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PubMed2006

Quorum sensing in Escherichia coli and Salmonella.

Quorum sensing in Escherichia coli and Salmonella has been an elusive topic for a long time. However, in the past 8 years, several research groups have demonstrated that these bacteria use several quorum-sensing systems, such as: the luxS/AI-2, AI-3/epinephrine/norepinephrine, indole, and the LuxR homolog SdiA to achieve intercellular signaling. The majority of these signaling systems are involved in interspecies communication, and the AI-3/epinephrine/norepinephrine signaling system is also involved in interkingdom communication. Both E. coli and Salmonella reside in the human intestine, which is the largest and most complex environment in the mammalian host. The observation that these bacteria evolved quorum-sensing systems primarily involved in interspecies communication may constitute an adaptation to this environment. The gastrointestinal tract harbors a high density and diversity of bacterial cells, with the majority of the flora residing in the colon (10(11)-10(12) bacterial cells/ml). Given the enormous number and diversity of bacteria inhabiting the gastrointestinal environment, it should not be surprising that the members of this community communicate amongst themselves and with the host itself to coordinate a variety of adaptive processes.

Bacterial Proteins↗

PubMed1997

A quorum-sensing system in the free-living photosynthetic bacterium Rhodobacter sphaeroides.

Rhodobacter sphaeroides is a free-living, photoheterotrophic bacterium known for its genomic and metabolic complexity. We have discovered that this purple photosynthetic organism possesses a quorum-sensing system. Quorum sensing occurs in a number of eukaryotic host-associated gram-negative bacteria. In these bacteria there are two genes required for quorum sensing, the luxR and luxI homologs, and there is an acylhomoserine lactone signal molecule synthesized by the product of the luxI homolog. In R. sphaeroides, synthesis of a novel homoserine lactone signal, 7,8-cis-N-(tetradecenoyl)homoserine lactone, is directed by a luxI homolog termed cerI. Two open reading frames immediately upstream of cerI are proposed to be components of the quorum-sensing system. The first of these is a luxR homolog termed cerR, and the second is a small open reading frame of 159 bp. Inactivation of cerI in R. sphaeroides results in mucoid colony formation on agar and formation of large aggregates of cells in liquid cultures. Clumping of CerI mutants in liquid culture is reversible upon addition of the acylhomoserine lactone signal and represents a phenotype unlike those controlled by quorum sensing in other bacteria.

4-Butyrolactone↗

PubMed2005

Vibrio fischeri uses two quorum-sensing systems for the regulation of early and late colonization factors.

Vibrio fischeri possesses two quorum-sensing systems, ain and lux, using acyl homoserine lactones as signaling molecules. We have demonstrated previously that the ain system activates luminescence gene expression at lower cell densities than those required for lux system activation and that both systems are essential for persistent colonization of the squid host, Euprymna scolopes. Here, we asked whether the relative contributions of the two systems are also important at different colonization stages. Inactivation of ain, but not lux, quorum-sensing genes delayed initiation of the symbiotic relationship. In addition, our data suggest that lux quorum sensing is not fully active in the early stages of colonization, implying that this system is not required until later in the symbiosis. The V. fischeri luxI mutant does not express detectable light levels in symbiosis yet initiates colonization as well as the wild type, suggesting that ain quorum sensing regulates colonization factors other than luminescence. We used a recently developed V. fischeri microarray to identify genes that are controlled by ain quorum sensing and could be responsible for the initiation defect. We found 30 differentially regulated genes, including the repression of a number of motility genes. Consistent with these data, ain quorum-sensing mutants displayed an altered motility behavior in vitro. Taken together, these data suggest that the sequential activation of these two quorum-sensing systems with increasing cell density allows the specific regulation of early colonization factors (e.g., motility) by ain quorum sensing, whereas late colonization factors (e.g., luminescence) are preferentially regulated by lux quorum sensing.

Aliivibrio fischeri↗

PubMed2004

The small RNA chaperone Hfq and multiple small RNAs control quorum sensing in Vibrio harveyi and Vibrio cholerae.

Quorum-sensing bacteria communicate with extracellular signal molecules called autoinducers. This process allows community-wide synchronization of gene expression. A screen for additional components of the Vibrio harveyi and Vibrio cholerae quorum-sensing circuits revealed the protein Hfq. Hfq mediates interactions between small, regulatory RNAs (sRNAs) and specific messenger RNA (mRNA) targets. These interactions typically alter the stability of the target transcripts. We show that Hfq mediates the destabilization of the mRNA encoding the quorum-sensing master regulators LuxR (V. harveyi) and HapR (V. cholerae), implicating an sRNA in the circuit. Using a bioinformatics approach to identify putative sRNAs, we identified four candidate sRNAs in V. cholerae. The simultaneous deletion of all four sRNAs is required to stabilize hapR mRNA. We propose that Hfq, together with these sRNAs, creates an ultrasensitive regulatory switch that controls the critical transition into the high cell density, quorum-sensing mode.

Amino Acid Sequence↗

PubMed1998

Starvation selection restores elastase and rhamnolipid production in a Pseudomonas aeruginosa quorum-sensing mutant.

The las quorum-sensing system of Pseudomonas aeruginosa controls the expression of elastase and rhamnolipid. We report that starvation can select a mutant producing these virulence factors in spite of a lasR deletion. Expression of the autoinducer synthase gene rhlI was increased in this suppressor mutant, suggesting compensation by the rhl system. These data show that P. aeruginosa can restore elastase and rhamnolipid production in the absence of a functional las quorum-sensing system.

Bacterial Proteins↗

PubMed2004

Quorum sensing regulates type III secretion in Vibrio harveyi and Vibrio parahaemolyticus.

In a process known as quorum sensing, bacteria communicate with one another by producing, releasing, detecting, and responding to signal molecules called autoinducers. Vibrio harveyi, a marine pathogen, uses two parallel quorum-sensing circuits, each consisting of an autoinducer-sensor pair, to control the expression of genes required for bioluminescence and a number of other target genes. Genetic screens designed to discover autoinducer-regulated targets in V. harveyi have revealed genes encoding components of a putative type III secretion (TTS) system. Using transcriptional reporter fusions and TTS protein localization studies, we show that the TTS system is indeed functional in V. harveyi and that expression of the genes encoding the secretion machinery requires an intact quorum-sensing signal transduction cascade. The newly completed genome of the closely related marine bacterium Vibrio parahaemolyticus, which is a human pathogen, shows that it possesses the genes encoding both of the V. harveyi-like quorum-sensing signaling circuits and that it also has a TTS system similar to that of V. harveyi. We show that quorum sensing regulates TTS in V. parahaemolyticus. Previous reports connecting quorum sensing to TTS in enterohemorrhagic and enteropathogenic Escherichia coli show that quorum sensing activates TTS at high cell density. Surprisingly, we find that at high cell density (in the presence of autoinducers), quorum sensing represses TTS in V. harveyi and V. parahaemolyticus.

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PubMed2003

LuxS quorum sensing: more than just a numbers game.

Quorum sensing is a process of bacterial cell-to-cell communication involving the production and detection of extracellular signaling molecules called autoinducers. Quorum sensing allows populations of bacteria to collectively control gene expression, and thus synchronize group behavior. Processes controlled by quorum sensing are typically ones that are unproductive unless many bacteria act together. Most autoinducers enable intraspecies communication; however, a recently discovered autoinducer AI-2 has been proposed to serve as a 'universal signal' for interspecies communication. Studies suggest that AI-2 encodes information in addition to specifics about cell number.

Bacteria↗

PubMed2002

Regulation of antimicrobial peptide production by autoinducer-mediated quorum sensing in lactic acid bacteria.

Several lactic acid bacteria produce peptides with antimicrobial activity. During the last few years, cell-cell communication has emerged as the key regulatory mechanism that controls the production of many of these antimicrobial peptides via a regulatory strategy denominated quorum sensing. Quorum sensing allows populationwide synchronised production of antimicrobial peptides as a function of cell density. The cell-cell communication phenomenon required for sensing of the cell density is mediated by secreted signalling molecules. These 'molecular messengers' accumulate in the environment as the cell density increases and activate signal transduction cascades that result in the production of antimicrobial peptides by the stimulated bacterial cell.

Bacterial Proteins↗

PubMed2005

Conservation of genes and processes controlled by the quorum response in bacteria: characterization of genes controlled by the quorum-sensing transcription factor ComA in Bacillus subtilis.

Quorum or diffusion responses in bacteria are mediated by secreted signalling molecules that accumulate extracellularly as cultures grow to high density. The regulatory response to these signalling molecules can result in dramatic changes in gene expression. In Bacillus subtilis, a quorum response is mediated by a secreted 10-amino-acid modified peptide (ComX pheromone) that activates a receptor histidine kinase (ComP) that activates a response regulator transcription factor (ComA). We have used DNA microarrays to identify genes controlled by the ComX-ComP-ComA quorum-sensing pathway. We found that ComX, ComP and ComA affect the same set of genes, indicating that the kinase ComP is the only receptor for the signalling molecule ComX, and that ComA is the only transcription factor activated directly by ComP, under the conditions tested. Expression of over 20 genes appears to be controlled directly by this signalling pathway, and expression of over 150 additional genes, including those involved in competence development, appears to be controlled indirectly. The genes affected appear to have three general functions: (i) to co-ordinate physiological changes involved in developmental pathways, (ii) to produce extracellular products under conditions in which high concentrations of product are needed to be effective and (iii) to enhance survival, growth and colonization under conditions of crowding or limited diffusion. Many of the genes and processes controlled by the quorum response in B. subtilis are also regulated by quorum sensing in Gram-positive and Gram-negative bacteria. The quorum-sensing signalling molecules and regulatory proteins are quite different between Gram-positives and Gram-negatives and the convergent physiological regulation of similar genes and processes indicate the important and conserved nature of the quorum response.

Arginine↗

PubMed2006

The impact of quorum sensing and swarming motility on Pseudomonas aeruginosa biofilm formation is nutritionally conditional.

The role of quorum sensing in Pseudomonas aeruginosa biofilm formation is unclear. Some researchers have shown that quorum sensing is important for biofilm development, while others have indicated it has little or no role. In this study, the contribution of quorum sensing to biofilm development was found to depend upon the nutritional environment. Depending upon the carbon source, quorum-sensing mutant strains (lasIrhlI and lasRrhlR) either exhibited a pronounced defect early in biofilm formation or formed biofilms identical to the wild-type strain. Quorum sensing was then shown to exert its nutritionally conditional control of biofilm development through regulation of swarming motility. Examination of pilA and fliM mutant strains further supported the role of swarming motility in biofilm formation. These data led to a model proposing that the prevailing nutritional conditions dictate the contributions of quorum sensing and swarming motility at a key juncture early in biofilm development.

Biofilms↗

PubMed2004

Pseudomonas aeruginosa with lasI quorum-sensing deficiency during corneal infection.

PURPOSE: To understand the importance of Pseudomonas aeruginosa quorum-sensing systems in the development of corneal infection, the genotypic characteristics and pathogenesis of seven ocular isolates with low-protease and acyl homoserine lactone (AHL) activity and quorum-sensing mutants of PAO1 deficient in lasI, lasR, or rhlR were investigated in the study. METHODS: The possession of the quorum-sensing genes lasI, lasR, rhlI, rhlR, and the quorum-sensing controlled genes lasB, aprA, and rhlAB in the clinical isolates were determined by polymerase chain reaction and Southern blot hybridization. Elastinolytic activity, controlled by the las system, was assayed using elastin Congo red and rhamnolipid production controlled by the rhl system was assessed using agar plates containing methylene blue/cetyltrimethyl ammonium bromide. Induction of keratitis was examined in a scarified inbred BALB/c mouse model. RESULTS: The clinical isolates Paer1 and -3 were lasI and lasR negative, and the isolates Paer2 and -4 were rhlR and rhlAB negative. The isolates Paer17, Paer26, 6294 and 6206 possessed all the genes examined. There was no rhamnolipid production in clinical isolates Paer2 and -4. The isolates Paer1 and -3 were virtually avirulent in the scarified mouse corneas. Using isogenic PAO1 mutants, strain lasI showed a markedly reduced virulence in the corneal infection model. The remainder of the clinical isolates and the lasR or rhlR mutant strains caused severe keratitis. CONCLUSIONS: These results indicate that quorum-sensing deficiency may occur naturally in clinical isolates, and the possession of lasI and hence a functional Las quorum-sensing system may be important in development of corneal infection.

Animals↗

PubMed2003

Molecular mechanisms of bacterial quorum sensing as a new drug target.

Bacterial quorum sensing evolved as a means for bacterial communities to rapidly and coordinately change genome expression patterns in response to environmental cues. Each cell in a community produces and responds to a signaling molecule called an autoinducer that serves as an indicator of the population density. A high concentration of autoinducer is associated with a large, often confined population, which requires altered gene expression for survival. Quorum sensing is one mechanism through which a bacterial population receives input from the environment and elicits an appropriate response. Because many pathogens require quorum sensing to produce virulence factors in response to association with a human host, the signaling pathway is a target for design of small-molecule inhibitors.

Animals↗

PubMed2006

Inhibition of bacterial quorum sensing by vanilla extract.

AIMS: The purpose of this study was to search for a novel quorum sensing inhibitor and analyse its inhibitory activity. METHODS AND RESULTS: Quorum sensing inhibition was monitored using the Tn-5 mutant, Chromobacterium violaceum CV026. Vanilla beans (Vanilla planifolia Andrews) were extracted using 75% (v/v) aqueous methanol and added to C. violaceum CV026 cultures. Inhibitory activity was measured by quantifying violacein production using a spectrophotometer. The results have revealed that vanilla extract significantly reduced violacein production in a concentration-dependent manner, indicating inhibition of quorum sensing. CONCLUSIONS: Vanilla, a widely used spice and flavour, can inhibit bacterial quorum sensing. SIGNIFICANCE AND IMPACT OF THE STUDY: The results suggest that the intake of vanilla-containing food materials might promote human health by inhibiting quorum sensing and preventing bacterial pathogenesis. Further studies are required to isolate specific substances from vanilla extract acting as quorum sensing inhibitors.

4-Butyrolactone↗

PubMed2003

Quorum sensing controls biofilm formation in Vibrio cholerae.

Multiple quorum-sensing circuits function in parallel to control virulence and biofilm formation in Vibrio cholerae. In contrast to other bacterial pathogens that induce virulence factor production and/or biofilm formation at high cell density in the presence of quorum-sensing autoinducers, V. cholerae represses these behaviours at high cell density. Consistent with this, we show here that V. cholerae strains 'locked' in the regulatory state mimicking low cell density are enhanced for biofilm production whereas mutants 'locked' in the regulatory state mimicking high cell density are incapable of producing biofilms. The quorum-sensing cascade we have identified in V. cholerae regulates the transcription of genes involved in exopolysaccharide production (EPS), and variants that produce EPS and form biofilms arise at high frequency from non-EPS, non-biofilm producing strains. Our data show that spontaneous mutation of the transcriptional regulator hapR is responsible for this effect. Several toxigenic strains of V. cholerae possess a naturally occurring frameshift mutation in hapR. Thus, the distinct environments occupied by this aquatic pathogen presumably include niches where cell-cell communication is crucial, as well as ones where loss of quorum sensing via hapR mutation confers a selective advantage. Bacterial biofilms could represent a complex habitat where such differentiation occurs.

Bacterial Proteins↗

PubMed2003

A novel, inducible, eukaryotic gene expression system based on the quorum-sensing transcription factor TraR.

Bacteria adapt their pattern of gene expression in response to a variety of external cues, including fluctuations in population density. This type of bacterial cell-to-cell communication is referred to as quorum-sensing. Quorum-sensing systems are present in many bacterial species and constitute a large collection of ligands and cognate receptors. The availability of such diversity offers interesting opportunities for biotechnological exploitation. We describe here the transformation of the quorum-sensing system of Agrobacterium tumefaciens into a transcription regulatory system that works in mammalian cells. The A. tumefaciens TraR protein was fused to the eukaryotic activation domain of NF-kappaB p65, generating a novel chimaeric transcriptional activator that stimulates gene transcription in different human cell lines from a minimal promoter containing the TraR DNA recognition sequence in the presence of the Agrobacterium quorum-sensing signal molecule N-(3-oxo-octanoyl)homoserine lactone (3-oxo-C(8)-HSL). The basal level of transcription was low in the absence of 3-oxo-C(8)-HSL, and gene expression was stimulated up to 1,000-fold at a saturating concentration of 3-oxo-C(8)-HSL.

Bacterial Proteins↗

PubMed2006

Quorum sensing and the lifestyle of Yersinia.

Bacterial cell-to-cell communication ('quorum sensing') is mediated by structurally diverse, small diffusible signal molecules which regulate gene expression as a function of cell population density. Many different Gram-negative animal, plant and fish pathogens employ N-acylhomoserine lactones (AHLs) as quorum sensing signal molecules which control diverse physiological processes including bioluminescence, swarming, antibiotic biosynthesis, plasmid conjugal transfer, biofilm development and virulence. AHL-dependent quorum sensing is highly conserved in both pathogenic and non-pathogenic members of the genus Yersinia. Yersinia pseudotuberculosis for example, produces at least eight different AHLs and possesses two homologues of the LuxI family of AHL synthases and two members of the LuxR family of AHL-dependent response regulators. In all Yersinia species so far examined, the genes coding for LuxR and LuxI homologues are characteristically arranged convergently and overlapping. In Y. pseudotuberculosis AHL-dependent quorum sensing is involved in the control of cell aggregation and swimming motility, the latter via the flagellar regulatory cascade. This is also the case for swimming and also swarming motility in Yersinia enterocolitica. Howeverthe role of AHL-dependent quorum sensing in Yersinia pestis remains to be determined.

4-Butyrolactone↗

PubMed1998

A negative regulator mediates quorum-sensing control of exopolysaccharide production in Pantoea stewartii subsp. stewartii.

Classical quorum-sensing (autoinduction) regulation, as exemplified by the lux system of Vibrio fischeri, requires N-acyl homoserine lactone (AHL) signals to stimulate cognate transcriptional activators for the cell density-dependent expression of specific target gene systems. For Pantoea stewartii subsp. stewartii, a bacterial pathogen of sweet corn and maize, the extracellular polysaccharide (EPS) stewartan is a major virulence factor, and its production is controlled by quorum sensing in a population density-dependent manner. Two genes, esaI and esaR, encode essential regulatory proteins for quorum sensing. EsaI is the AHL signal synthase, and EsaR is the cognate gene regulator. esaI, DeltaesaR, and DeltaesaI-esaR mutations were constructed to establish the regulatory role of EsaR. We report here that strains containing an esaR mutation produce high levels of EPS independently of cell density and in the absence of the AHL signal. Our data indicate that quorum-sensing regulation in P. s. subsp. stewartii, in contrast to most other described systems, uses EsaR to repress EPS synthesis at low cell density, and that derepression requires micromolar amounts of AHL. In addition, derepressed esaR strains, which synthesize EPS constitutively at low cell densities, were significantly less virulent than the wild-type parent. This finding suggests that quorum sensing in P. s. subsp. stewartii may be a mechanism to delay the expression of EPS during the early stages of infection so that it does not interfere with other mechanisms of pathogenesis.

4-Butyrolactone↗

PubMed2003

Vibrio harveyi quorum sensing: a coincidence detector for two autoinducers controls gene expression.

In a process called quorum sensing, bacteria communicate with one another by exchanging chemical signals called autoinducers. In the bioluminescent marine bacterium Vibrio harveyi, two different auto inducers (AI-1 and AI-2) regulate light emission. Detection of and response to the V.harveyi autoinducers are accomplished through two two-component sensory relay systems: AI-1 is detected by the sensor LuxN and AI-2 by LuxPQ. Here we further define the V.harveyi quorum-sensing regulon by identifying 10 new quorum-sensing-controlled target genes. Our examination of signal processing and integration in the V.harveyi quorum-sensing circuit suggests that AI-1 and AI-2 act synergistically, and that the V.harveyi quorum-sensing circuit may function exclusively as a 'coincidence detector' that discriminates between conditions in which both autoinducers are present and all other conditions.

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