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

Pseudomonas aeruginosa, Candida albicans, and device-related nosocomial infections: implications, trends, and potential approaches for control

For many years, device-associated infections and particularly device-associated nosocomial infections have been of considerable concern. Recently, this concern was heightened as a result of increased antibiotic resistance among the common causal agents of nosocomial infections, the appearance of new strains which are intrinsically resistant to the antibiotics of choice, and the emerging understanding of the role biofilms may play in device-associated infections and the development of increased antibiotic resistance. Pseudomonas aeruginosa and Candida albicans are consistently identified as some of the more important agents of nosocomial infections. In light of the recent information regarding device-associated nosocomial infections, understanding the nature of P. aeruginosa and C. albicans infections is increasingly important. These two microorganisms demonstrate: (1) an ability to form biofilms on the majority of devices employed currently, (2) increased resistance/tolerance to antibiotics when associated with biofilms, (3) documented infections noted for virtually all indwelling devices, (4) opportunistic pathogenicity, and (5) persistence in the hospital environment. To these five demonstrated characteristics, two additional areas of interest are emerging: (a) the as yet unclear relationship of these two microorganisms to those species of highly resistant Pseudomonas spp and Candida spp that are of increasing concern with device-related infections, and (b) the recent research showing the dynamic interaction of P. aeruginosa and C. albicans in patients with cystic fibrosis. An understanding of these two opportunistic pathogens in the context of their ecosystems/biofilms also has significant potential for the development of novel and effective approaches for the control and treatment of device-associated infections.

Bioinformatics analysis of the locus for enterocyte effacement provides novel insights into type-III secretion

Background

Like many other pathogens, enterohaemorrhagic and enteropathogenic strains of Escherichia coli employ a type-III secretion system to translocate bacterial effector proteins into host cells, where they then disrupt a range of cellular functions. This system is encoded by the locus for enterocyte effacement. Many of the genes within this locus have been assigned names and functions through homology with the better characterised Ysc-Yop system from Yersinia spp. However, the functions and homologies of many LEE genes remain obscure.

Results

We have performed a fresh bioinformatics analysis of the LEE. Using PSI-BLAST we have been able to identify several novel homologies between LEE-encoded and Ysc-Yop-associated proteins: Orf2/YscE, Orf5/YscL, rORF8/EscI, SepQ/YscQ, SepL/YopN-TyeA, CesD2/LcrR. In addition, we highlight homology between EspA and flagellin, and report many new homologues of the chaperone CesT.

Conclusion

We conclude that the vast majority of LEE-encoded proteins do indeed possess homologues and that homology data can be used in combination with experimental data to make fresh functional predictions.

Cell-to-cell signalling in Escherichia coli and Salmonella enterica

Cell-to-cell signalling in prokaryotes that leads to co-ordinated behaviour has been termed quorum sensing. This type of signalling can have profound impacts on microbial community structure and host-microbe interactions. The Gram-negative quorum-sensing systems were first discovered and extensively characterized in the marine Vibrios. Some components of the Vibrio systems are present in the classical genetic model organisms Escherichia coli and Salmonella enterica. Both organisms encode a signal receptor of the LuxR family, SdiA, but not a corresponding signal-generating enzyme. Instead, SdiA of Salmonella detects and responds to signals generated only by other microbial species. Conversely, E. coli and Salmonella encode the signal-generating component of a second system (a LuxS homologue that generates AI-2), but the sensory apparatus for AI-2 differs substantially from the Vibrio system. The only genes currently known to be regulated by AI-2 in Salmonella encode an active uptake and modification system for AI-2. Therefore, it is not yet clear whether Salmonella uses AI-2 as a signal molecule or whether AI-2 has some other function. In E. coli, the functions of both SdiA and AI-2 are unclear due to pleiotropy. Genetic strategies to identify novel signalling systems have been performed with E. coli and Providencia stuartii. Several putative signalling systems have been identified, one that uses indole as a signal and another that releases what appears to be a peptide. The latter system has homologues in E. coli and Salmonella, as well as other bacteria, plants and animals. In fact, the protease components from Providencia and Drosophila are functionally interchangeable.

Chemical signaling among bacteria and its inhibition

Generations of chemists and biologists have conducted research on natural products and other metabolites produced by bacteria and other microorganisms. This has led to an explosion in knowledge concerning the mechanism by which such natural products are made, ultimately allowing custom redesign of many of these molecules for increased potency and selectivity as therapeutic drugs. Along the way, scientists have begun to appreciate that the bacterial world is teeming with life on a scale hardly conceivable, with constant communication within the bacterial world and with outside neighbors, such as plants and mammals. Only in recent years have some of the signaling molecules that comprise these elaborate forms of communication been characterized in any sort of chemical detail, which has in turn peaked interest in the intricate biology of this micro-world and its interactions with the macro-world.

Quorum sensing and swarming migration in bacteria

Bacterial cells can produce and sense signal molecules, allowing the whole population to initiate a concerted action once a critical concentration (corresponding to a particular population density) of the signal has been reached, a phenomenon known as quorum sensing. One of the possible quorum sensing-regulated phenotypes is swarming, a flagella-driven movement of differentiated swarmer cells (hyperflagellated, elongated, multinucleated) by which bacteria can spread as a biofilm over a surface. The glycolipid or lipopeptide biosurfactants thereby produced function as wetting agent by reducing the surface tension. Quorum sensing systems are almost always integrated into other regulatory circuits. This effectively expands the range of environmental signals that influence target gene expression beyond population density. In this review, we first discuss the regulation of AHL-mediated surface migration and the involvement of other low-molecular-mass signal molecules (such as the furanosyl borate diester AI-2) in biosurfactant production of different bacteria. In addition, population density-dependent regulation of swarmer cell differentiation is reviewed. Also, several examples of interspecies signalling are reported. Different signal molecules either produced by bacteria (such as other AHLs and diketopiperazines) or excreted by plants (such as furanones, plant signal mimics) might influence the quorum sensing-regulated swarming behaviour in bacteria different from the producer. On the other hand, specific bacteria can reduce the local available concentration of signal molecules produced by others. In the last part, the role and regulation of a surface-associated movement in biofilm formation is discussed. Here we also describe how quorum sensing may disperse existing biofilms and control the interaction between bacteria and higher organisms (such as the Rhizobium-bean symbiosis).

Quorum sensing: a primer for food microbiologists

Quorum sensing is a signaling mechanism through which bacteria modulate a number of cellular functions (genes), including sporulation, biofilm formation, bacteriocin production, virulence responses, as well as others. Quorum sensing is a mechanism of cell-to-cell communication and is mediated by extracellular chemical signals generated by the bacteria when specific cell densities are reached. When the concentration of the signal (and cell population) is sufficiently high, the target gene or genes are either activated or repressed. Quorum sensing increases the ability of the bacteria to have access to nutrients or to more favorable environmental niches and enhances bacterial defenses against eukaryotic hosts, competing bacteria, and environmental stresses. The physiological and clinical aspects of quorum sensing have received considerable attention and have been studied at the molecular level. Little is known, however, on the role of quorum sensing in food spoilage or in the growth and/or toxin production of pathogens present in food. A number of compounds have been isolated or synthesized that antagonize quorum sensors, and application of these antagonists may potentially be useful in inhibiting the growth or virulence mechanisms of bacteria in different environments, including food. It is important that food microbiologists have an awareness and an understanding of the mechanisms involved in bacterial quorum sensing, since strategies targeting quorum sensing may offer a means to control the growth of undesirable bacteria in foods.

Pathways leading from BarA/SirA to motility and virulence gene expression in Salmonella

The barA and sirA genes of Salmonella enterica serovar Typhimurium encode a two-component sensor kinase and a response regulator, respectively. This system increases the expression of virulence genes and decreases the expression of motility genes. In this study, we examined the pathways by which SirA affects these genes. We found that the master regulator of flagellar genes, flhDC, had a positive regulatory effect on the primary regulator of intestinal virulence determinants, hilA, but that hilA had no effect on flhDC. SirA was able to repress flhDC in a hilA mutant and activate hilA in an flhDC mutant. Therefore, although the flhDC and hilA regulatory cascades interact, sirA affects each of them independently. A form of BarA lacking the two N-terminal membrane-spanning domains, BarA198, autophosphorylates in the presence of ATP and transfers the phosphate to purified SirA. Phosphorylated SirA was found to directly bind the hilA and hilC promoters in gel mobility shift assays but not the flhD, fliA, hilD, and invF promoters. Given that the CsrA/csrB system is known to directly affect flagellar gene expression, we tested the hypothesis that SirA affects flagellar gene expression indirectly by regulating csrA or csrB. The sirA gene did not regulate csrA but did activate csrB expression. Consistent with these results, phosphorylated SirA was found to directly bind the csrB promoter but not the csrA promoter. We propose a model in which SirA directly activates virulence expression via hilA and hilC while repressing the flagellar regulon indirectly via csrB.

Recipient-induced transfer of the symbiotic plasmid pRL1JI in Rhizobium leguminosarum bv. viciae is regulated by a quorum-sensing relay

Analysis of the regulation of plasmid transfer genes on the symbiotic plasmid pRL1JI in Rhizobium leguminosarum bv. viciae has revealed a novel regulatory relay that is specifically poised to detect an N-acyl-homoserine lactone (AHL) made by different cells (potential recipients of pRL1JI). Adjacent to the traI-trbBCDEJKLFGHI plasmid transfer operon on pRL1JI are two regulatory genes, bisR and traR, which encode LuxR-type quorum-sensing regulators required for conjugation. Potential recipients of pRL1JI induce the traI-trb operon and plasmid transfer via a quorum-sensing relay involving BisR, TraR and the traI-trb operon in donor cells. BisR induces expression of traR in response to N-(3-hydroxy-7-cis-tetradecenoyl)-l-homoserine lactone (3-OH-C14:1-HSL), which is produced by CinI in potential recipient strains. In donor strains (carrying pRL1JI), BisR represses the expression of the chromosomal gene cinI; this repression results in a very low level of formation of 3-OH-C14:1-HSL and hence relatively low levels of expression of traR and the traI-trb operon in strains carrying pRL1JI. However, if 3-OH-C14:1-HSL from potential recipients is present, then traR and plasmid transfer are induced. The induction of traR occurs at very low concentrations of 3-OH-C14:1-HSL (around 1 nm). TraR then induces the traI-trb operon in a quorum-sensing dependent manner in re-sponse to the TraI-made AHLs, N-(3-oxo-octanoyl)-l-homoserine lactone and N-(octanoyl)-l-homoserine lactone. The resulting autoinduction results in high levels of expression of the traI-trb operon. Premature expression of the traI-trb operon is reduced by TraM, which probably titres out TraR preventing expression of traI when there are low levels of traR expression. Expression of traR in stationary phase cells is limited by feedback inhibition mediated by TraI-made AHLs.

Control of rpoS transcription in Escherichia coli and Pseudomonas: why so different?

In Escherichia coli, the stationary phase alternative sigma factor sigmas controls the expression of genes involved cell survival in response to cessation of growth (stationary phase) and provides cross-protection to various stresses. Levels of sigmas increase dramatically at the onset of stationary phase and are regulated at the transcriptional, post-transcriptional and post-translational level, making this one of the most complex regulatory systems in bacteria. The basic mechanisms for the control of translation and sigmas proteolysis have been understood. However, studies on the transcriptional control in E. coli lag behind and are controversial. The cAMP-CRP complex and the two component BarA/UvrY system have been implicated and, ppGpp and polyphosphate appear to have a signalling role. sigmas has also been reported to be a general stress regulator in the fluorescent pseudomonads (Pseudomonas aeruginosa, P. fluorescens and P. putida) and recent studies on sigmas regulation highlight that transcriptional regulation in these bacteria apparently plays a major role. Global regulatory systems, the GacA/GacS two component system and quorum sensing all affect rpoS expression, as does the TetR family PsrA regulator that directly binds to- and activates the rpoS promoter in stationary phase. This striking difference in regulation between E. coli and Pseudomonas can be partly attributed to the differences in the functional role of sigmas in the two bacterial species. This report will review mainly recent studies on rpoS transcriptional regulation and will try to rationalize the current knowledge into a working model.

Production of the signalling molecule, autoinducer-2, by Neisseria meningitidis: lack of evidence for a concerted transcriptional response

Neisseria meningitidis is a Gram-negative bacterium which is an important causative agent of septicaemia and meningitis. LuxS has been shown to be involved in the biosynthesis of a quorum sensing molecule, autoinducer-2 (AI-2), known to play a role in virulence in Escherichia coli, as well as other bacteria. Evidence that serogroup B of N. meningitidis produces AI-2, along with the observation that a luxS mutant of this strain had attenuated virulence in an infant rat model of bacteraemia, led to further investigation of the role of this quorum sensing molecule in N. meningitidis. In this study, it is demonstrated that AI-2 is not involved in regulating growth of meningococci, either in culture or in contact with epithelial cells. Furthermore, transcriptional profiling using DNA microarrays shows an absence of the concerted regulation seen in other bacteria. Taken together, these data suggest that in N. meningitidis, AI-2 may be a metabolic by-product and not a cell-to-cell signalling molecule.

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 in plant-pathogenic bacteria

Quorum sensing (QS) allows bacteria to assess their local population density and/or physical confinement via the secretion and detection of small, diffusible signal molecules. This review describes how phytopathogenic bacteria have incorporated QS mechanisms into complex regulatory cascades that control genes for pathogenicity and colonization of host surfaces. Traits regulated by QS include the production of extracellular polysaccharides, degradative enzymes, antibiotics, siderophores, and pigments, as well as Hrp protein secretion, Ti plasmid transfer, motility, biofilm formation, and epiphytic fitness. Since QS regulatory systems are often required for pathogenesis, interference with QS signaling may offer a means of controlling bacterial diseases of plants. Several bacterial pathogens of plants that have been intensively studied and have revealed information of both fundamental and practical importance are reviewed here: Agrobacterium tumefaciens, Pantoea stewartii, Erwinia carotovora, Ralstonia solanacearum, Pseudomonas syringae, Pseudomonas aeruginosa, and Xanthomonas campestris.

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.

Bacterial autoinduction: looking outside the cell for new metabolic engineering targets

Recent evidence has demonstrated that cell-to-cell signaling is a fundamental activity carried out by numerous microorganisms. A number of specialized processes are reported to be regulated by density-dependent signaling molecules including antibiotic production, bioluminescence, biofilm formation, genetic competence, sporulation, swarming motility and virulence. However, a more centralized role for quorum sensing is emerging where quorum signaling pathways overlap with stress and starvation circuits to regulate cellular adaptation to changing environmental conditions. The interplay of these phenomena is especially critical in the expression of recombinant proteins where elicitation of stress responses can dramatically impact cellular productivity.

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

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

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.

Regulatory circuitry of the CsrA/CsrB and BarA/UvrY systems of Escherichia coli

The global regulator CsrA (carbon storage regulator) is an RNA binding protein that coordinates central carbon metabolism, activates flagellum biosynthesis and motility, and represses biofilm formation in Escherichia coli. CsrA activity is antagonized by the untranslated RNA CsrB, to which it binds and forms a globular ribonucleoprotein complex. CsrA indirectly activates csrB transcription, in an apparent autoregulatory mechanism. In the present study, we elucidate the intermediate regulatory circuitry of this system. Mutations affecting the BarA/UvrY two-component signal transduction system decreased csrB transcription but did not affect csrA'-'lacZ expression. The uvrY defect was severalfold more severe than that of barA. Both csrA and uvrY were required for optimal barA expression. The latter observation suggests an autoregulatory loop for UvrY. Ectopic expression of uvrY suppressed the csrB-lacZ expression defects caused by uvrY, csrA, or barA mutations; csrA suppressed csrA or barA defects; and barA complemented only the barA mutation. Purified UvrY protein stimulated csrB-lacZ expression approximately sixfold in S-30 transcription-translation reactions, revealing a direct effect of UvrY on csrB transcription. Disruption of sdiA, which encodes a LuxR homologue, decreased the expression of uvrY'-'lacZ and csrB-lacZ fusions but did not affect csrA'-'lacZ. The BarA/UvrY system activated biofilm formation. Ectopic expression of uvrY stimulated biofilm formation by a csrB-null mutant, indicative of a CsrB-independent role for UvrY in biofilm development. Collectively, these results demonstrate that uvrY resides downstream from csrA in a signaling pathway for csrB and that CsrA stimulates UvrY-dependent activation of csrB expression by BarA-dependent and -independent mechanisms.

Quorum sensing in plant-associated bacteria

N-acyl homoserine lactone (AHL)-mediated quorum sensing by bacteria regulates traits that are involved in symbiotic, pathogenic and surface-associated relationships between microbial populations and their plant hosts. Recent advances demonstrate deviations from the classic LuxR/LuxI paradigm, which was first developed in Vibrio. For example, LuxR homologs can repress as well as activate gene expression, and non-AHL signals and signal mimics can affect the expression of genes that are controlled by quorum sensing. Many bacteria utilize multiple quorum-sensing systems, and these may be modulated via post-transcriptional and other global regulatory mechanisms. Microbes inhabiting plant surfaces also produce and respond to a diverse mixture of AHL signals. The production of AHL mimics by plants and the identification of AHL degradative pathways suggest that bacteria and plants utilize this method of bacterial communication as a key control point for influencing the outcome of their interactions.

RsmA and the quorum-sensing signal, N-[3-oxohexanoyl]-L-homoserine lactone, control the levels of rsmB RNA in Erwinia carotovora subsp. carotovora by affecting its stability

RsmA (for regulator of secondary metabolism), RsmC, and rsmB RNA, the components of a posttranscriptional regulatory system, control extracellular protein production and pathogenicity in Erwinia carotovora subsp. carotovora. RsmA, an RNA binding protein, acts as a negative regulator by promoting message decay. rsmB RNA, on the other hand, acts as a positive regulator by neutralizing the effect of RsmA. RsmC modulates the levels of RsmA and rsmB RNA by positively regulating rsmA and negatively controlling rsmB. The level of rsmB RNA is substantially higher in RsmA(+) bacteria than in RsmA(-) mutants. We show that rsmB RNA is more stable in the presence of RsmA than in its absence. RsmA does not stimulate the expression of an rsmB-lacZ transcriptional fusion; in fact, the beta-galactosidase level is somewhat higher in RsmA(-) bacteria than in RsmA(+) bacteria. We also investigated the basis for increased levels of rsmA and rsmB RNAs in the absence of the quorum-sensing signal, N-[3-oxohexanoyl]-L-homoserine lactone (OHL). The absence of OHL activates transcription of rsmA but not of rsmB. Instead, increased stability of rsmB RNA in the presence of RsmA accounts for the elevated levels of the rsmB RNA in OHL(-) bacteria. Mutant studies disclosed that while RsmA, OHL, and RsmC control the levels of rsmB RNA, high levels of rsmB RNA occur in the absence of RsmC or OHL only in RsmA(+) bacteria, indicating a critical role for RsmA in modulating the levels of rsmB RNA. The findings reported here firmly establish that the quorum-sensing signal is channeled in E. carotovora subsp. carotovora via the rsmA-rsmB posttranscriptional regulatory system.

Quorum sensing: an emerging target for antibacterial chemotherapy?

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

Disruption of bacterial quorum sensing by other organisms

Higher plants and algae produce compounds that mimic quorum sensing: signals used by bacteria to regulate the expression of many genes and behaviors. Similarly, various bacteria can stimulate, inhibit or inactivate quorum sensing in other bacteria. These discoveries offer new opportunities to manipulate bacterial quorum sensing in applications relevant to medicine, agriculture and the environment.