Quorum sensing in plant-pathogenic bacteria

Involvement of a quorum-sensing-regulated lipase secreted by a clinical isolate of Burkholderia glumae in severe disease symptoms in rice

Burkholderia glumae is an emerging rice pathogen in several areas around the world. Closely related Burkholderia species are important opportunistic human pathogens for specific groups of patients, such as patients with cystic fibrosis and patients with chronic granulomatous disease. Here we report that the first clinical isolate of B. glumae, strain AU6208, has retained its capability to be very pathogenic to rice. As previously reported for rice isolate B. glumae BGR1 (and also for the clinical isolate AU6208), TofI or TofR acyl homoserine lactone (AHL) quorum sensing played a pivotal role in rice virulence. We report that AHL quorum sensing in B. glumae AU6208 regulates secreted LipA lipase and toxoflavin, the phytotoxin produced by B. glumae. B. glumae AU6208 lipA mutants were no longer pathogenic to rice, indicating that the lipase is an important virulence factor. It was also established that type strain B. glumae ATCC 33617 did not produce toxoflavin and lipase and was nonpathogenic to rice. It was determined that in strain ATCC 33617 the LuxR family quorum-sensing sensor/regulator TofR was inactive. Introducing the tofR gene of B. glumae AU6208 in strain ATCC 33617 restored its ability to produce toxoflavin and the LipA lipase. This study extends the role of AHL quorum sensing in rice pathogenicity through the regulation of a lipase which was demonstrated to be a virulence factor. It is the first report of a clinical B. glumae isolate retaining strong rice pathogenicity and finally determined that B. glumae can undergo phenotypic conversion through a spontaneous mutation in the tofR regulator.

Pseudomonas corrugata contains a conserved N-acyl homoserine lactone quorum sensing system; its role in tomato pathogenicity and tobacco hypersensitivity response

Pseudomonas corrugata is a phytopathogenic bacterium, causal agent of tomato pith necrosis, yet it is an ubiquitous bacterium that is part of the microbial community in the soil and in the rhizosphere of different plant species. Although it is a very heterogeneous species, all the strains tested were able to produce short chain acyl homoserine lactone (AHL) quorum sensing signal molecules. The main AHL produced was N-hexanoyl-L-homoserine lactone (C(6)-AHL). An AHL quorum sensing system, designated PcoI/PcoR, was identified and characterized. The role of the quorum sensing system in the expression of a variety of traits was evaluated. Inactivation of pcoI abolished the production of AHLs. The pcoR mutant, but not the pcoI mutant, was impaired in swarming, unable to cause a hypersensitivity response on tobacco and resulted in a reduced tomato pith necrosis phenotype. The pcoI mutant showed a reduced antimicrobial activity against various fungi and bacteria when assayed on King's B medium. These results demonstrate that the AHL quorum sensing in Ps. corrugata regulates traits that contribute to virulence, antimicrobial activity and fitness. This is the first report of genes of Ps. corrugata involved in the disease development and biological control activity.

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.

Quorum-sensing regulation in rhizobia and its role in symbiotic interactions with legumes

Legume-nodulating bacteria (rhizobia) usually produce N-acyl homoserine lactones, which regulate the induction of gene expression in a quorum-sensing (or population-density)-dependent manner. There is significant diversity in the types of quorum-sensing regulatory systems that are present in different rhizobia and no two independent isolates worked on in detail have the same complement of quorum-sensing genes. The genes regulated by quorum sensing appear to be rather diverse and many are associated with adaptive aspects of physiology that are probably important in the rhizosphere. It is evident that some aspects of rhizobial physiology related to the interaction between rhizobia and legumes are influenced by quorum sensing. However, it also appears that the legumes play an active role, both in terms of interfering with the rhizobial quorum-sensing systems and responding to the signalling molecules made by the bacteria. In this article, we review the diversity of quorum-sensing regulation in rhizobia and the potential role of legumes in influencing and responding to this signalling system.

The Pseudomonas putida Lon protease is involved in N-acyl homoserine lactone quorum sensing regulation

BACKGROUND

In Pseudomonas putida and Pseduomonas aeruginosa, the similar PpuR/RsaL/PpuI and LasR/RsaL/LasI acyl homoserine lactones (AHLs) quorum sensing (QS) systems have been shown to be under considerable regulation by other global regulators. A major regulator is the RsaL protein which strongly directly represses the transcription of the P. putida ppuI and P. aeruginosa lasI AHL synthases. In this study we screened a transposon mutant bank of P. putida in order to identify if any other regulators were involved in negative regulation of AHL QS.

RESULTS

In our screen we identified three Tn5 mutants which displayed overproduction of AHLs in P. putida strain WCS358. Two of the mutants had a Tn5 located in the rsaL gene, whereas in one mutant the transposon was located in the lon protease gene. Lon proteases play important roles in protein quality control via degradation of misfolded proteins. It was determined that in the P. putida lon mutant, AHL levels, PpuR levels and ppuI promoter activity all increased significantly; we therefore postulated that PpuR is a target for Lon. The Lon protease had no effect on AHL production in P. aeruginosa.

CONCLUSION

The Lon protease is a negative regulator of AHL production in P. putida WCS358. The Lon protease has also been shown by others to influence AHL QS in Vibrio fischeri and Agrobacterium tumefaciens and can thus become an important regulator of AHL QS timing and regulation in bacteria.

Interactions between plants and beneficial Pseudomonas spp.: exploiting bacterial traits for crop protection

Specific strains of fluorescent Pseudomonas spp. inhabit the environment surrounding plant roots and some even the root interior. Introducing such bacterial strains to plant roots can lead to increased plant growth, usually due to suppression of plant pathogenic microorganisms. We review the modes of action and traits of these beneficial Pseudomonas bacteria involved in disease suppression. The complex regulation of biological control traits in relation to the functioning in the root environment is discussed. Understanding the complexity of the interactions is instrumental in the exploitation of beneficial Pseudomonas spp. in controlling plant diseases.

Characterization of a putative Xylella fastidiosa diffusible signal factor by HRGC-EI-MS

Xylella fastidiosa (X.f.) is a plant pathogen with high levels of genomic similarity to Xanthomonas campestris pv. campestris (X.c.c.). It has been shown that X. fastidiosa synthesizes a putative diffusible signal factor (X.f.-DSF) that activates regulation of pathogenicity factor (rpf) genes in a X.c.c. reporter system, which might be involved in the regulation of pathogenesis associated genes as in X.c.c., as well as in quorum-sensing. The nature of the X.f.-DSF is not known, whereas the X.c.c.-DSF has been identified as cis-11-methyl-2-dodecenoic acid. In this work, the chemical nature of a putative X.f.-DSF molecule, able to restore endoglucanase activity in a X.c.c. rpfF mutant, was investigated as if it was a fatty acid derivative. Bioassays with X.c.c. reporter bacterium and X.f. culture extracts, based on endoglucanase restoration activity, were also carried out in order to confirm the DSFs molecules similarities. For this reason, a gas chromatography-mass spectrometry method was developed with standard fatty acids methyl esters mixtures. The retention time, as well as the fragmentation patterns, of each standard was used to identify the DSF molecule synthesized by X.f. in the culture medium. Typical ester fragmentation patterns (the derivatized analyte) were observed, such as: McLafferty rearrangement and migration of the Hdelta followed by 1,4-hydrogen shift and cleavage of the bond Cbeta-Cgamma, confirming the nature of this molecule. This confirmation was corroborated by the common peaks in both spectra. Besides, the observed retention time reinforces our conclusion since it corresponds to a methyl ester with 15 carbons. Since the X.f.-DSF molecule was tentatively identified as 12-methyl-tetradecanoic acid (by mass spectra library comparison), this standard compound was also analyzed, strongly suggesting that this is the identification of such a molecule. To our knowledge, this is the first time a DSF produced by X.f. has been characterized.

Oryza sativa rice plants contain molecules that activate different quorum-sensing N-acyl homoserine lactone biosensors and are sensitive to the specific AiiA lactonase

Gram-negative bacteria most often use N-acyl homoserine lactones (AHLs) as intercellular quorum-sensing signal molecules. In this study, it was demonstrated that rice plants contain AHL mimic molecules that are very sensitive to the highly specific AiiA lactonase enzyme and can activate three different AHL bacterial biosensors, indicating that the compounds have a homoserine lactone structure and could be AHLs. The possible source and biological significance of this finding are discussed.

PsrA, the Pseudomonas sigma regulator, controls regulators of epiphytic fitness, quorum-sensing signals, and plant interactions in Pseudomonas syringae pv. tomato strain DC3000

Pseudomonas syringae pv. tomato strain DC3000, a pathogen of tomato and Arabidopsis, occurs as an epiphyte. It produces N-acyl homoserine lactones (AHLs) which apparently function as quorum-sensing signals. A Tn5 insertion mutant of DC3000, designated PsrA(-) (Psr is for Pseudomonas sigma regulator), overexpresses psyR (a LuxR-type regulator of psyI) and psyI (the gene for AHL synthase), and it produces a ca. 8-fold-higher level of AHL than does DC3000. The mutant is impaired in its ability to elicit the hypersensitive reaction and is attenuated in its virulence in tomato. These phenotypes correlate with reduced expression of hrpL, the gene for an alternate sigma factor, as well as several hrp and hop genes during early stages of incubation in a Hrp-inducing medium. PsrA also positively controls rpoS, the gene for an alternate sigma factor known to control various stress responses. By contrast, PsrA negatively regulates rsmA1, an RNA-binding protein gene known to function as negative regulator, and aefR, a tetR-like gene known to control AHL production and epiphytic fitness in P. syringae pv. syringae. Gel mobility shift assays and other lines of evidence demonstrate a direct interaction of PsrA protein with rpoS promoter DNA and aefR operator DNA. In addition, PsrA negatively autoregulates and binds the psrA operator. In an AefR(-) mutant, the expression of psyR and psyI and AHL production are lower than those in DC3000, the AefR(+) parent. In an RpoS(-) mutant, on the other hand, the levels of AHL and transcripts of psyR and psyI are much higher than those in the RpoS(+) parent, DC3000. We present evidence, albeit indirect, that the RpoS effect occurs via psyR. Thus, AefR positively regulates AHL production, whereas RpoS has a strong negative effect. We show that AefR and RpoS do not regulate PsrA and that the PsrA effect on AHL production is exerted via its cumulative, but independent, effects on both AefR and RpoS.

Rhizobia and plant-pathogenic bacteria: common infection weapons

Plant-interacting micro-organisms can establish either mutualistic or pathogenic associations. Although the outcome is completely different, common molecular mechanisms that mediate communication between the interacting partners seem to be involved. Specifically, nitrogen-fixing bacterial symbionts of legume plants, collectively termed rhizobia, and phytopathogenic bacteria have adopted similar strategies and genetic traits to colonize, invade and establish a chronic infection in the plant host. Quorum-sensing signals and identical two-component regulatory systems are used by these bacteria to coordinate, in a cell density-dependent manner or in response to changing environmental conditions, the expression of important factors for host colonization and infection. The success of invasion and survival within the host also requires that rhizobia and pathogens suppress and/or overcome plant defence responses triggered after microbial recognition, a process in which surface polysaccharides, antioxidant systems, ethylene biosynthesis inhibitors and virulence genes are involved.

The epidemiology and management of bacterial blight (Pseudomonas syringae pv. pisi) of field pea (Pisum sativum) in Australia: a review

Bacterial blight caused by Pseudomonas syringae pv. pisi is an important, but sporadic, disease of field peas (Pisum sativum) in Australia. The presence of P. syringae pv. pisi reduces the profitability of peas due to yield loss and, in some cases, it also limits Australia’s export of peas to some countries. Pseudomonoas syringae pv. pisi is primarily a seed-borne pathogen, but infected pea trash can be an important source of inoculum. Alternative hosts and soil are not regarded as epidemiologically important sources of inoculum. P. syringae pv. pisi survives, multiplies and spreads epiphytically in pea crops. Epiphytic populations of P. syringae pv. pisi only become pathogenic following crop damage caused by frost or severe weather conditions. Frost damage is especially important because the ice nucleating activity of P. syringae pv. pisi initiates frost damage at higher temperatures than occurs in the absence of the bacterium. In addition early-sown crops are more prone to damage from bacterial blight than crops sown later in the season. Pseudomonas syringae pv. pisi consists of seven identified races. One of these (Race 6) lacks all avirulence genes and is common around the world and in Australia. Globally, Race 2 and Race 6 predominate; however, in Australia, Race 3 predominates due to the widespread cultivation of cultivars susceptible to Race 3, but resistant to Race 2. Resistance to Race 6 within P. sativum has not been found but attempts are being made to incorporate a race non-specific resistance identified from P. abyssinicum into field pea. Bacterial blight can be successfully controlled using an integrated disease management strategy incorporating crop rotation, pathogen-free seed, avoidance of planting in areas prone to frequent frosts or extreme wet weather, crop hygiene and avoiding early sowing. Seed treatment and application of foliar bactericides have limited use in control of this disease.

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.

Purification, characterization, and gene sequence of michiganin A, an actagardine-like lantibiotic produced by the tomato pathogen Clavibacter michiganensis subsp. michiganensis

Members of the actinomycete genus Clavibacter are known to produce antimicrobial compounds, but so far none of these compounds has been purified and characterized. We have isolated an antimicrobial peptide, michiganin A, from the tomato pathogen Clavibacter michiganensis subsp. michiganensis, using ammonium sulfate precipitation followed by cation-exchange and reversed-phase chromatography steps. Upon chemical derivatization of putative dehydrated amino acids and lanthionine bridges by alkaline ethanethiol, Edman degradation yielded sequence information that proved to be sufficient for cloning of the gene by a genome-walking strategy. The mature unmodified peptide consists of 21 amino acids, SSSGWLCTLTIECGTIICACR. All of the threonine residues undergo dehydration, and three of them interact with cysteines via thioether bonds to form methyllanthionine bridges. Michiganin A resembles actagardine, a type B lantibiotic with a known three-dimensional structure, produced by Actinoplanes liguriae, which is a filamentous actinomycete. The DNA sequence of the gene showed that the michiganin A precursor contains an unusual putative signal peptide with no similarity to well-known secretion signals and only very limited similarity to the (only two) available leader peptides of other type B lantibiotics. Michiganin A inhibits the growth of Clavibacter michiganensis subsp. sepedonicus, the causal agent of ring rot of potatoes, with MICs in the low nanomolar range. Thus, michiganin A may have some potential in biological control of potato ring rot.

A plasmid-borne truncated luxI homolog controls quorum-sensing systems and extracellular carbohydrate production in Methylobacterium extorquens AM1

A cryptic plasmid of Methylobacterium extorquens AM1 was found to encode tslI, a truncated luxI homolog. tslI was shown to be expressed and to control transcription of the acyl-homoserine lactone (HSL) synthase gene msaI and thus, indirectly, acyl-HSL production. In addition, tslI was found to positively regulate extracellular polysaccharide production.

High Variation in Pathogenicity of Genetically Closely Related Strains of Xanthomonas albilineans, the Sugarcane Leaf Scald Pathogen, in Guadeloupe

ABSTRACT Pathogenicity of 75 strains of Xanthomonas albilineans from Guadeloupe was assessed by inoculation of sugarcane cv. B69566, which is susceptible to leaf scald, and 19 of the strains were selected as representative of the variation in pathogenicity observed based on stalk colonization. In vitro production of albicidin varied among these 19 strains, but the restriction fragment length polymorphism pattern of their albicidin biosynthesis genes was identical. Similarly, no genomic variation was found among strains by pulsed-field gel electrophoresis. Some variation among strains was found by amplified fragment length polymorphism, but no relationship between this genetic variation and variation in pathogenicity was found. Only 3 (pilB, rpfA, and xpsE) of 40 genes involved in pathogenicity of bacterial species closely related to X. albilineans could be amplified by polymerase chain reaction from total genomic DNA of all nine strains tested of X. albilineans differing in pathogenicity in Guadeloupe. Nucleotide sequences of these genes were 100% identical among strains, and a phylogenetic study with these genes and housekeeping genes efp and ihfA suggested that X. albilineans is on an evolutionary road between the X. campestris group and Xylella fastidiosa, another vascular plant pathogen. Sequencing of the complete genome of Xanthomonas albilineans could be the next step in deciphering molecular mechanisms involved in pathogenicity of X. albilineans.

Bacterial elicitation and evasion of plant innate immunity

Recent research on plant responses to bacterial attack has identified extracellular and intracellular host receptors that recognize conserved pathogen-associated molecular patterns and more specialized virulence proteins, respectively. These findings have shed light on our understanding of the molecular mechanisms by which bacteria elicit host defences and how pathogens have evolved to evade or suppress these defences.

Disruption of N-acyl homoserine lactone-mediated cell signaling and iron acquisition in epiphytic bacteria by leaf surface compounds

Since N-acyl homoserine lactones (AHLs) are key mediators of cell density-dependent regulation of traits involved in virulence and epiphytic fitness in gram-negative bacteria such as Pseudomonas syringae, a variety of plant species were examined to determine their production of leaf surface compounds that could interact with these signaling systems. Leaf washings of 17 of 52 plant species tested stimulated or inhibited AHL-dependent traits in at least one of the bacterial reporter strains used. The active compounds from most plants could be distinguished from known AHLs due to different patterns of mobility during C8 and C18 reverse-phase thin-layer chromatography (TLC) and normal-phase TLC compared to the patterns for authentic bacterial AHLs. All plant extracts were also tested to determine their abilities to sequester iron and trigger bacterial siderophore synthesis on a medium containing abundant iron. Leaf washings from 16 of the 52 plant species, as well as tannic acid solutions, stimulated pyoverdine synthesis in P. syringae in a high-iron medium. These preparations also inhibited the growth of a P. syringae mutant unable to produce pyoverdine siderophores but not the growth of the wild-type bacterium. The stimulation of siderophore production and the growth inhibition by plant extracts and purified tannins were both reversed by addition of ferric chloride to culture media, indicating that iron was made unavailable by the compounds released onto the leaf surface.

Comparative genomics of host-specific virulence in Pseudomonas syringae

While much study has gone into characterizing virulence factors that play a general role in disease, less work has been directed at identifying pathogen factors that act in a host-specific manner. Understanding these factors will help reveal the variety of mechanisms used by pathogens to suppress or avoid host defenses. We identified candidate Pseudomonas syringae host-specific virulence genes by searching for genes whose distribution among natural P. syringae isolates was statistically associated with hosts of isolation. We analyzed 91 strains isolated from 39 plant hosts by DNA microarray-based comparative genomic hybridization against an array containing 353 virulence-associated (VA) genes, including 53 type III secretion system effectors (T3SEs). We identified individual genes and gene profiles that were significantly associated with strains isolated from cauliflower, Chinese cabbage, soybean, rice, and tomato. We also identified specific horizontal gene acquisition events associated with host shifts by mapping the array data onto the core genome phylogeny of the species. This study provides the largest suite of candidate host-specificity factors from any pathogen, suggests that there are multiple ways in which P. syringae isolates can adapt to the same host, and provides insight into the evolutionary mechanisms underlying host adaptation.

Production of quorum-sensing-related signal molecules by epiphytic bacteria inhabiting wheat heads

The production of quorum-sensing-related signal molecules (QSRMs) among culturable bacteria comprising the community on wheat heads was investigated. The taxonomic position of 186 bacterial isolates obtained from ten heads was inferred based on 16S rRNA gene sequences, and their QSRM production was determined using two bioreporter strains of N-acylhomoserine lactones. Approximately 33% of isolates produced QSRMs, though the proportion of QSRM-producing isolates on a wheat head was significantly negatively correlated with population size. Most of the producing isolates were Pantoea species, most commonly Pantoea ananatis. Furthermore, the proportion of Pantoea ananatis that produced QSRMs was significantly negatively correlated with the number of bacterial genera (community richness) on each head. Finally, community richness was positively correlated with population size. Qualitative analysis using thin-layer-chromatography revealed that the QSRMs of Pantoea isolates were composed of at least two compounds. This is the first report indicating that Pantoea ananatis isolates inhabiting wheat heads are capable of producing QSRMs. QSRM production by Pantoea spp. may contribute to the predominance of this genus on wheat heads, particularly at relatively low population densities and community diversity.

Erwinia carotovora subspecies produce duplicate variants of ExpR, LuxR homologs that activate rsmA transcription but differ in their interactions with N-acylhomoserine lactone signals

The N-acylhomoserine lactone (AHL) signaling system comprises a producing system that includes acylhomoserine synthase (AhlI, a LuxI homolog) and a receptor, generally a LuxR homolog. AHL controls exoprotein production in Erwinia carotovora and consequently the virulence for plants. In previous studies we showed that ExpR, a LuxR homolog, is an AHL receptor and that it activates transcription of rsmA, the gene encoding an RNA binding protein which is a global negative regulator of exoproteins and secondary metabolites. An unusual finding was that the transcriptional activity of ExpR was neutralized by AHL. We subsequently determined that the genomes of most strains of E. carotovora subspecies tested possess two copies of the expR gene: expR1, which was previously studied, and expR2, which was the focus of this study. Comparative analysis of the two ExpR variants of E. carotovora subsp. carotovora showed that while both variants activated rsmA transcription, there were significant differences in the patterns of their AHL interactions, the rsmA sequences to which they bound, and their relative efficiencies of activation of rsmA transcription. An ExpR2- mutant produced high levels of exoproteins and reduced levels of RsmA in the absence of AHL. This contrasts with the almost complete inhibition of exoprotein production and the high levels of RsmA production in an AhlI- mutant that was ExpR1-. Our results suggest that ExpR2 activity is responsible for regulating exoprotein production primarily by modulating the levels of an RNA binding protein.

Quorum sensing and phenazines are involved in biofilm formation by Pseudomonas chlororaphis (aureofaciens) strain 30-84

The biological control bacterium Pseudomonas chlororaphis (aureofaciens) strain 30-84 employs two quorum sensing (QS) systems: PhzR/PhzI regulates the production of the antibiotics phenazine-1-carboxylic acid, 2-hydroxy-phenazine-1-carboxylic acid, and 2-hydroxy-phenazine, whereas CsaR/CsaI regulates currently unknown aspects of the cell surface. Previously characterized derivatives of strain 30-84 with mutations in each QS system and in the phenazine biosynthetic genes were screened for their ability to form surface-attached biofilm populations in vitro, using microtiter plate and flow cell biofilm assays, and on seeds and roots. Results from in vitro, seed, and root studies demonstrated that the PhzR/PhzI and the CsaR/CsaI QS regulatory systems contribute to the establishment of biofilms, with mutations in PhzR/PhzI having a significantly greater effect than mutations in CsaR/CsaI. Interestingly, phenazine antibiotic production was necessary for biofilm formation to the same extent as the PhzR/PhzI QS system, suggesting the loss of phenazines was responsible for the majority of the biofilm defect in these mutants. In vitro analysis indicated that genetic complementation or AHL addition to the growth medium restored the ability of the AHL synthase phzI mutant to form biofilms. However, only phenazine addition or genetic complementation of the phenazine biosynthetic mutation in trans restored biofilm formation by mutants defective in the transcriptional activator phzR or the phzB structural mutant. QS and phenazine production were also involved in the establishment of surface-attached populations on wheat seeds and plant roots, and, as observed in vitro, the addition of AHL extracts restored the ability of phzI mutants, but not phzR mutants, to form surface attached populations on seeds. Similarly, the presence of the wild type in mixtures with the mutants restored the ability of the mutants to colonize wheat roots, demonstrating that AHL and/or phenazine production by the wild-type population could complement the AHL- and phenazine-deficient mutants in situ. Together, these data demonstrate that both QS systems are involved in the formation of surface-attached populations required for biofilm formation by P. chlororaphis strain 30-84, and indicate a new role for phenazine antibiotics in rhizosphere community development beyond inhibition of other plant-associated microorganisms.

Bacterial endophytes and their interactions with hosts

Recent molecular studies on endophytic bacterial diversity have revealed a large richness of species. Endophytes promote plant growth and yield, suppress pathogens, may help to remove contaminants, solubilize phosphate, or contribute assimilable nitrogen to plants. Some endophytes are seedborne, but others have mechanisms to colonize the plants that are being studied. Bacterial mutants unable to produce secreted proteins are impaired in the colonization process. Plant genes expressed in the presence of endophytes provide clues as to the effects of endophytes in plants. Molecular analysis showed that plant defense responses limit bacterial populations inside plants. Some human pathogens, such as Salmonella spp., have been found as endophytes, and these bacteria are not removed by disinfection procedures that eliminate superficially occurring bacteria. Delivery of endophytes to the environment or agricultural fields should be carefully evaluated to avoid introducing pathogens.

The rcsA promoter of Pantoea stewartii subsp. stewartii features a low-level constitutive promoter and an EsaR quorum-sensing-regulated promoter

The upstream region of the Pantoea stewartii rcsA gene features two promoters, one for constitutive basal-level expression and a second autoregulated promoter for induced expression. The EsaR quorum-sensing repressor binds to a site centered between the two promoters, blocking transcription elongation from the regulated promoter under noninducing conditions.

Anti-quorum sensing activity of medicinal plants in southern Florida

Bacterial intercellular communication, or quorum sensing (QS), controls the pathogenesis of many medically important organisms. Anti-QS compounds are known to exist in marine algae and have the ability to attenuate bacterial pathogenicity. We hypothesized that terrestrial plants traditionally used as medicines may also produce anti-QS compounds. To test this hypothesis, 50 medicinal plants from southern Florida were screened for anti-QS activity using two biomonitor strains, Chromobacterium violaceum and Agrobacterium tumefaciens. Of these plants, six showed QS inhibition: Conocarpus erectus L. (Combretaceae), Chamaecyce hypericifolia (L.) Millsp. (Euphorbiaceae), Callistemon viminalis (Sol. ex Gaertn.) G. Don (Myrtaceae), Bucida burceras L. (Combretaceae), Tetrazygia bicolor (Mill.) Cogn. (Melastomataceae), and Quercus virginiana Mill. (Fagaceae). This study introduces not only a new mode of action and possible validation for traditional plant use, but also a potentially new therapeutic direction for the treatment of bacterial infections.

Quorum-sensing system influences root colonization and biological control ability in Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a biocontrol agent isolated from a wheat take-all decline soil in China. This strain produces several antifungal compounds, such as 2,4-diacetylphloroglucinol (2,4-DAPG), hydrogen cyanide and siderophore(s). Our recent work revealed that strain 2P24 employs a quorum-sensing system to regulate its biocontrol activity. In this study, we identified a quorum-sensing system consisting of PcoR and PcoI of the LuxR-LuxI family from strain 2P24. Deletion of pcoI from 2P24 abolishes the production of the quorum-sensing signals, but does not detectably affect the production of antifungal metabolites. However, the mutant is significantly defective in biofilm formation, colonization on wheat rhizosphere and biocontrol ability against wheat take-all, whilst complementation of pcoI restores the biocontrol activity to the wild-type level. Our data indicate that quorum sensing is involved in regulation of biocontrol activity in P. fluorescens 2P24.

Regulation of quorum sensing in Pseudomonas

Bacteria use small signal molecules in order to monitor their population density and coordinate gene regulation in a process called quorum sensing. In Gram-negative bacteria, the most common signal molecules are acylated homoserine lactones. Several Pseudomonas species produce acylated homoserine lactones that control important functions including pathogenicity and plant growth promotion. Many reports indicate that the quorum sensing systems of Pseudomonas are significantly regulated and interconnected with regulons of other global regulators. The integration of quorum sensing into additional regulatory circuits increases the range of environmental and metabolic signals beyond that of cell density, as well as further tuning the timing of the response. This review will focus on the regulation of quorum sensing in Pseudomonas, highlighting a complex response that might serve a given species to adapt in its particular environment.

Halomonas maura is a physiologically versatile bacterium of both ecological and biotechnological interest

Halomonas maura is a bacterium of great metabolic versatility. We summarise in this work some of the properties that make it a very interesting microorganism both from an ecological and biotechnological point of view. It plays an active role in the nitrogen cycle, is capable of anaerobic respiration in the presence of nitrate and has recently been identified as a diazotrophic bacterium. Of equal interest is mauran, the exopolysaccharide produced by H. maura, which contributes to the formation of biofilms and thus affords the bacterium advantages in the colonisation of its saline niches. Mauran is highly viscous, shows thixotropic and pseudoplastic behaviour, has the capacity to capture heavy metals and exerts a certain immunomodulator effect in medicine. All these attributes have prompted us to make further investigations into its molecular characteristics. To date we have described 15 open reading frames (ORF's) related to exopolysaccharide production, nitrogen fixation and nitrate reductase activity among others.

Dual control of quorum sensing by two TraM-type antiactivators in Agrobacterium tumefaciens octopine strain A6

Agrobacterium tumefaciens wild-type strains have a unique quorum-sensing (QS)-dependent Ti plasmid conjugative transfer phenotype in which QS signaling is activated by corresponding conjugative opine inducers. Strain K588, with a nopaline-type chromosomal background harboring an octopine-type Ti plasmid, however, is a spontaneous mutant displaying a constitutive phenotype in QS. In this study, we show that a single amino acid mutation (L54P) in the QS antiactivator TraM encoded by the traM gene of Ti plasmid is responsible for the constitutive phenotype of strain K588. Introduction of the L54P point mutation to the TraM of wild-type strain A6 by allelic replacement, however, failed to generate the expected constitutive phenotype in this octopine-type strain. Intriguingly, the QS-constitutive phenotype appeared when the pTiA6 carrying the mutated traM was placed in the chromosomal background of the nopaline-type strain C58C1RS, suggesting an unknown inhibitory factor(s) encoded by the chromosomal background of strain A6 but not by C58C1RS. Low-stringency Southern blotting analysis showed that strain A6, but not strain C58 and its derivatives, contains a second traM homologue. The homologue, designated traM2, has 64% and 65% identities with traM at the DNA and peptide levels, respectively. Similar to TraM, TraM2 is a potent antiactivator that functions by blocking TraR, the QS activator, from specific binding to the tra gene promoters. Deletion of traM2 in strain A6 harboring the mutated traM confers a constitutive QS phenotype. The results demonstrate that the QS system in strain A6 is subjected to the dual control of TraM and TraM2.

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.

The Rcs phosphorelay: a complex signal transduction system

RcsC, RcsB, and RcsA were first identified as a sensor kinase, a response regulator, and an auxiliary regulatory protein, respectively, regulating the genes of capsular polysaccharide synthesis. Recent advances have demonstrated that these proteins are part of a complex phosphorelay, in which phosphate travels from the histidine kinase domain in RcsC to a response regulator domain in the same protein; from there to a phosphotransfer protein, RcsD; and from there to RcsB. In addition to capsule synthesis, which requires the unstable regulatory protein RcsA, RcsB also stimulates transcription of a small RNA, RprA; the cell division gene ftsZ; and genes encoding membrane and periplasmic proteins, including the osmotically inducible genes osmB and osmC. The Rcs system appears to play an important role in the later stages of biofilm development; induction of Rcs signaling by surfaces is consistent with this role.

Variations on a theme: diverse N-acyl homoserine lactone-mediated quorum sensing mechanisms in gram-negative bacteria

Many Gram-negative bacteria employ a mechanism of cell-cell communication known as quorum sensing (QS). The role of QS is to enable the cells in a culture to coordinate their gene expression profile with changes in the population cell density. The best characterized mechanisms of QS employ N-acylated homoserine lactones (AHLs) as signalling molecules. These AHLs are made by enzymes known as LuxI homologs, and accumulate in the culture supernatant at a rate proportional to the increase in cell density. Once the AHL concentration exceeds a certain threshold value, these ligands bind to intracellular receptors known as LuxR homologs. The latter are transcriptional regulators, whose activity alters upon binding the AHL ligand, thereby eliciting a change in gene transcription. Over the last five years, it has become increasingly obvious that this is a rather simplistic view of AHL-dependent QS, and that in fact, there is considerable diversity in the way in which LuxI-R homologs operate. The aim of the current review is to describe these variations on the basic theme, and to show how functional genomics is revolutionizing our understanding of QS-controlled regulons.

The role of root exudates in rhizosphere interactions with plants and other organisms

The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

Variation in Albicidin Biosynthesis Genes and in Pathogenicity of Xanthomonas albilineans, the Sugarcane Leaf Scald Pathogen

ABSTRACT Total genomic DNA from 137 strains of Xanthomonas albilineans from worldwide locations was hybridized with two DNA probes that together harbor the entire 49-kb albicidin biosynthesis gene cluster and two additional 3-kb genomic regions required for albicidin production. Fourteen haplotypes and two major genetic groups (albicidin [ALB]-restriction fragment length polymorphism [RFLP] A and ALB-RFLP B) were identified, and strains that were isolated after recent outbreaks of leaf scald disease belonged to group ALB-RFLP B. Albicidin genetic diversity was very similar to the previously described genetic diversity of the pathogen based on the whole genome. No relationship was found between variability of albicidin biosynthesis genes and the amount of albicidin produced in vitro by X. albilineans. Leaf scald-susceptible sugarcane cv. H70-144 was inoculated with 20 strains of the pathogen belonging to different ALB-RFLP haplotypes. Among them, 10 strains from Guadeloupe belonged to the same ALB-RFLP group but differed in the amount of albicidin produced in vitro. Strains were distributed in at least three different pathogenicity groups based on symptom severity and pathogen population density in the stalk. These two pathogenicity factors varied concurrently; however, no relationship between variation in albicidin biosynthesis genes, variation in the amount of albicidin produced in vitro, and variation in pathogenicity of X. albilineans was found. Further investigation is necessary to identify other genes involved in pathogenicity of X. albilineans.

Fitness of human enteric pathogens on plants and implications for food safety

The continuous rise in the number of outbreaks of foodborne illness linked to fresh fruit and vegetables challenges the notion that enteric pathogens are defined mostly by their ability to colonize the intestinal habitat. This review describes the epidemiology of produce-associated outbreaks of foodborne disease and presents recently acquired knowledge about the behavior of enteric pathogens on plants, with an emphasis on Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes. The growth and survival of enteric pathogens on plants are discussed in the light of knowledge and concepts in plant microbial ecology, including epiphytic fitness, the physicochemical nature of plant surfaces, biofilm formation, and microbe-microbe and plant-microbe interactions. Information regarding the various stresses that affect the survival of enteric pathogens and the molecular events that underlie their interactions in the plant environment provides a good foundation for assessing their role in the infectious dose of the pathogens when contaminated fresh produce is the vehicle of illness.

Comparative genomics reveals what makes an enterobacterial plant pathogen

The bacterial family Enterobacteriaceae contains some of the most devastating human and animal pathogens, including Escherichia coli, Salmonella enterica and species of Yersinia and Shigella. These are among the best-studied of any organisms, yet there is much to be learned about the nature and evolution of interactions with their hosts and with the wider environment. Comparative and functional genomics have fundamentally improved our understanding of their modes of adaptation to different ecological niches and the genes that determine their pathogenicity. In addition to animal pathogens, Enterobacteriaceae include important plant pathogens, such as Erwinia carotovora subsp. atroseptica (Eca), the first plant-pathogenic enterobacterium to be sequenced. This review focuses on genomic comparisons between Eca and other enterobacteria, with particular emphasis on the differences that exemplify or explain the plant-associated lifestyle(s) of Eca. Horizontal gene transfer in Eca may directly have led to the acquisition of a number of determinants that mediate its interactions, pathogenic or otherwise, with plants, offering a glimpse into its evolutionary divergence from animal-pathogenic enterobacteria.

Comparative analysis of two classes of quorum-sensing signaling systems that control production of extracellular proteins and secondary metabolites in Erwinia carotovora subspecies

In Erwinia carotovora subspecies, N-acyl homoserine lactone (AHL) controls the expression of various traits, including extracellular enzyme/protein production and pathogenicity. We report here that E. carotovora subspecies possess two classes of quorum-sensing signaling systems defined by the nature of the major AHL analog produced as well as structural and functional characteristics of AHL synthase (AhlI) and AHL receptor (ExpR). Class I strains represented by E. carotovora subsp. atroseptica strain Eca12 and E. carotovora subsp. carotovora strains EC153 and SCC3193 produce 3-oxo-C8-HL (N-3-oxooctanoyl-l-homoserine lactone) as the major AHL analog as well as low but detectable levels of 3-oxo-C6-HL (N-3-oxohexanoyl-l-homoserine lactone). In contrast, the members of class II (i.e., E. carotovora subsp. betavasculorum strain Ecb168 and E. carotovora subsp. carotovora strains Ecc71 and SCRI193) produce 3-oxo-C6-HL as the major analog. ExpR species of both classes activate rsmA (Rsm, repressor of secondary metabolites) transcription and bind rsmA DNA. Gel mobility shift assays with maltose-binding protein (MBP)-ExpR(71) and MBP-ExpR(153) fusion proteins show that both bind a 20-mer sequence present in rsmA. The two ExpR functions (i.e., expR-mediated activation of rsmA expression and ExpR binding with rsmA DNA) are inhibited by AHL. The AHL effects are remarkably specific in that expR effect of EC153, a strain belonging to class I, is counteracted by 3-oxo-C8-HL but not by 3-oxo-C6-HL. Conversely, the expR effect of Ecc71, a strain belonging to class II, is neutralized by 3-oxo-C6-HL but not by 3-oxo-C8-HL. The AHL responses correlated with expR-mediated inhibition of exoprotein and secondary metabolite production.

Methylobacterium extorquensAM1 produces a novel type of acyl-homoserine lactone with a double unsaturated side chain under methylotrophic growth conditions

Acyl-homoserine lactones (acyl-HSLs) have emerged as important regulatory molecules for many gram-negative bacteria. We have found that Methylobacterium extorquens AM1, a member of the pink-pigmented facultative methylotrophs commonly present on plant surfaces, produces several acyl-HSLs depending upon the carbon source. A novel HSL was discovered with a double unsaturated carbon chain (N-(tetradecenoyl)) (C14:2) and characterized by MS and proton NMR. This long-chain acyl-HSL is synthesized by MlaI that also directs synthesis of C14:1-HSL. The Alphaproteobacterium also produces N-hexanoyl-HSL (C6-HSL) and N-octanoyl-HSL (C8-HSL) via MsaI.

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.

Inter-kingdom signaling: deciphering the language of acyl homoserine lactones

Bacteria use small secreted chemicals or peptides as auto-inducers to coordinately regulate gene expression within a population in a process called quorum sensing. Quorum sensing controls several important functions in different bacterial species, including the production of virulence factors and biofilm formation in Pseudomonas aeruginosa and bioluminescence in Vibrio fischeri. Many gram-negative bacterial species use acyl homoserine lactones as auto-inducers that function as ligands for transcriptional regulatory proteins. Several recent reports indicate that bacterial acyl homoserine lactones can also affect gene expression in host cells. Direct signaling also appears to function in the opposite direction as some eukaryotic cell types produce mimics that interact with quorum sensing systems in bacteria. Here, we will describe the evidence to support the existence of bi-directional inter-kingdom signaling via acyl homoserine lactones and eukaryotic mimics and discuss the potential molecular mechanisms that mediate these responses. The functional consequences of inter-kingdom signaling will be discussed in relation to both pathogenic and non-pathogenic bacterial-host interactions.

Insights into genome plasticity and pathogenicity of the plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria revealed by the complete genome sequence

The gram-negative plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria is the causative agent of bacterial spot disease in pepper and tomato plants, which leads to economically important yield losses. This pathosystem has become a well-established model for studying bacterial infection strategies. Here, we present the whole-genome sequence of the pepper-pathogenic Xanthomonas campestris pv. vesicatoria strain 85-10, which comprises a 5.17-Mb circular chromosome and four plasmids. The genome has a high G+C content (64.75%) and signatures of extensive genome plasticity. Whole-genome comparisons revealed a gene order similar to both Xanthomonas axonopodis pv. citri and Xanthomonas campestris pv. campestris and a structure completely different from Xanthomonas oryzae pv. oryzae. A total of 548 coding sequences (12.2%) are unique to X. campestris pv. vesicatoria. In addition to a type III secretion system, which is essential for pathogenicity, the genome of strain 85-10 encodes all other types of protein secretion systems described so far in gram-negative bacteria. Remarkably, one of the putative type IV secretion systems encoded on the largest plasmid is similar to the Icm/Dot systems of the human pathogens Legionella pneumophila and Coxiella burnetii. Comparisons with other completely sequenced plant pathogens predicted six novel type III effector proteins and several other virulence factors, including adhesins, cell wall-degrading enzymes, and extracellular polysaccharides.

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.

ExpR, a LuxR homolog of Erwinia carotovora subsp. carotovora, activates transcription of rsmA, which specifies a global regulatory RNA-binding protein

N-acyl homoserine lactone (AHL) is required by Erwinia carotovora subspecies for the expression of various traits, including extracellular enzyme and protein production and pathogenicity. Previous studies with E. carotovora subsp. carotovora have shown that AHL deficiency causes the production of high levels of RsmA, an RNA binding protein that functions as a global negative regulator of extracellular enzymes and proteins and secondary metabolites (Rsm, regulator of secondary metabolites). We document here that ExpR, a putative AHL receptor belonging to the LuxR family of regulators, activates RsmA production. In the absence of AHL, an ExpR(+) E. carotovora subsp. carotovora strain compared to its ExpR(-) mutant, produces higher levels of rsmA RNA and better expresses an rsmA-lacZ transcriptional fusion. Moreover, the expression of the rsmA-lacZ fusion in Escherichia coli is much higher in the presence of expR(71) (the expR gene of E. carotovora subsp. carotovora strain Ecc71) than in its absence. We also show that purified preparation of MBP-ExpR(71) binds (MBP, maltose binding protein) rsmA DNA. By contrast, MBP-ExpR(71) does not bind ahlI (gene for AHL synthase), pel-1 (gene for pectate lyase), or rsmB (gene for regulatory RNA that binds RsmA), nor does ExpR(71) activate expression of these genes. These observations strongly suggest transcriptional activation of rsmA resulting from a direct and specific interaction between ExpR(71) and the rsmA promoter. Several lines of evidence establish that N-3-oxohexanoyl-L-homoserine lactone (3-oxo-C6-HL), the major AHL analog produced by E. carotovora subsp. carotovora strain Ecc71, inhibits ExpR(71)-mediated activation of rsmA expression. These findings for the first time establish that the expR effect in E. carotovora subsp. carotovora is channeled via RsmA, a posttranscriptional regulator of E. carotovora subspecies, and AHL neutralizes this ExpR effect.

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.

Systems analysis of a quorum sensing network: design constraints imposed by the functional requirements, network topology and kinetic constants

Understanding the relationship between the structural organization of intracellular decision networks and the observable phenotypes they control is one of the exigent problems of modern systems biology. Here we perform a systems analysis of a prototypic quorum sensing network whose operation allows bacterial populations to activate certain patterns of gene expression cooperatively. We apply structural perturbations to the model and analyze the resulting changes in the network behavior with the aim to identify the contribution of individual network elements to the functional fitness of the whole network. Specifically, we demonstrate the importance of the dimerization of the transcription factor and the presence of the auxiliary positive feedback loop on the switch-like behavior of the network and the stability of its "on" and "off" states under the influence of molecular noise.

Transition to quorum sensing in an Agrobacterium population: A stochastic model

Understanding of the intracellular molecular machinery that is responsible for the complex collective behavior of multicellular populations is an exigent problem of modern biology. Quorum sensing, which allows bacteria to activate genetic programs cooperatively, provides an instructive and tractable example illuminating the causal relationships between the molecular organization of gene networks and the complex phenotypes they control. In this work we—to our knowledge for the first time—present a detailed model of the population-wide transition to quorum sensing using the example of Agrobacterium tumefaciens. We construct a model describing the Ti plasmid quorum-sensing gene network and demonstrate that it behaves as an “on–off” gene expression switch that is robust to molecular noise and that activates the plasmid conjugation program in response to the increase in autoinducer concentration. This intracellular model is then incorporated into an agent-based stochastic population model that also describes bacterial motion, cell division, and chemical communication. Simulating the transition to quorum sensing in a liquid medium and biofilm, we explain the experimentally observed gradual manifestation of the quorum-sensing phenotype by showing that the transition of individual model cells into the “on” state is spread stochastically over a broad range of autoinducer concentrations. At the same time, the population-averaged values of critical autoinducer concentration and the threshold population density are shown to be robust to variability between individual cells, predictable and specific to particular growth conditions. Our modeling approach connects intracellular and population scales of the quorum-sensing phenomenon and provides plausible answers to the long-standing questions regarding the ecological and evolutionary significance of the phenomenon. Thus, we demonstrate that the transition to quorum sensing requires a much higher threshold cell density in liquid medium than in biofilm, and on this basis we hypothesize that in Agrobacterium quorum sensing serves as the detector of biofilm formation.

Understanding the interplay between the extracellular environment and intracellular decision circuitry of a cell is important but is an arduous goal to achieve since many interacting factors, difficult to measure and control in experiment, are involved. The authors address this problem by means of computational modeling using the example of a bacterial population that cooperatively switches on a common gene expression program if a certain critical population density is achieved. They developed a detailed model of the intracellular control network and demonstrated that it can operate as an “on–off” gene expression switch that is sensitive to environmental control and yet highly robust to intracellular molecular noise. The population-wide transition is further modeled using a novel method in which each bacterium is given a unique copy of an intracellular network. This approach, which allows monitoring of both the dynamics of individual cells and population behavior, provides an explanation for the gradual appearance of the transition to the “on” state that has been observed in experiments, and quantitatively predicts the critical value of the population density at which this transition occurs. Unexpectedly, a comparison of the cell densities required for the transition in different environmental conditions brought about a hypothesis regarding the previously elusive ecological and evolutionary function of this cooperative phenomenon.