS358 gacA gene was cloned and inactivated in the genome. It was determined that the three global regulatory systems are closely linked, with quorum sensing and RpoS regulating each other and GacA positively regulating ppuI expression. Studies of the regulation of AHL quorum-sensing systems have lagged behind other studies and are important for understanding how these systems are integrated into the overall growth phase and metabolic status of the cells.

N-Sulfonyl homoserine lactones as antagonists of bacterial quorum sensing

A series of 11 new analogues of N-acylhomoserine lactones in which the carboxamide bond was replaced by a sulfonamide one, has been synthesised. These compounds were evaluated for their ability to competitively inhibit the action of 3-oxohexanoyl-L-homoserine lactone, the natural ligand of the quorum sensing transcriptional regulator LuxR, which in turn activates expression of bioluminescence in the model bacterium Vibrio fischeri. Several compounds were found to display antagonist activity. Molecular modeling suggests that the latter prevent a cascade of structural rearrangements necessary for the formation of the active LuxR dimer.

Regulation of Vibrio anguillarum empA metalloprotease expression and its role in virulence

Atlantic salmon (Salmo salar) were challenged with Vibrio anguillarum strains M93Sm and NB10 and empA null mutants M99 and NB12. Both wild types were virulent when administered by intraperitoneal (i.p.) injection or anal intubation. NB12 was avirulent via either route of infection. M99 virulence was attenuated when delivered by intubation, but fully virulent by i.p. injection. Northern blot analysis revealed empA expression in M93Sm and NB10 cells incubated in mucus, while incubation in Luria-Bertani broth plus 2% NaCl (LB20) induced empA expression only in NB10. Nucleotide differences between M93Sm and NB10 empA sequences were found in regions located 207 and 229 bp upstream of the empA translational start. Reverse transcription-PCR and 5' rapid amplification of cDNA ends revealed the empA transcriptional start site 85 bp upstream of the translational start for both strains. A putative sigma(S)-dependent promoter was identified upstream of the transcriptional start in both strains. Site-directed mutagenesis was used to create rpoS mutants of M93Sm and NB10. Neither rpoS mutant exhibited protease activity. Since empA is expressed during stationary phase, the effects of conditioned medium on protease activity were examined. M99 conditioned LB20 supernatants stimulated protease activity in NB10 while allowing M93Sm to produce protease in LB20. Neither acyl homoserine lactones nor AI-2 induced protease activity. Conditioned LB20 supernatant from a V. anguillarum luxS mutant caused a more rapid induction of protease activity in wild-type cells. Our data show that expression of empA is differentially regulated in V. anguillarum strains NB10 and M93Sm and requires sigma(S), quorum-sensing molecules, and gastrointestinal mucus.

The global regulator genes from biocontrol strain Serratia plymuthica IC1270: cloning, sequencing, and functional studies

The biocontrol activity of various fluorescent pseudomonads towards plant-pathogenic fungi is dependent upon the GacA/GacS-type two-component system of global regulators and the RpoS transcription sigma factor. In particular, these components are required for the production of antifungal antibiotics and exoenzymes. To investigate the effects of these global regulators on the expression of biocontrol factors by plant-associated bacteria other than Pseudomonas spp., gacA/gacS and rpoS homologues were cloned from biocontrol strain IC1270 of Serratia plymuthica, which produces a set of antifungal compounds, including chitinolytic enzymes and the antibiotic pyrrolnitrin. The nucleotide and deduced protein sequence alignments of the cloned gacA/gacS-like genes-tentatively designated grrA (global response regulation activator) and grrS (global response regulation sensor) and of the cloned rpoS gene revealed 64 to 93% identity with matching genes and proteins of the enteric bacteria Escherichia coli, Pectobacterium carotovora subsp. carotovora, and Serratia marcescens. grrA, grrS, and rpoS gene replacement mutants of strain IC1270 were deficient in the production of pyrrolnitrin, an exoprotease, and N-acylhomoserine lactone quorum-sensing signal molecules. However, neither mutant appeared to differ from the parental strain in the production of siderophores, and only grrA and grrS mutants were deficient in the production of a 58-kDa endochitinase, representing the involvement of other sigma factors in the regulation of strain IC1270's chitinolytic activity. Compared to the parental strain, the grrA, grrS, and rpoS mutants were markedly less capable of suppressing Rhizoctonia solani and Pythium aphanidermatum under greenhouse conditions, indicating the dependence of strain IC1270's biocontrol property on the GrrA/GrrS and RpoS global regulators.

Use of Sinorhizobium meliloti as an indicator for specific detection of long-chain N-acyl homoserine lactones

Population-density-dependent gene expression in gram-negative bacteria involves the production of signal molecules characterized as N-acyl homoserine lactones (AHLs). The synthesis of AHLs by numerous microorganisms has been identified by using biosensor strains based on the Agrobacterium tumefaciens and Chromobacterium violaceum quorum-sensing systems. The symbiotic nitrogen-fixing bacterium Sinorhizobium meliloti is rapidly becoming a model organism for the study of quorum sensing. This organism harbors at least three different quorum-sensing systems (Sin, Mel, and Tra), which play a role in its symbiotic relationship with its host plant, alfalfa. The Sin system is distinguished among them for the production of long-chain AHLs, including C(18)-HL, the longest AHL reported so far. In this work, we show that construction of a sinI::lacZ transcriptional fusion results in a strain that detects long-chain AHLs with exquisite sensitivity. Overexpression of the SinR regulator protein from a vector promoter increases its sensitivity without loss of specificity. We also show that the resulting indicator strain can recognize long-chain AHLs produced by unrelated bacteria such as Paracoccus denitrificans and Rhodobacter capsulatus. This S. meliloti indicator strain should serve as a tool for the specific detection of long-chain AHLs in new systems.

Potato plants genetically modified to produce N-acylhomoserine lactones increase susceptibility to soft rot erwiniae

Many gram-negative bacteria employ N-acylhomoserine lactones (AHL) to regulate diverse physiological processes in concert with cell population density (quorum sensing [QS]). In the plant pathogen Erwinia carotovora, the AHL synthesized via the carI/expI genes are responsible for regulating the production of secreted plant cell wall-degrading exoenzymes and the antibiotic carbapen-3-em carboxylic acid. We have previously shown that targeting the product of an AHL synthase gene (yenI) from Yersinia enterocolitica to the chloroplasts of transgenic tobacco plants caused the synthesis in planta of the cognate AHL signaling molecules N-(3-oxohexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL), which in turn, were able to complement a carI-QS mutant. In the present study, we demonstrate that transgenic potato plants containing the yenI gene are also able to express AHL and that the presence and level of these AHL in the plant increases susceptibility to infection by E. carotovora. Susceptibility is further affected by both the bacterial level and the plant tissue under investigation.

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.

Fast screening method for detection of acyl-HSL-degrading soil isolates

A reliable method was developed for screening of bacteria isolates capable of degrading acyl-HSLs, the signal molecules in quorum-sensing-mediated processes of many Proteobacteria. The microtiter assay was based on the use of a GFP-marked Escherichia coli strain, which fluoresces upon the presence of acyl-HSLs. Measurement of GFP fluorescence with a Molecular Imager FX scanner (fluorometer) detected isolates capable of degrading acyl-HSLs. The potential of this method was demonstrated by isolation of different bacteria from a potato rhizosphere able to inactivate synthetic and natural acyl HSLs produced by Pectobacterium carotovorum subsp. carotovorum (Pcc) (Erwinia carotovora subsp. carotovora (Ecc)).

Programmable cells: interfacing natural and engineered gene networks

Novel cellular behaviors and characteristics can be obtained by coupling engineered gene networks to the cell's natural regulatory circuitry through appropriately designed input and output interfaces. Here, we demonstrate how an engineered genetic circuit can be used to construct cells that respond to biological signals in a predetermined and programmable fashion. We employ a modular design strategy to create Escherichia coli strains where a genetic toggle switch is interfaced with: (i) the SOS signaling pathway responding to DNA damage, and (ii) a transgenic quorum sensing signaling pathway from Vibrio fischeri. The genetic toggle switch endows these strains with binary response dynamics and an epigenetic inheritance that supports a persistent phenotypic alteration in response to transient signals. These features are exploited to engineer cells that form biofilms in response to DNA-damaging agents and cells that activate protein synthesis when the cell population reaches a critical density. Our work represents a step toward the development of "plug-and-play" genetic circuitry that can be used to create cells with programmable behaviors.

Influence of environmental conditions on the production of phenazine-1-carboxamide by Pseudomonas chlororaphis PCL1391

Pseudomonas chlororaphis PCL1391 produces the secondary metabolite phenazine-1-carboxamide (PCN), which is an antifungal metabolite required for biocontrol activity of the strain. Identification of conditions involved in PCN production showed that some carbon sources and all amino acids tested promote PCN levels. Decreasing the pH from 7 to 6 or decreasing the growth temperature from 21 to 16 degrees C decreased PCN production dramatically. In contrast, growth at 1% oxygen as well as low magnesium concentrations increased PCN levels. Salt stress, low concentrations of ferric iron, phosphate, sulfate, and ammonium ions reduced PCN levels. Fusaric acid, a secondary metabolite produced by the soilborne Fusarium spp. fungi, also reduced PCN levels. Different nitrogen sources greatly influenced PCN levels. Analysis of autoinducer levels at conditions of high and low PCN production demonstrated that, under all tested conditions, PCN levels correlate with autoinducer levels, indicating that the regulation of PCN production by environmental factors takes place at or before autoinducer production. Moreover, the results show that autoinducer production not only is induced by a high optical density but also can be induced by certain environmental conditions. We discuss our findings in relation to the success of biocontrol in the field.

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

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

Chlamydomonas reinhardtii secretes compounds that mimic bacterial signals and interfere with quorum sensing regulation in bacteria

The unicellular soil-freshwater alga Chlamydomonas reinhardtii was found to secrete substances that mimic the activity of the N-acyl-L-homoserine lactone (AHL) signal molecules used by many bacteria for quorum sensing regulation of gene expression. More than a dozen chemically separable but unidentified substances capable of specifically stimulating the LasR or CepR but not the LuxR, AhyR, or CviR AHL bacterial quorum sensing reporter strains were detected in ethyl acetate extracts of C. reinhardtii culture filtrates. Colonies of C. reinhardtii and Chlorella spp. stimulated quorum sensing-dependent luminescence in Vibrio harveyi, indicating that these algae may produce compounds that affect the AI-2 furanosyl borate diester-mediated quorum sensing system of Vibrio spp. Treatment of the soil bacterium Sinorhizobium meliloti with a partially purified LasR mimic from C. reinhardtii affected the accumulation of 16 of the 25 proteins that were altered in response to the bacterium's own AHL signals, providing evidence that the algal mimic affected quorum sensing-regulated functions in this wild-type bacterium. Peptide mass fingerprinting identified 32 proteins affected by the bacterium's AHLs or the purified algal mimic, including GroEL chaperonins, the nitrogen regulatory protein PII, and a GTP-binding protein. The algal mimic was able to cancel the stimulatory effects of bacterial AHLs on the accumulation of seven of these proteins, providing evidence that the secretion of AHL mimics by the alga could be effective in disruption of quorum sensing in naturally encountered bacteria.

Detection of N-(3-oxohexanoyl)-L-homoserine lactone in mice infected with Yersinia enterocolitica serotype O8

Yersinia enterocolitica synthesizes N-acyl-L-homoserine lactone (AHL) signal molecules via the LuxR-LuxI homologues YenR-YenI. In this study we checked two prototypes of mouse-virulent Y. enterocolitica serotype O8 strains WA-314 and 8081 for AHL production in vitro and in vivo (mouse infection model). We used thin-layer chromatography in combination with the Escherichia coli AHL biosensor to identify the AHL species produced. We detected only OHHL [N-(3-oxohexanoyl)-L-homoserine lactone] and not HHL (N-hexanoyl-L-homoserine lactone) produced by Y. enterocolitica O8 in culture supernatant or infected mouse tissue. This is the first report demonstrating AHL production by yersiniae during infection.

Production of substances by Medicago truncatula that affect bacterial quorum sensing

Earlier work showed that higher plants produce unidentified compounds that specifically stimulate or inhibit quorum sensing (QS) regulated responses in bacteria. The ability of plants to produce substances that affect QS regulation may provide plants with important tools to manipulate gene expression and behavior in the bacteria they encounter. In order to examine the kinds of QS active substances produced by the model legume M. truncatula, young seedlings and seedling exudates were systematically extracted with various organic solvents, and the extracts were fractionated by reverse phase C18 high-performance liquid chromatography. M. truncatula appears to produce at least 15 to 20 separable substances capable of specifically stimulating or inhibiting responses in QS reporter bacteria, primarily substances that affect QS regulation dependent on N-acyl homoserine lactone (AHL) signals. The secretion of AHL QS mimic activities by germinating seeds and seedlings was found to change substantially with developmental age. The secretion of some mimic activities may be dependent upon prior exposure of the plants to bacteria.

Proteomic analysis of wild-type Sinorhizobium meliloti responses to N-acyl homoserine lactone quorum-sensing signals and the transition to stationary phase

Proteome analysis revealed that two long-chain N-acyl homoserine lactones (AHLs) produced by Sinorhizobium meliloti 1021 induced significant differences in the accumulation of more than 100 polypeptides in early-log-phase cultures of the wild type. Fifty-six of the corresponding proteins have been identified by peptide mass fingerprinting. The proteins affected by addition of these two AHLs had diverse functions in carbon and nitrogen metabolism, energy cycles, metabolite transport, DNA synthesis, and protein turnover. Two hours of exposure to 3-oxo-C(16:1)-homoserine lactone (3-oxo-C(16:1)-HL) affected the accumulation of 40 of the 56 identified proteins, whereas comparable exposure to C(14)-HL affected 13 of the 56 proteins. Levels of four proteins were affected by both AHLs. Exposure to 3-oxo-C(16:1)-HL for 8 h affected the accumulation of 17 proteins, 12 of which had reduced accumulation. Of the 80 proteins identified as differing in accumulation between early-log- and early-stationary-phase cultures, only 13 were affected by exposure to 3-oxo-C(16:1)-HL or C(14)-HL. These results provide a foundation for future studies of the functions regulated by AHL quorum sensing in S. meliloti and help to establish proteomic analysis as a powerful global approach to the identification of quorum-sensing regulatory patterns in wild-type bacteria.

Phenazines and their role in biocontrol by Pseudomonas bacteria

Various rhizosphere bacteria are potential (micro)biological pesticides which are able to protect plants against diseases and improve plant yield. Knowledge of the molecular mechanisms that govern these beneficial plant-microbe interactions enables optimization, enhancement and identification of potential synergistic effects in plant protection. The production of antifungal metabolites, induction of systemic resistance, and the ability to compete efficiently with other resident rhizobacteria are considered to be important prerequisites for the optimal performance of biocontrol agents. Intriguing aspects in the molecular mechanisms of these processes have been discovered recently. Phenazines and phloroglucinols are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. This review focuses on the current state of knowledge on biocontrol by phenazine-producing Pseudomonas strains and the action, biosynthesis, and regulation mechanisms of the production of microbial phenazines.

Extensive and specific responses of a eukaryote to bacterial quorum-sensing signals

Many bacteria use N-acyl homoserine lactone (AHL) signals to coordinate the behavior of individual cells in a local population. The successful infection of eukaryotic hosts by bacteria seems to depend particularly on such AHL-mediated "quorum-sensing" regulation. We have used proteome analysis to show that a eukaryotic host, the model legume Medicago truncatula, is able to detect nanomolar to micromolar concentrations of bacterial AHLs from both symbiotic (Sinorhizobium meliloti) and pathogenic (Pseudomonas aeruginosa) bacteria, and that it responds in a global manner by significant changes in the accumulation of over 150 proteins, 99 of which have been identified by peptide mass fingerprinting. The accumulation of specific proteins and isoforms depended on AHL structure, concentration, and time of exposure. AHLs were also found to induce tissue-specific activation of beta-glucuronidase (GUS) reporter fusions to an auxin-responsive and three chalcone synthase promoters, consistent with AHL-induced changes in the accumulation of auxin-responsive and flavonoid synthesis proteins. In addition, exposure to AHLs was found to induce changes in the secretion of compounds by the plants that mimic quorum-sensing signals and thus have the potential to disrupt quorum sensing in associated bacteria. Our results indicate that eukaryotes have an extensive range of functional responses to AHLs that may play important roles in the beneficial or pathogenic outcomes of eukaryote-prokaryote interactions.

Detection of other microbial species by Salmonella: expression of the SdiA regulon

Salmonella, Escherichia, and Klebsiella do not encode any recognized type of N-acylhomoserine lactone (AHL) synthase, and consistent with this, they do not synthesize AHLs under any conditions tested. However, they do encode an AHL receptor of the LuxR family, named SdiA. MudJ fusions in four loci are known to respond to plasmid-encoded sdiA in Salmonella, but only the rck locus has been described. Here we report the location and sequence analysis of the remaining three loci. The srg-6::MudJ is within gtgA of the gifsy-2 prophage, and the srg-7::MudJ is within PSLT61 of the virulence plasmid. Both fusions are in the antisense orientation. The third fusion, srgE5::MudJ, is within a horizontally acquired gene of unknown function at 33.6 centisomes that we have named srgE. Previously, sdiA expressed from its natural position in the chromosome was demonstrated to activate a plasmid-based transcriptional fusion to the rck promoter in response to AHL production by other bacterial species. However, the MudJ fusions did not respond to chromosomal sdiA. Here we report that MudJ fusions to three of the four loci (not srg-6) are activated by AHL in an sdiA-dependent manner during growth in motility agar (0.25% agar) but not during growth in top agar (0.7% agar) or on agar plates (1.2% agar). In motility agar, the srgE promoter responds to sdiA at 30 degrees C and higher while the rck and srg-7 promoters respond only at 37 or 42 degrees C. Substantial AHL-independent SdiA activity was observed at 30 degrees C but not at 37 degrees C.

Quorum-sensing-directed protein expression in Serratia proteamaculans B5a

N-Acyl-L-homoserine-lactone-producing Serratia species are frequently encountered in spoiling foods of vegetable and protein origin. The role of quorum sensing in the food spoiling properties of these bacteria is currently being investigated. A set of luxR luxI homologous genes encoding a putative quorum sensor was identified in the N-(3-oxo-hexanoyl)-L-homoserine lactone (3-oxo-C6-HSL)-producing Serratia proteamaculans strain B5a. The 3-oxo-C6-HSL synthase SprI showed 79 % similarity with EsaI from Pantoea stewartii and the putative regulatory protein SprR was 86 % similar to the SpnR of Serratia marcescens. Proteome analysis suggested that the presence of at least 39 intracellular proteins was affected by the 3-oxo-C6-HSL-based quorum sensing system. The lipB-encoded secretion system was identified as one target gene of the quorum sensing system. LipB was required for the production of extracellular lipolytic and proteolytic activities, thus rendering the production of food-deterioration-relevant exoenzymes indirectly under the control of quorum sensing. Strain B5a caused quorum-sensing-controlled spoilage of milk. Furthermore, chitinolytic activity was controlled by quorum sensing. This control appeared to be direct and not mediated via LipB. The data presented here demonstrate that quorum-sensing-controlled exoenzymic activities affect food quality.

Transcriptional modulation of bacterial gene expression by subinhibitory concentrations of antibiotics

Antibiotics such as erythromycin and rifampicin, at low concentrations, alter global bacterial transcription patterns as measured by the stimulation or inhibition of a variety of promoter-lux reporter constructs in a Salmonella typhimurium library. Analysis of a 6,500-clone library indicated that as many as 5% of the promoters may be affected, comprising genes for a variety of functions, as well as a significant fraction of genes with no known function. Studies of a selection of the reporter clones showed that stimulation varied depending on the nature of the antibiotic, the promoter, and what culture medium was used; the response differed on solid as compared with liquid media. Transcription was markedly reduced in antibiotic-resistant hosts, but the presence of mutations deficient in stress responses such as SOS or universal stress did not prevent antibiotic-induced modulation. The results show that small molecules may have contrasting effects on bacteria depending on their concentration: either the modulation of bacterial metabolism by altering transcription patterns or the inhibition of growth by the inhibition of specific target functions. Both activities could play important roles in the regulation of microbial communities. These studies indicate that the detection of pharmaceutically useful natural product inhibitors could be effectively achieved by measuring activation of transcription at low concentrations in high-throughput assays using appropriate bacterial promoter-reporter constructs.

Identification and characterization of an N-acylhomoserine lactone-dependent quorum-sensing system in Pseudomonas putida strain IsoF

Recent reports have shown that several strains of Pseudomonas putida produce N-acylhomoserine lactones (AHLs). These signal molecules enable bacteria to coordinately express certain phenotypic traits in a density-dependent manner in a process referred to as quorum sensing. In this study we have cloned a genomic region of the plant growth-promoting P. putida strain IsoF that, when present in trans, provoked induction of a bioluminescent AHL reporter plasmid. Sequence analysis identified a gene cluster consisting of four genes: ppuI and ppuR, whose predicted amino acid sequences are highly similar to proteins of the LuxI-LuxR family, an open reading frame (ORF) located in the intergenic region between ppuI and ppuR with significant homology to rsaL from Pseudomonas aeruginosa, and a gene, designated ppuA, present upstream of ppuR, the deduced amino acid sequence of which shows similarity to long-chain fatty acid coenzyme A ligases from various organisms. Using a transcriptional ppuA::luxAB fusion we demonstrate that expression of ppuA is AHL dependent. Furthermore, transcription of the AHL synthase ppuI is shown to be subject to quorum-sensing regulation, creating a positive feedback loop. Sequencing of the DNA regions flanking the ppu gene cluster indicated that the four genes form an island in the suhB-PA3819 intergenic region of the currently sequenced P. putida strain KT2440. Moreover, we provide evidence that the ppu genes are not present in other AHL-producing P. putida strains, indicating that this gene cluster is so far unique for strain IsoF. While the wild-type strain formed very homogenous biofilms, both a ppuI and a ppuA mutant formed structured biofilms with characteristic microcolonies and water-filled channels. These results suggest that the quorum-sensing system influences biofilm structural development.

Genetic analysis of functions involved in the late stages of biofilm development in Burkholderia cepacia H111

Burkholderia cepacia and Pseudomonas aeruginosa often co-exist as mixed biofilms in the lungs of patients suffering from cystic fibrosis (CF). Here, we report the isolation of 13 random mini-Tn5 insertion mutants of B. cepacia H111 that are defective in biofilm formation on a polystyrene surface. We show that the screening procedure used in this study is biased towards mutants defective in the late stages of biofilm development. A detailed quantitative analysis of the biofilm structures formed by wild-type and mutant strains revealed that the isolated mutants are impaired in their abilities to develop a typical three-dimensional biofilm structure. Molecular investigations showed that the genes required for biofilm maturation fall into several classes: (i). genes encoding for surface proteins; (ii). genes involved in the biogenesis and maintenance of an integral outer membrane; and (iii). genes encoding regulatory factors. It is shown that three of the regulatory mutants produce greatly reduced amounts of N-octanoylhomoserine lactone (C8-HSL). This compound serves as the major signal molecule of the cep quorum-sensing system. As this density-dependent regulatory system is involved in the regulation of biofilm maturation, we investigated the interplay between the three regulatory genes and the quorum-sensing cascade. The results of these investigations show that the identified genes encode for regulatory elements that are positioned upstream of the cep system, indicating that the quorum-sensing system of B. cepacia is a major checkpoint for biofilm formation.

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.

A LuxR homolog controls production of symbiotically active extracellular polysaccharide II by Sinorhizobium meliloti

Production of complex extracellular polysaccharides (EPSs) by the nitrogen-fixing soil bacterium Sinorhizobium meliloti is required for efficient invasion of root nodules on the host plant alfalfa. Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate infection thread initiation and extension (root nodule invasion) on alfalfa. Of these three polysaccharides, the only symbiotically active polysaccharide produced by S. meliloti wild-type strain Rm1021 is succinoglycan. The expR101 mutation is required to turn on production of symbiotically active forms of EPS II in strain Rm1021. In this study, we have determined the nature of the expR101 mutation in S. meliloti. The expR101 mutation, a spontaneous dominant mutation, results from precise, reading frame-restoring excision of an insertion sequence from the coding region of expR, a gene whose predicted protein product is highly homologous to the Rhizobium leguminosarum bv. viciae RhiR protein and a number of other homologs of Vibrio fischeri LuxR that function as receptors for N-acylhomoserine lactones (AHLs) in quorum-sensing regulation of gene expression. S. meliloti ExpR activates transcription of genes involved in EPS II production in a density-dependent fashion, and it does so at much lower cell densities than many quorum-sensing systems. High-pressure liquid chromatographic fractionation of S. meliloti culture filtrate extracts revealed at least three peaks with AHL activity, one of which activated ExpR-dependent expression of the expE operon.

The LuxR family protein SpnR functions as a negative regulator of N-acylhomoserine lactone-dependent quorum sensing in Serratia marcescens

Serratia marcescens SS-1 produces at least four N-acylhomoserine lactones (AHLs) which were identified using high-resolution mass spectrometry and chemical synthesis, as N-(3-oxohexanoyl) homo-serine lactone (3-oxo-C6-HSL), N-hexanoyl- (C6-HSL), N-heptanoyl (C7-HSL) and N-octanoyl- (C8-HSL) homoserine lactone. These AHLs are synthesized via the LuxI homologue SpnI, and regulate via the LuxR homologue SpnR, the production of the red pigment, prodigiosin, the nuclease, NucA, and a biosurfactant which facilitates surface translocation. spnR overexpression and spnR gene deletion show that SpnR, in contrast to most LuxR homologues, acts as a negative regulator. spnI overexpression, the provision of exogenous AHLs and spnI gene deletion suggest that SpnR is de-repressed by 3-oxo-C6-HSL. In addition, long chain AHLs antagonize the biosurfactant-mediated surface translocation of S. marcescens SS-1. Upstream of spnI there is a gene which we have termed spnT. spnI and spnT form an operon and although database searches failed to reveal any spnT homologues, overexpression of this novel gene negatively affected both sliding motility and prodigiosin production.

Plant-dependent genotypic and phenotypic diversity of antagonistic rhizobacteria isolated from different Verticillium host plants

To study the effect of plant species on the abundance and diversity of bacterial antagonists, the abundance, the phenotypic diversity, and the genotypic diversity of rhizobacteria isolated from potato, oilseed rape, and strawberry and from bulk soil which showed antagonistic activity towards the soilborne pathogen Verticillium dahliae Kleb. were analyzed. Rhizosphere and soil samples were taken five times over two growing seasons in 1998 and 1999 from a randomized field trial. Bacterial isolates were obtained after plating on R2A (Difco, Detroit, Mich.) or enrichment in microtiter plates containing high-molecular-weight substrates followed by plating on R2A. A total of 5,854 bacteria isolated from the rhizosphere of strawberry, potato, or oilseed rape or bulk soil from fallow were screened by dual testing for in vitro antagonism towards VERTICILLIUM: The proportion of isolates with antagonistic activity was highest for strawberry rhizosphere (9.5%), followed by oilseed rape (6.3%), potato (3.7%), and soil (3.3%). The 331 Verticillium antagonists were identified by their fatty acid methyl ester profiles. They were characterized by testing their in vitro antagonism against other pathogenic fungi; their glucanolytic, chitinolytic, and proteolytic activities; and their BOX-PCR fingerprints. The abundance and composition of Verticillium antagonists was plant species dependent. A rather high proportion of antagonists from the strawberry rhizosphere was identified as Pseudomonas putida B (69%), while antagonists belonging to the Enterobacteriaceae (Serratia spp., Pantoea agglomerans) were mainly isolated from the rhizosphere of oilseed rape. For P. putida A and B plant-specific genotypes were observed, suggesting that these bacteria were specifically enriched in each rhizosphere.

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.

Advancing the quorum in Pseudomonas aeruginosa: MvaT and the regulation of N-acylhomoserine lactone production and virulence gene expression

Pseudomonas aeruginosa regulates the production of many exoproteins and secondary metabolites via a hierarchical quorum-sensing cascade through LasR and RhlR and their cognate signal molecules N-(3-oxododecanoyl)-L-homoserine lactone (3O-C12-HSL) and N-(butanoyl)-L-homoserine lactone (C4-HSL). In this study, we found that transcription of the quorum sensing-regulated genes lecA (coding for PA-IL lectin), lasB (coding for elastase), and rpoS appeared to be growth phase dependent and their expression could not be advanced to the logarithmic phase in cells growing in batch culture by the addition of exogenous C4-HSL and 3O-C12-HSL. To identify novel regulators responsible for this growth phase dependency, a P. aeruginosa lecA::lux reporter strain was subjected to random transposon mutagenesis. A number of mutants affected in lecA expression were found that exhibited altered production of multiple quorum sensing-dependent phenotypes. While some mutations were mapped to new loci such as clpA and mvaT and a putative efflux system, a number of mutations were also mapped to known regulators such as lasR, rhlR, and rpoS. MvaT was identified as a novel global regulator of virulence gene expression, as a mutation in mvaT resulted in enhanced lecA expression and pyocyanin production. This mutant also showed altered swarming ability and production of the LasB and LasA proteases, 3O-C12-HSL, and C4-HSL. Furthermore, addition of exogenous 3O-C12-HSL and C4-HSL to the mvaT mutant significantly advanced lecA expression, suggesting that MvaT is involved in the growth phase-dependent regulation of the lecA gene.

New synthetic analogues of N-acyl homoserine lactones as agonists or antagonists of transcriptional regulators involved in bacterial quorum sensing

A series of 22 novel synthetic N-acyl-homoserine lactone analogues has been evaluated for both their inducing activity and their ability to competitively inhibit the action of 3-oxo-hexanoyl-L-homoserine lactone, the natural inducer of bioluminescence in the bacterium Vibrio fischeri. In the newly synthesized analogues, the extremity of the acyl chain was modified by introducing ramified alkyl, cycloalkyl or aryl substituents at the C-4 position. Most of the analogues bearing either acyclic or cyclic alkyl substituents showed inducing activity. In contrast, the phenyl substituted analogues displayed significant antagonist activity. We hypothesized that the antagonist activity of the phenyl compounds may result from the interaction between the aryl group and aromatic amino acids of the LuxR receptor, preventing it from adopting the active dimeric form.

VanT, a homologue of Vibrio harveyi LuxR, regulates serine, metalloprotease, pigment, and biofilm production in Vibrio anguillarum

Vibrio anguillarum possesses at least two N-acylhomoserine lactone (AHL) quorum-sensing circuits, one of which is related to the luxMN system of Vibrio harveyi. In this study, we have cloned an additional gene of this circuit, vanT, encoding a V. harveyi LuxR-like transcriptional regulator. A V. anguillarum Delta vanT null mutation resulted in a significant decrease in total protease activity due to loss of expression of the metalloprotease EmpA, but no changes in either AHL production or virulence. Additional genes positively regulated by VanT were identified from a plasmid-based gene library fused to a promoterless lacZ. Three lacZ fusions (serA::lacZ, hpdA-hgdA::lacZ, and sat-vps73::lacZ) were identified which exhibited decreased expression in the Delta vanT strain. SerA is similar to 3-phosphoglycerate dehydrogenases and catalyzes the first step in the serine-glycine biosynthesis pathway. HgdA has identity with homogentisate dioxygenases, and HpdA is homologous to 4-hydroxyphenylpyruvate dioxygenases (HPPDs) involved in pigment production. V. anguillarum strains require an active VanT to produce high levels of an L-tyrosine-induced brown color via HPPD, suggesting that VanT controls pigment production. Vps73 and Sat are related to Vibrio cholerae proteins encoded within a DNA locus required for biofilm formation. A V. anguillarum Delta vanT mutant and a mutant carrying a polar mutation in the sat-vps73 DNA locus were shown to produce defective biofilms. Hence, a new member of the V. harveyi LuxR transcriptional activator family has been characterized in V. anguillarum that positively regulates serine, metalloprotease, pigment, and biofilm production.

LuxS-dependent quorum sensing in Porphyromonas gingivalis modulates protease and haemagglutinin activities but is not essential for virulence

Porphyromonas gingivalis is a Gram-negative black-pigmented obligate anaerobe implicated in the aetiology of human periodontal disease. The virulence of P. gingivalis is associated with the elaboration of the cysteine proteases Arg-gingipain (Rgp) and Lys-gingipain (Kgp), which are produced at high bacterial cell densities. To determine whether quorum sensing plays a role in the regulation of Rgp and Kgp, biosensors capable of detecting either N-acylhomoserine lactone (AHLs) or the luxS-dependent autoinducer (AI-2) quorum-sensing signalling molecules in spent culture supernatants were first employed. While no AHLs could be detected, the Vibrio harveyi BB170 biosensor was activated by spent P. gingivalis W50 culture supernatants. The P. gingivalis luxS gene was cloned and demonstrated to restore AI-2 production in the Escherichia coli luxS mutant DH5alpha. Mutation of luxS abolished AI-2 production in P. gingivalis. Western blotting using antibodies raised against the recombinant protein revealed that LuxS levels increased throughout growth even though AI-2 activity was only maximally detected at the mid-exponential phase of growth and disappeared by the onset of stationary phase. Similar results were obtained with E. coli DH5alpha transformed with luxS, suggesting that AI-2 production is not limited by a lack of LuxS protein. Analysis of Rgp and Kgp protease activities revealed that the P. gingivalis luxS mutant produced around 45% less Rgp and 30% less Kgp activity than the parent strain. In addition, the luxS mutant exhibited a fourfold reduction in haemagglutinin titre. However, these reductions in virulence determinant levels were insufficient to attenuate the luxS mutant in a murine lesion model of P. gingivalis infection.

H-NS-dependent activation of pectate lyases synthesis in the phytopathogenic bacterium Erwinia chrysanthemi is mediated by the PecT repressor

Production of the main virulence determinant pectate lyases (Pels) of the phytopathogenic bacterium Erwinia chrysanthemi is modulated by a complex regulatory network involving the repressor proteins KdgR, PecS and PecT and the activator systems Pir, ExpI-ExpR and CRP. Of these regulators, CRP and PecT are particularly important since the absence of CRP or a slight overproduction of PecT leads to a drastic reduction in synthesis of Pel species. Recently, it has been shown that production of Pel species is strongly reduced in an E. chrysanthemi hns mutant, suggesting an activator function of the nucleoid-associated protein H-NS in the expression of the pel genes. Here, we report that the reduced synthesis of Pel species in the hns mutant results from a negative control, exerted by H-NS, on the transcription of the regulatory gene pecT. This H-NS/PecT cascade regulation is one of the first elucidations of a positive effect of H-NS on target gene expression. Moreover, we found that H-NS also represses the expression of expI, expR and pel genes. H-NS control is the result of H-NS binding to extended regions within the pecT, expI, expR and pel genes. Investigation of the simultaneous binding of CRP, RNA polymerase (RNAP) and H-NS on the pelD gene revealed that these three proteins form a nucleoprotein com-plex. Together, these data indicate that, by exerting a negative control at multiple levels, H-NS plays a crucial role in the E. chrysanthemi pel regulatory network.

Nonenzymatic turnover of an Erwinia carotovora quorum-sensing signaling molecule

The production of virulence factors and carbapenem antibiotic in the phytopathogen Erwinia carotovora is under the control of quorum sensing. The quorum-sensing signaling molecule, N-(3-oxohexanoyl)-L-homoserine lactone (OHHL), accumulates in log-phase culture supernatants of E. carotovora but diminishes in concentration during the stationary phase. In this study, we show that the diminution in OHHL was not due to sequestration of the ligand by the cells, although some partitioning did occur. Rather, it was caused by degradation of the molecule. The rate of stationary-phase degradation of OHHL was as rapid as the rate of log-phase accumulation of the ligand, but it was nonenzymatic and led to a decrease in the expression of selected genes known to be under the control of quorum sensing. The degradation of OHHL was dependent on the pH of the supernatant, which increased as the growth curve progressed in cultures grown in Luria-Bertani medium from pH 7 to approximately 8.5. OHHL became unstable over a narrow pH range (pH 7 to 8). Instability was increased at high temperatures even at neutral pH but could be prevented at the growth temperature (30 degrees C) by buffering the samples at pH 6.8. These results may provide a rationale for the observation that an early response of plants which are under attack by Erwinia is to activate a proton pump which alkalizes the site of infection to a pH of >8.2.

Direct detection of N-acylhomoserine lactones in cystic fibrosis sputum

Pseudomonas aeruginosa and Burkholderia cepacia cause destructive lung disease in cystic fibrosis (CF) patients. Both pathogens employ 'quorum sensing', i.e. cell-to-cell communication, via diffusible N-acyl-L-homoserine lactone (AHL) signal molecules, to regulate the production of a number of virulence determinants in vitro. However, to date, evidence that quorum sensing systems are functional and play a role in vivo is lacking. This study presents the first direct evidence for the presence of AHLs in CF sputum. A total of 42 samples from 25 CF patients were analysed using lux-based Escherichia coli AHL biosensors. AHLs were detected in sputum from patients colonised by P. aeruginosa or B. cepacia but not Staphylococcus aureus. Furthermore, using liquid chromatography-mass spectrometry and thin layer chromatography, we confirmed the presence of N-hexanoylhomoserine lactone and N-(3-oxododecanoyl)homoserine lactone respectively in sputum samples from patients colonised by P. aeruginosa.

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.

Visualization of N-acylhomoserine lactone-mediated cell-cell communication between bacteria colonizing the tomato rhizosphere

Given that a large proportion of the bacteria colonizing the roots of plants is capable of producing N-acyl-L-homoserine lactone (AHL) molecules, it appears likely that these bacterial pheromones may serve as signals for communication between cells of different species. In this study, we have developed and characterized novel Gfp-based monitor strains that allow in situ visualization of AHL-mediated communication between individual cells in the plant rhizosphere. For this purpose, three Gfp-based AHL sensor plasmids that respond to different spectra of AHL molecules were transferred into AHL-negative derivatives of Pseudomonas putida IsoF and Serratia liquefaciens MG1, two strains that are capable of colonizing tomato roots. These AHL monitor strains were used to visualize communication between defined bacterial populations in the rhizosphere of axenically grown tomato plants. Furthermore, we integrated into the chromosome of AHL-negative P. putida strain F117 an AHL sensor cassette that responds to the presence of long-chain AHLs with the expression of Gfp. This monitor strain was used to demonstrate that the indigenous bacterial community colonizing the roots of tomato plants growing in nonsterile soil produces AHL molecules. The results strongly support the view that AHL signal molecules serve as a universal language for communication between the different bacterial populations of the rhizosphere consortium.

N-acylhomoserine-lactone-mediated communication between Pseudomonas aeruginosa and Burkholderia cepacia in mixed biofilms

Pseudomonas aeruginosa and Burkholderia cepacia are capable of forming mixed biofilms in the lungs of cystic fibrosis patients. Both bacteria employ quorum-sensing systems, which rely on N-acylhomoserine lactone (AHL) signal molecules, to co-ordinate expression of virulence factors with the formation of biofilms. As both bacteria utilize the same class of signal molecules the authors investigated whether communication between the species occurs. To address this issue, novel Gfp-based biosensors for non-destructive, in situ detection of AHLs were constructed and characterized. These sensors were used to visualize AHL-mediated communication in mixed biofilms, which were cultivated either in artificial flow chambers or in alginate beads in mouse lung tissue. In both model systems B. cepacia was capable of perceiving the AHL signals produced by P. aeruginosa, while the latter strain did not respond to the molecules produced by B. cepacia. Measurements of extracellular proteolytic activities of defined quorum-sensing mutants grown in media complemented with AHL extracts prepared from culture supernatants of various wild-type and mutant strains supported the view of unidirectional signalling between the two strains.

Halogenated furanones from the red alga, Delisea pulchra, inhibit carbapenem antibiotic synthesis and exoenzyme virulence factor production in the phytopathogen Erwinia carotovora

The plant pathogen Erwinia carotovora regulates expression of virulence factors and antibiotic production via an N-3-oxohexanoyl-L-homoserine lactone (3-oxo-C6-HSL) dependent quorum sensing mechanism. The marine alga Delisea pulchra produces halogenated furanones known to antagonise 3-oxo-C6-HSL activity. We have tested the effects of a halogenated furanone on the production of carbapenem, cellulase and protease in E. carotovora. Despite differences in the regulatory mechanisms controlling carbapenem and exoenzyme production each was inhibited by the algal metabolite. We present evidence to suggest that the furanone dependent inhibition of carbapenem production is a result of the disruption of the 3-oxo-C6-HSL dependent expression of the carABCDEFGH operon.

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.