Modulation of luminescence operon expression by N-octanoyl-L-homoserine lactone in ainS mutants of Vibrio fischeri

Crosstalk enables mutual activation of coupled quorum sensing pathways through "jump-start" and "push-start" mechanisms

Many quorum sensing microbes produce more than one chemical signal and detect them using interconnected pathways that crosstalk with each other. While there are many hypotheses for the advantages of sensing multiple signals, the prevalence and functional significance of crosstalk between pathways are much less understood. We explore the effect of intracellular signal crosstalk using a simple model that captures key features of typical quorum sensing pathways: multiple pathways in a hierarchical configuration, operating with positive feedback, with crosstalk at the receptor and promoter levels. We find that crosstalk enables activation or inhibition of one output by the non-cognate signal, broadens the dynamic range of the outputs, and allows one pathway to modulate the feedback circuit of the other. Our findings show how crosstalk between quorum sensing pathways can be viewed not as a detriment to the processing of information, but as a mechanism that enhances the functional range of the full regulatory system. When positive feedback systems are coupled through crosstalk, several new modes of activation or deactivation become possible.

Differential Equations Models to Study Quorum Sensing

Mathematical models to study quorum sensing (QS) have become an important tool to explore all aspects of this type of bacterial communication. A wide spectrum of mathematical tools and methods such as dynamical systems, stochastics, and spatial models can be employed. In this chapter, we focus on giving an overview of models consisting of differential equations (DE), which can be used to describe changing quantities, for example, the dynamics of one or more signaling molecule in time and space, often in conjunction with bacterial growth dynamics. The chapter is divided into two sections: ordinary differential equations (ODE) and partial differential equations (PDE) models of QS. Rates of change are represented mathematically by derivatives, i.e., in terms of DE. ODE models allow describing changes in one independent variable, for example, time. PDE models can be used to follow changes in more than one independent variable, for example, time and space. Both types of models often consist of systems (i.e., more than one equation) of equations, such as equations for bacterial growth and autoinducer concentration dynamics. Almost from the onset, mathematical modeling of QS using differential equations has been an interdisciplinary endeavor and many of the works we revised here will be placed into their biological context.

Comparative analysis reveals regulatory motifs at the ainS/ainR pheromone-signaling locus of Vibrio fischeri

Vibrio fischeri uses the AinS/AinR pheromone-signaling system to control bioluminescence and other symbiotic colonization factors. The Ain system is thought to initiate cell-cell signaling at moderate cell densities and to prime the LuxI/LuxR signaling system. Here we compared and analyzed the ain locus from two V. fischeri strains and a Vibrio salmonicida strain to explore ain regulation. The ainS and ainR genes were predicted to constitute an operon, which we corroborated using RT-PCR. Comparisons between strains revealed a stark area of conservation across the ainS-ainR junction, including a large inverted repeat in ainR. We found that this inverted repeat in cis can affect accumulation of the AinS-generated pheromone N-octanoyl homoserine lactone, which may account for the previously unexplained low-signal phenotype of a ∆ainR mutant, although the mechanism behind this regulation remains elusive. We also extended the previous observation of a possible “lux box” LuxR binding site upstream of ainS by showing the conservation of this site as well as a second putative lux box. Using a plasmid-based reporter we found that LuxR can mediate repression of ainS, providing a negative feedback mechanism in the Ain/Lux signaling cascade. Our results provide new insights into the regulation, expression, and evolution of ainSR.

Host-selected mutations converging on a global regulator drive an adaptive leap towards symbiosis in bacteria

Host immune and physical barriers protect against pathogens but also impede the establishment of essential symbiotic partnerships. To reveal mechanisms by which beneficial organisms adapt to circumvent host defenses, we experimentally evolved ecologically distinct bioluminescent Vibrio fischeri by colonization and growth within the light organs of the squid Euprymna scolopes. Serial squid passaging of bacteria produced eight distinct mutations in the binK sensor kinase gene, which conferred an exceptional selective advantage that could be demonstrated through both empirical and theoretical analysis. Squid-adaptive binK alleles promoted colonization and immune evasion that were mediated by cell-associated matrices including symbiotic polysaccharide (Syp) and cellulose. binK variation also altered quorum sensing, raising the threshold for luminescence induction. Preexisting coordinated regulation of symbiosis traits by BinK presented an efficient solution where altered BinK function was the key to unlock multiple colonization barriers. These results identify a genetic basis for microbial adaptability and underscore the importance of hosts as selective agents that shape emergent symbiont populations.

Specificity and complexity in bacterial quorum-sensing systems

Quorum sensing (QS) is a microbial cell-to-cell communication process that relies on the production and detection of chemical signals called autoinducers (AIs) to monitor cell density and species complexity in the population. QS allows bacteria to behave as a cohesive group and coordinate collective behaviors. While most QS receptors display high specificity to their AI ligands, others are quite promiscuous in signal detection. How do specific QS receptors respond to their cognate signals with high fidelity? Why do some receptors maintain low signal recognition specificity? In addition, many QS systems are composed of multiple intersecting signaling pathways: what are the benefits of preserving such a complex signaling network when a simple linear ‘one-to-one’ regulatory pathway seems sufficient to monitor cell density? Here, we will discuss different molecular mechanisms employed by various QS systems that ensure productive and specific QS responses. Moreover, the network architectures of some well-characterized QS circuits will be reviewed to understand how the wiring of different regulatory components achieves different biological goals.

This review focuses on the specificity and complexity of quorum-sensing circuits in both Gram-negative and Gram-positive bacterial species.

Time-dependent hormesis of chemical mixtures: A case study on sulfa antibiotics and a quorum-sensing inhibitor of Vibrio fischeri

Sulfa antibiotics (SAs) and quorum-sensing inhibitor (QSI) may pose potential ecological risks because mixed using of them has been proposed to inhibit bacteria from generating antibiotic resistance. This study investigated the time-dependent hormesis of single and binary mixtures of QSI and SAs of Vibrio fischeri (V. fischeri) for 0-24 h. Although the low-dose SAs stimulated the expression of LuxR protein, the high-dose SAs could inhibit bacteria growth by competitively binding to dihydropteroate synthase. Moreover, AinR protein was bound to Benzofuran-3(2H)-one (B3O) with low concentration, thus the N-octanoyl homoserine lactone signal molecules (C8) has chance to bind to LuxR protein to promote light emission. The hormesis effect induced by the mixtures could be deduced that SAs promoted the expression of LuxR protein and B3O increases the chance of C8 binding to LuxR. Our findings facilitate new insight into the mechanistic study of hormesis and ecological risks of the chemical mixtures.

Modeling Analysis of Signal Sensitivity and Specificity by Vibrio fischeri LuxR Variants

The LuxR protein of the bacterium Vibrio fischeri belongs to a family of transcriptional activators that underlie pheromone-mediated signaling by responding to acyl-homoserine lactones (-HSLs) or related molecules. V. fischeri produces two acyl-HSLs, N-3-oxo-hexanoyl-HSL (3OC6-HSL) and N-octanoyl-HSL (C8-HSL), each of which interact with LuxR to facilitate its binding to a "lux box" DNA sequence, thereby enabling LuxR to activate transcription of the lux operon responsible for bioluminescence. We have investigated the HSL sensitivity of four different variants of V. fischeri LuxR: two derived from wild-type strains ES114 and MJ1, and two derivatives of LuxRMJ1 generated by directed evolution. For each LuxR variant, we measured the bioluminescence induced by combinations of C8-HSL and 3OC6-HSL. We fit these data to a model in which the two HSLs compete with each other to form multimeric LuxR complexes that directly interact with lux to activate bioluminescence. The model reproduces the observed effects of HSL combinations on the bioluminescence responses directed by LuxR variants, including competition and non-monotonic responses to C8-HSL and 3OC6-HSL. The analysis yields robust estimates for the underlying dissociation constants and cooperativities (Hill coefficients) of the LuxR-HSL complexes and their affinities for the lux box. It also reveals significant differences in the affinities of LuxRMJ1 and LuxRES114 for 3OC6-HSL. Further, LuxRMJ1 and LuxRES114 differed sharply from LuxRs retrieved by directed evolution in the cooperativity of LuxR-HSL complex formation and the affinity of these complexes for lux. These results show how computational modeling of in vivo experimental data can provide insight into the mechanistic consequences of directed evolution.

The autoinducer synthases LuxI and AinS are responsible for temperature-dependent AHL production in the fish pathogen Aliivibrio salmonicida

Background

Quorum sensing (QS) is a cell-to-cell communication system used by bacteria to regulate activities such as virulence, bioluminescence and biofilm formation. The most common QS signals in Gram-negative bacteria are N-acyl-homoserine lactones (AHLs). Aliivibrio salmonicida is the etiological agent of cold water vibriosis in Atlantic salmon, a disease which occurs mainly during seasons when the seawater is below 12°C. In this work we have constructed several mutants of A. salmonicida LFI1238 in order to study the LuxI/LuxR and AinS/AinR QS systems with respect to AHL production and biofilm formation.

Results

Using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) we found that LuxI in A. salmonicida LFI1238 is responsible for producing seven of the different AHLs, whereas AinS is responsible for producing only one. The production of these various AHLs is dependent on both cell density and growth temperature. The AHLs were efficiently produced when wild type LFI1238 was grown at 6 or 12°C, however at 16°C AHL production decreased dramatically, and LFI1238 produced less than 5% of the maximum concentrations observed at 6°C. LitR, the master regulator of QS, was found to be a positive regulator of AinS-dependent AHL production, and to a lesser extent LuxI-dependent AHL production. This implies a connection between the two systems, and both systems were found to be involved in regulation of biofilm formation. Finally, inactivation of either luxR1 or luxR2 in the lux operon significantly reduced production of LuxI-produced AHLs.

Conclusion

LuxI and AinS are the autoinducer synthases responsible for the eight AHLs in A. salmonicida. AHL production is highly dependent on growth temperature, and a significant decrease was observed when the bacterium was grown at a temperature above its limit for disease outbreak. Numerous AHLs could offer the opportunity for fine-tuning responses to changes in the environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0402-z) contains supplementary material, which is available to authorized users.

Substrate specificity and function of the pheromone receptor AinR in Vibrio fischeri ES114

Two distinct but interrelated pheromone-signaling systems, LuxI/LuxR and AinS/AinR, positively control bioluminescence in Vibrio fischeri. Although each system generates an acyl-homoserine lactone (AHL) signal, the protein sequences of LuxI/LuxR and AinS/AinR are unrelated. AinS and LuxI generate the pheromones N-octanoyl-AHL (C8-AHL) and N-3-oxo-hexanoyl-AHL (3OC6-AHL), respectively. LuxR is a transcriptional activator that responds to 3OC6-AHL, and to a lesser extent to C8-AHL. AinR is hypothesized to respond to C8-AHL and, based on homology to Vibrio harveyi LuxN, to mediate the repression of a Qrr regulatory RNA. However, a ΔainR mutation decreased luminescence, which was not predicted based on V. harveyi LuxN, raising the possibility of a distinct regulatory mechanism for AinR. Here we show that ainR can complement a luxN mutant, suggesting functional similarity. Moreover, in V. fischeri, we observed ainR-dependent repression of a Pqrr-lacZ transcriptional reporter in the presence of C8-AHL, consistent with its hypothesized regulatory role. The system appears quite sensitive, with a half-maximal effect on a Pqrr reporter at 140 pM C8-AHL. Several other AHLs with substituted and unsubstituted acyl chains between 6 and 10 carbons also displayed an AinR-dependent effect on Pqrr-lacZ; however, AHLs with acyl chains of four carbons or 12 or more carbons lacked activity. Interestingly, 3OC6-AHL also affected expression from the qrr promoter, but this effect was largely luxR dependent, indicating a previously unknown connection between these systems. Finally, we propose a preliminary explanation for the unexpected luminescence phenotype of the ΔainR mutant.

Cyclic AMP receptor protein regulates pheromone-mediated bioluminescence at multiple levels in Vibrio fischeri ES114

Bioluminescence in Vibrio fischeri ES114 is activated by autoinducer pheromones, and this regulation serves as a model for bacterial cell-cell signaling. As in other bacteria, pheromone concentration increases with cell density; however, pheromone synthesis and perception are also modulated in response to environmental stimuli. Previous studies suggested that expression of the pheromone-dependent bioluminescence activator LuxR is regulated in response to glucose by cyclic AMP (cAMP) receptor protein (CRP) (P. V. Dunlap and E. P. Greenberg, J. Bacteriol. 164:45-50, 1985; P. V. Dunlap and E. P. Greenberg, J. Bacteriol. 170:4040-4046, 1988; P. V. Dunlap, J. Bacteriol. 171:1199-1202, 1989; and W. F. Friedrich and E. P. Greenberg, Arch. Microbiol. 134:87-91, 1983). Consistent with this model, we found that bioluminescence in V. fischeri ES114 is modulated by glucose and stimulated by cAMP. In addition, a Δcrp mutant was ∼100-fold dimmer than ES114 and did not increase luminescence in response to added cAMP, even though cells lacking crp were still metabolically capable of producing luminescence. We further discovered that CRP regulates not only luxR but also the alternative pheromone synthase gene ainS. We found that His-tagged V. fischeri CRP could bind sequences upstream of both luxR and ainS, supporting bioinformatic predictions of direct regulation at both promoters. Luminescence increased in response to cAMP if either the ainS or luxR system was under native regulation, suggesting cAMP-CRP significantly increases luminescence through both systems. Finally, using transcriptional reporters in transgenic Escherichia coli, we elucidated two additional regulatory connections. First, LuxR-independent basal transcription of the luxI promoter was enhanced by CRP. Second, the effect of CRP on the ainS promoter depended on whether the V. fischeri regulatory gene litR was also introduced. These results suggest an integral role for CRP in pheromone signaling that goes beyond sensing cell density.

Quorum sensing in the squid-Vibrio symbiosis

Quorum sensing is an intercellular form of communication that bacteria use to coordinate group behaviors such as biofilm formation and the production of antibiotics and virulence factors. The term quorum sensing was originally coined to describe the mechanism underlying the onset of luminescence production in cultures of the marine bacterium Vibrio fischeri. Luminescence and, more generally, quorum sensing are important for V. fischeri to form a mutualistic symbiosis with the Hawaiian bobtail squid, Euprymna scolopes. The symbiosis is established when V. fischeri cells migrate via flagella-based motility from the surrounding seawater into a specialized structure injuvenile squid called the light organ. The cells grow to high cell densities within the light organ where the infection persists over the lifetime of the animal. A hallmark of a successful symbiosis is the luminescence produced by V. fischeri that camouflages the squid at night by eliminating its shadow within the water column. While the regulatory networks governing quorum sensing are critical for properly regulating V. fischeri luminescence within the squid light organ, they also regulate luminescence-independent processes during symbiosis. In this review, we discuss the quorum-sensing network of V. fischeri and highlight its impact at various stages during host colonization.

The iron-dependent regulator fur controls pheromone signaling systems and luminescence in the squid symbiont Vibrio fischeri ES114

Bacteria often use pheromones to coordinate group behaviors in specific environments. While high cell density is required for pheromones to achieve stimulatory levels, environmental cues can also influence pheromone accumulation and signaling. For the squid symbiont Vibrio fischeri ES114, bioluminescence requires pheromone-mediated regulation, and this signaling is induced in the host to a greater extent than in culture, even at an equivalent cell density. Our goal is to better understand this environment-specific control over pheromone signaling and bioluminescence. Previous work with V. fischeri MJ1 showed that iron limitation induces luminescence, and we recently found that ES114 encounters a low-iron environment in its host. Here we show that ES114 induces luminescence at lower cell density and achieves brighter luminescence in low-iron media. This iron-dependent effect on luminescence required ferric uptake regulator (Fur), which we propose influences two pheromone signaling master regulators, LitR and LuxR. Genetic and bioinformatic analyses suggested that under low-iron conditions, Fur-mediated repression of litR is relieved, enabling more LitR to perform its established role as an activator of luxR. Interestingly, Fur may similarly control the LitR homolog SmcR of Vibrio vulnificus. These results reveal an intriguing regulatory link between low-iron conditions, which are often encountered in host tissues, and pheromone-dependent master regulators.

LuxU connects quorum sensing to biofilm formation in Vibrio fischeri

Biofilm formation by Vibrio fischeri is a complex process involving multiple regulators, including the sensor kinase (SK) RscS and the response regulator (RR) SypG, which control the symbiosis polysaccharide (syp) locus. To identify other regulators of biofilm formation in V. fischeri, we screened a transposon library for mutants defective in wrinkled colony formation. We identified LuxQ as a positive regulator of syp-dependent biofilm formation. LuxQ is a member of the Lux phosphorelay and is predicted to control bioluminescence in concert with the SK AinR, the phosphotransferase LuxU and the RR LuxO. Of these, LuxU was the only other regulator that exerted a substantial impact on biofilm formation. We propose a model in which the Lux pathway branches at LuxU to control both bioluminescence and biofilm formation. Furthermore, our evidence suggests that LuxU functions to regulate syp transcription, likely by controlling SypG activity. Finally, we found that, in contrast to its predicted function, the SK AinR has little impact on bioluminescence under our conditions. Thus, this study reveals a novel connection between the Lux and Syp pathways in V. fischeri, and furthers our understanding of how the Lux pathway regulates bioluminescence in this organism.

Influence of cations and anions on the induction of cell density-independent luminescence in Photorhabdus luminescens

Bioluminescence is emitted by various living organisms, including bacteria. While the induction mechanism in marine luminescent bacteria, such as Vibrio fischeri and V. harveyi, has been well characterized, this mechanism has not been studied in detail in the non-marine luminescent bacterium Photorhabdus luminescens. Therefore, we investigated the effect of cations and anions on the induction of luminescence by P. luminescens. Cultivation of cells in an inorganic salts solution (ISS) containing KCl, CaCl2 , MgCl2 , NaHCO3 , and MgSO4 resulted in a rapid increase in luminescence intensity. Moreover, the induction of luminescence in the ISS medium was not dependent on cell density, since cell densities remained unchanged during 48 h. Furthermore, we found that compounds containing K(+) , Mg(2+) , and HCO3(-) were necessary to induce cell density-independent luminescence. The intensity of luminescence per cell cultured in medium containing KCl, MgCl2 , and NaHCO3 was approximately 100-fold higher than that cultured in NB. In contrast, when cells actively grew in normal growth condition, the intensity of luminescence per cell was not increased even in the presence of K(+) , Mg(2+) , and HCO3(-) . Thus, these results suggest that the luminescence of P. luminescens is regulated by 2 independent cell density-dependent and -independent mechanisms.

Noise and crosstalk in two quorum-sensing inputs of Vibrio fischeri

Background

One of the puzzles in bacterial quorum sensing is understanding how an organism integrates the information gained from multiple input signals. The marine bacterium Vibrio fischeri regulates its bioluminescence through a quorum sensing mechanism that receives input from three pheromone signals, including two acyl homoserine lactone (HSL) signals. While the role of the 3-oxo-C6 homoserine lactone (3OC6HSL) signal in activating the lux genes has been extensively studied and modeled, the role of the C8 homoserine lactone (C8HSL) is less obvious, as it can either activate luminescence or block its activation. It remains unclear how crosstalk between C8HSL and 3OC6HSL affects the information that the bacterium obtains through quorum sensing.

Results

We have used microfluidic methods to measure the response of individual V.fischeri cells to combinations of C8HSL and 3OC6HSL. By measuring the fluorescence of individual V.fischeri cells containing a chromosomal gfp-reporter for the lux genes, we study how combinations of exogenous HSLs affect both the population average and the cell-to-cell variability of lux activation levels. At the level of a population average, the crosstalk between the C8HSL and 3OC6HSL inputs is well-described by a competitive inhibition model. At the level of individual cells, the heterogeneity in the lux response depends only on the average degree of activation, so that the noise in the output is not reduced by the presence of the second HSL signal. Overall we find that the mutual information between the signal inputs and the lux output is less than one bit. A nonlinear correlation between fluorescence and bioluminescence outputs from lux leads to different noise properties for these reporters.

Conclusions

The lux genes in V.fischeri do not appear to distinguish between the two HSL inputs, and even with two signal inputs the regulation of lux is extremely noisy. Hence the role of crosstalk from the C8HSL input may not be to improve sensing precision, but rather to suppress the sensitivity of the switch for as long as possible during colony growth.

Quorum sensing and Bacterial Pathogenicity: From Molecules to Disease

Quorum sensing in prokaryotic biology refers to the ability of a bacterium to sense information from other cells in the population when they reach a critical concentration (i.e. a Quorum) and communicate with them. The “language” used for this intercellular communication is based on small, self-generated signal molecules called as autoinducers. Quorum sensing is thought to afford pathogenic bacteriaa mechanism to minimize host immune responses by delaying theproduction of tissue-damaging virulence factors until sufficientbacteria have amassed and are prepared to overwhelm host defensemechanisms and establish infection. Quorum sensing systems are studied in a large number of gram-negative bacterial species belonging to α, β, and γ subclasses of proteobacteria. Among the pathogenic bacteria, Pseudomonas aeruginosa is perhaps the best understood in terms of the virulence factors regulated and the role the Quorum sensing plays in pathogenicity. Presently, Quorum sensing is considered as a potential novel target for antimicrobial therapy to control multi/all drug-resistant infections. This paper reviews Quorum sensing in gram positive and gram negative bacteria and its role in biofilm formation.

Requirements for sulfur in cell density-independent induction of luminescence in Vibrio fischeri under nutrient-starved conditions

Despite the universal requirement for sulfur in living organisms, it is not known whether the luminescence of Vibrio fischeri is sulfur-dependent and how sulfur affects the intensity of its luminescence. In this study, we investigated the requirement for sulfur in V. fischeri luminescence under nutrient-starved conditions. Full induction of V. fischeri luminescence required MgSO(4); in artificial seawater cultures that lacked sufficient MgSO(4), its luminescence was not fully induced. This induction of luminescence was not dependent on autoinduction because the cell density of V. fischeri did not reach the critical threshold concentration. In addition to MgSO(4), this cell density-independent luminescence was induced or maintained by nontoxic concentrations of l-cysteine, sulfate, sulfite, and thiosulfate. Moreover, the addition of N -3-oxo-hexanoyl homoserine lactone and N -octanoyl homoserine lactone, which are known autoinducers in V. fischeri, did not induce luminescence under these conditions. This result suggested that the underlying mechanism of luminescence may be different from the known autoinduction mechanism.

Characterization of quorum sensing signals in coral-associated bacteria

Marine environment habitats, such as the coral mucus layer, are abundant in nutrients and rich with diverse populations of microorganisms. Since interactions among microorganisms found in coral mucus can be either mutualistic or competitive, understanding quorum sensing-based acyl homoserine lactone (AHL) language may shed light on the interaction between coral-associated microbial communities in the native host. More than 100 bacterial isolates obtained from different coral species were screened for their ability to produce AHL. When screening the isolated coral bacteria for AHL induction activity using the reporter strains Escherichia coli K802NR-pSB1075 and Agrobacterium tumefaciens KYC55, we found that approximately 30% of the isolates tested positive. Thin layer chromatography separation of supernatant extracts revealed different AHL profiles, with detection of at least one active compound in the supernatant of those bacterial extracts being able to induce AHL activity in the two different bioreporter strains. The active extract of bacterial isolate 3AT 1-10-4 was subjected to further analysis by preparative thin layer chromatography and liquid chromatography tandem mass spectrometry. One of the compounds was found to correspond with N-(3-hydroxydecanoyl)-L-homoserine lactone. 16S rRNA gene sequencing of the isolates with positive AHL activity affiliated them with the Vibrio genus. Understanding the ecological role of AHL in the coral environment and its regulatory circuits in the coral holobiont-associated microbial community will further expand our knowledge of such interactions.

Extraction, purification and identification of bacterial signal molecules based on N-acyl homoserine lactones

Bacteria possess an extraordinary repertoire for intercellular communication and social behaviour. This repertoire for bacterial communication, termed as quorum sensing (QS), depends on specific diffusible signal molecules. There are many different kinds of signal molecules in the bacterial community. Among those signal molecules, N‐acyl homoserine lactones (HSLs, in other publications also referred to as AHLs, acy‐HSLs etc.) are often employed as QS signal molecules for many Gram‐negative bacteria. Due to the specific structure and tiny amount of those HSL signal molecules, the characterization of HSLs has been the subject of extensive investigations in the last decades and has become a paradigm for bacteria intercellular signalling. In this article, different methods, including extraction, purification and characterization of HSLs, are reviewed. The review provides an insight into identification and characterization of new HSLs and other signal molecules for bacterial intercellular communication.

Bacterial quorum-sensing network architectures

Quorum sensing is a cell-cell communication process in which bacteria use the production and detection of extracellular chemicals called autoinducers to monitor cell population density. Quorum sensing allows bacteria to synchronize the gene expression of the group, and thus act in unison. Here, we review the mechanisms involved in quorum sensing with a focus on the Vibrio harveyi and Vibrio cholerae quorum-sensing systems. We discuss the differences between these two quorum-sensing systems and the differences between them and other paradigmatic bacterial signal transduction systems. We argue that the Vibrio quorum-sensing systems are optimally designed to precisely translate extracellular autoinducer information into internal changes in gene expression. We describe how studies of the V. harveyi and V. cholerae quorum-sensing systems have revealed some of the fundamental mechanisms underpinning the evolution of collective behaviors.

Interplay of two quorum sensing regulation systems of Vibrio fischeri

Many bacteria developed a possibility to recognise aspects of their environment or to communicate with each other by chemical signals. An important strategy is the so-called quorum sensing (QS), a regulatory mechanism for the gene expression, where the bacteria measure their own cell density by means of this signalling pathway. One of the best-studied species using QS is the marine luminescent bacterium Vibrio fischeri which is considered here as a model organism. The two main regulatory pathways (lux and ain) are combined to a regulation system, the dynamics is modelled by an ODE system. This system is analysed thoroughly, considering stationary states, dynamical behaviour and the possible biological meaning of it. The influence of different parameter values on the behaviour is examined, the same basic system is able to reflect the peculiarities of different bacteria strains (respectively, their mutants).

Pleiotropic role of quorum-sensing autoinducer 2 in Photorhabdus luminescens

Bacterial virulence is an integrative process that may involve quorum sensing. In this work, we compared by global expression profiling the wild-type entomopathogenic Photorhabdus luminescens subsp. laumondii TT01 to a luxS-deficient mutant unable to synthesize the type 2 quorum-sensing inducer AI-2. AI-2 was shown to regulate more than 300 targets involved in most compartments and metabolic pathways of the cell. AI-2 is located high in the hierarchy, as it controls the expression of several transcriptional regulators. The regulatory effect of AI-2 appeared to be dose dependent. The luxS-deficient strain exhibited decreased biofilm formation and increased type IV/V pilus-dependent twitching motility. AI-2 activated its own synthesis and transport. It also modulated bioluminescence by regulating the synthesis of spermidine. AI-2 was further shown to increase oxidative stress resistance, which is necessary to overcome part of the innate immune response of the host insect involving reactive oxygen species. Finally, we showed that the luxS-deficient strain had attenuated virulence against the lepidopteran Spodoptera littoralis. We concluded that AI-2 is involved mainly in early steps of insect invasion in P. luminescens.

Direct quantification of N-(3-oxo-hexanoyl)-L-homoserine lactone in culture supernatant using a whole-cell bioreporter

The autoinducer N-(3-oxo-hexanoyl)-L-homoserine lactone (3-oxo-C6-HSL) plays a significant role in the quorum-sensing system of the marine bacterium Vibrio fischeri. Upon forming a transcriptional activation complex with LuxR, 3-oxo-C6-HSL induces transcription of the luxICDABEG operon, leading to the increased production of both the 3-oxo-C6-HSL synthase (LuxI) and the bioluminescent proteins. In order to quantitatively analyze this regulatory mechanism, a novel approach was developed to measure 3-oxo-C6-HSL concentrations in V. fischeri cell culture supernatant. A bioluminescent strain of Escherichia coli that responds to 3-oxo-C6-HSL was used as a bioreporter. Although a linear response of the bioreporter to exogenously added synthetic 3-oxo-C6-HSL was found over several orders of magnitude, we show that bioreporter performance was dramatically impacted by variations in the supernatants using samples from a V. fischeri LuxI- strain. However, when maintained in the same supernatant background, the normalized peak bioluminescence maintained a linear response to 3-oxo-C6-HSL concentrations. Therefore, a standard additions technique was developed in which a known concentration of 3-oxo-C6-HSL was added to supernatant samples from wild-type V. fischeri cultures, and the incremental increase of the normalized peak bioluminescence relative to the untreated sample was determined. The concentration of 3-oxo-C6-HSL in the supernatant of the unknown sample was then quantified from the slope of the response between the normalized bioluminescent peaks with and without the addition of 3-oxo-C6-HSL. Advantages of this method are that it is rapid, does not require concentration or extraction, uses a small sample volume (ca. 2 ml), and accounts for effects caused by the composition of the supernatant. Furthermore, the findings can be broadly applicable to other bioreporter systems involving variable background conditions.

Molecular characterization of autoinduction of bioluminescence in the Microtox indicator strain Vibrio fischeri ATCC 49387

Repeated attempts to clone the luxI from Vibrio fischeri ATCC 49387 failed to produce a clone carrying a functional LuxI. Sequence data from the clones revealed the presence of a polymorphism when compared with previously published luxI sequences, prompting further characterization of bioluminescence regulation in V. fischeri ATCC 49387. Further investigation of V. fischeri ATCC 49387 revealed that its LuxI protein lacks detectable LuxI activity due to the presence of a glutamine residue at position 125 in the deduced amino acid sequence. Specific bioluminescence in V. fischeri ATCC 49387 increases with increasing cell density, indicative of a typical autoinduction response. However, conditioned medium from this strain does not induce bioluminescence in an ATCC 49387 luxR-plux-based acyl homoserine lactone reporter strain, but does induce bioluminescence in ATCC 49387. It has been previously shown that a V. fischeri MJ-1 luxI mutant exhibits autoinduction of bioluminescence through N-octanoyl-L-homoserine lactone, the product of the AinS autoinducer synthase. However, a bioreporter based on luxR-plux from V. fischeri ATCC 49387 responded poorly to conditioned medium from V. fischeri ATCC 49387 and also responded poorly to authentic N-octanoyl-DL-homoserine lactone. A similar MJ-1-based bioreporter showed significant induction under the same conditions. A putative ainS gene cloned from ATCC 49387, unlike luxI from ATCC 49387, expresses V. fischeri autoinducer synthase activity in Escherichia coli. This study suggests that a regulatory mechanism independent of LuxR and LuxI but possibly involving AinS is responsible for the control of autoinduction of bioluminescence in V. fischeri ATCC 49387.

Vibrio fischeri LuxS and AinS: comparative study of two signal synthases

Vibrio fischeri possesses two acyl-homoserine lactone quorum-sensing systems, ain and lux, both of which are involved in the regulation of luminescence gene expression and are required for persistent colonization of the squid host, Euprymna scolopes. We have previously demonstrated that the ain system induces luminescence at cell densities that precede lux system activation. Our data suggested that the ain system both relieves repression and initially induces the lux system, thereby achieving sequential induction of gene expression by these two systems. Analysis of the V. fischeri genome revealed the presence of a putative third system based on the enzyme LuxS, which catalyzes the synthesis of the Vibrio harveyi autoinducer 2 (AI-2). In this study, we investigated the impact of V. fischeri LuxS on luminescence and colonization competence in comparison to that of the ain system. Similar to the ain system, inactivation of the AI-2 system decreased light production in culture, but not in the squid host. However, while an ainS mutant produces no detectable light in culture, a luxS mutant expressed approximately 70% of wild-type luminescence levels. A mutation in luxS alone did not compromise symbiotic competence of V. fischeri; however, levels of colonization of an ainS luxS double mutant were reduced to 50% of the already diminished level of ainS mutant colonization, suggesting that these two systems regulate colonization gene expression synergistically through a common pathway. Introduction of a luxO mutation into the luxS and ainS luxS background could relieve both luminescence and colonization defects, consistent with a model in which LuxS, like AinS, regulates gene expression through LuxO. Furthermore, while luxS transcription appeared to be constitutive and the AI-2 signal concentration did not change dramatically, our data suggest that ainS transcription is autoregulated, resulting in an over 2,000-fold increase in signal concentration as culture density increased. Taken together, these data indicate that V. fischeri LuxS affects both luminescence regulation and colonization competence; however, its quantitative contribution is small when compared to that of the AinS signal.

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).

The Vibrio fischeri quorum-sensing systems ain and lux sequentially induce luminescence gene expression and are important for persistence in the squid host

Bacterial quorum sensing using acyl-homoserine lactones (acyl-HSLs) as cell-density dependent signalling molecules is important for the transcriptional regulation of many genes essential in the establishment and the maintenance of bacteria-host associations. Vibrio fischeri, the symbiotic partner of the Hawaiian bobtail squid Euprymna scolopes, possesses two distinct acyl-HSL synthase proteins, LuxI and AinS. Whereas the cell density-dependent regulation of luminescence by the LuxI-produced signal is a well-described phenomenon, and its role in light organ symbiosis has been defined, little is known about the ain system. We have investigated the impact of the V. fischeri acyl-HSL synthase AinS on both luminescence and symbiotic colonization. Through phenotypic studies of V. fischeri mutants we have found that the AinS-signal is the predominant inducer of luminescence expression in culture, whereas the impact of the LuxI-signal is apparent only at the high cell densities occurring in symbiosis. Furthermore, our studies revealed that ainS regulates activities essential for successful colonization of E. scolopes, i.e. the V. fischeri ainS mutant failed to persist in the squid light organ. Mutational inactivation of the transcriptional regulator protein LuxO in the ainS mutant partially or completely reversed all the observed phenotypes, demonstrating that the AinS-signal regulates expression of downstream genes through the inactivation of LuxO. Taken together, our results suggest that the two quorum-sensing systems in V. fischeri, ain and lux, sequentially induce the expression of luminescence genes and possibly other colonization factors.

LuxO controls luxR expression in Vibrio harveyi: evidence for a common regulatory mechanism in Vibrio

Quorum-sensing control of luminescence in Vibrio harveyi, which involves an indirect autoinducer-mediated phosphorelay signal transduction system, contrasts with the prototypical quorum-sensing system of Vibrio fischeri, in which the autoinducer and the transcriptional activator LuxR directly activate lux operon expression. In V. harveyi, a regulator not homologous to V. fischeri LuxR and also designated LuxR (LuxRvh), binds specifically to the lux operon promoter region and activates the expression of luminescence. A direct connection has not been identified previously between V. harveyi LuxRvh and the autoinducer-mediated phosphorelay system. Here, we demonstrate by mobility shift assays and measurement of luxRvh mRNA levels with luxO+ and luxO- cells that the central response regulator of the V. harveyi phosphorelay system (LuxO) represses the level of LuxRvh. Expression of a luxRvh-bearing plasmid strongly stimulated luminescence of a luxO- mutant but had no effect on luminescence of wild-type luxO+ cells, indicating tight regulation of luxRvh by LuxO. Furthermore, luxO null mutants of V. fischeri MJ-1 and two autoinducer mutants, MJ-211 (luxI-) and MJ-215 (luxI-ainS-), emitted more light and exhibited more elevated levels of litR, a newly identified V. harveyi luxRvh homologue, than their luxO+ counterparts. These results suggest that activity of the autoinducer-mediated phosphorelay system is coupled to LuxRvh/LitR control of luminescence through LuxO in V. harveyi and V. fischeri. The presence of homologues of V. harveyi LuxRvh, LuxO and other phosphorelay system proteins in various Vibrio species and the control of LuxRvh and its homologues by LuxO identified here in V. harveyi and V. fischeri and recently in Vibrio cholerae suggest that the luxO-luxRvh couple is a central feature of this quorum-sensing system in members of the genus Vibrio.

Production of Acylated Homoserine Lactones by Different Serotypes of Vibrio anguillarum Both in Culture and During Infection of Rainbow Trout

Onehundred and forty-eight out of onehundred and fifty strains of Vibrio anguillarum isolated from vibriosis in Danish marine aquaculture produced bacterial communication signals, acylated homoserine lactones, eliciting a response in the Agrobacterium tumefaciens (pZLR4) monitoring system. One strain, a serotype O4, induced a strong response in the Chromobacterium violaceum (CV026) monitoring system. Profiles of AHLs determined by TLC separation revealed the presence of at least four AHLs and a compound similar to N-3-oxo-decanoyl homoserine lactone (3-oxo-C10-HSL) was present in all strains. The production rate of the presumed 3-oxo-C10-HSL followed the growth rate of V. anguillarum whereas the production rate of a small AHL (Rf value of 0.74) increased faster than the growth rate of V. anguillarum indicating autoinduction. AHLs were produced by all serotypes (O1 to O10) and by non-typable strains. During infection with V. anguillarum, AHLs could be extracted from liver, kidney and muscle of rainbow trout and AHLs were detected both in vitro and in vivo when cell numbers reached 10(7) per ml or gram. Preliminary investigations of interactions between AHLs and the fish immune system were carried out determining oxidative burst of fish macrophages exposed to 3-oxo-C10-HSL. No activation or suppression of the superoxide anion production in the head kidney macrophages was seen when treated with the AHL compound in concentrations of 1 nM-10 microM. Our data show that AHLs are produced by almost all V. anguillarum strains and that no clear pattern relating AHL production to disease or virulence appear.

Characterization of the Sinorhizobium meliloti sinR/sinI locus and the production of novel N-acyl homoserine lactones

Sinorhizobium meliloti is a soil bacterium which can establish a nitrogen-fixing symbiosis with the legume Medicago sativa. Recent work has identified a pair of genes, sinR and sinI, which represent a potential quorum-sensing system and are responsible for the production of N-acyl homoserine lactones (AHLs) in two S. meliloti strains, Rm1021 and Rm41. In this work, we characterize the sinRI locus and show that these genes are responsible for the synthesis of several long-chain AHLs ranging from 12 to 18 carbons in length. Four of these, 3-oxotetradecanoyl HL, 3-oxohexadecenoyl HL, hexadecenoyl HL, and octadecanoyl HL, have novel structures. This is the first report of AHLs having acyl chains longer than 14 carbons. We show that a disruption in sinI eliminates these AHLs and that a sinR disruption results in only basal levels of the AHLs. Moreover, the same sinI and sinR mutations also lead to a decrease in the number of pink nodules during nodulation assays, as well as a slight delay in the appearance of pink nodules, indicating a role for quorum sensing in symbiosis. We also show that sinI and sinR mutants are still capable of producing several short-chain AHLs, one of which was identified as octanoyl HL. We believe that these short-chain AHLs are evidence of a second quorum-sensing system in Rm1021, which we refer to here as the mel system, for "S. meliloti."

Genetically programmed autoinducer destruction reduces virulence gene expression and swarming motility in Pseudomonas aeruginosa PAO1

Virulence in the opportunistic human pathogen Pseudomonas aeruginosa is controlled by cell density via diffusible signalling molecules ('autoinducers') of the N-acylhomoserine lactone (AHL) type. Two Bacillus sp. isolates (A23 and A24) with AHL-degrading activity were identified among a large collection of rhizosphere bacteria. From isolate A24 a gene was cloned which was similar to the aiiA gene, encoding an AHL lactonase in another Bacillus strain. Expression of the aiiA homologue from isolate A24 in P. aeruginosa PAO1 reduced the amount of the quorum sensing signal N-oxododecanoyl-L-homoserine lactone and completely prevented the accumulation of the second AHL signal, N-butyryl-L-homoserine lactone. This strongly reduced AHL content correlated with a markedly decreased expression and production of several virulence factors and cytotoxic compounds such as elastase, rhamnolipids, hydrogen cyanide and pyocyanin, and strongly reduced swarming. However, no effect was observed on flagellar swimming or on twitching motility, and aiiA expression did not affect bacterial adhesion to a polyvinylchloride surface. In conclusion, introduction of an AHL degradation gene into P. aeruginosa could block cell-cell communication and exoproduct formation, but failed to interfere with surface colonization.

Regulation of gene expression by cell-to-cell communication: acyl-homoserine lactone quorum sensing

Quorum sensing is an example of community behavior prevalent among diverse bacterial species. The term "quorum sensing" describes the ability of a microorganism to perceive and respond to microbial population density, usually relying on the production and subsequent response to diffusible signal molecules. A significant number of gram-negative bacteria produce acylated homoserine lactones (acyl-HSLs) as signal molecules that function in quorum sensing. Bacteria that produce acyl-HSLs can respond to the local concentration of the signaling molecules, and high population densities foster the accumulation of inducing levels of acyl-HSLs. Depending upon the bacterial species, the physiological processes regulated by quorum sensing are extremely diverse, ranging from bioluminescence to swarming motility. Acyl-HSL quorum sensing has become a paradigm for intercellular signaling mechanisms. A flurry of research over the past decade has led to significant understanding of many aspects of quorum sensing including the synthesis of acyl-HSLs, the receptors that recognize the acyl-HSL signal and transduce this information to the level of gene expression, and the interaction of these receptors with the transcriptional machinery. Recent studies have begun to integrate acyl-HSL quorum sensing into global regulatory networks and establish its role in developing and maintaining the structure of bacterial communities.

Quorum sensing in bacteria

Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density. Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression. Gram-positive and Gram-negative bacteria use quorum sensing communication circuits to regulate a diverse array of physiological activities. These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation. In general, Gram-negative bacteria use acylated homoserine lactones as autoinducers, and Gram-positive bacteria use processed oligo-peptides to communicate. Recent advances in the field indicate that cell-cell communication via autoinducers occurs both within and between bacterial species. Furthermore, there is mounting data suggesting that bacterial autoinducers elicit specific responses from host organisms. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quorum sensing systems differ, in every case the ability to communicate with one another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Presumably, this process bestows upon bacteria some of the qualities of higher organisms. The evolution of quorum sensing systems in bacteria could, therefore, have been one of the early steps in the development of multicellularity.

Feedback regulation of the Bradyrhizobium japonicum nodulation genes

Lipochitin Nod signals are produced by rhizobia and are required for the establishment of a nitrogen-fixing symbiosis with a legume host. The nodulation genes encode products required for the synthesis of this signal and are induced in response to plant-produced flavonoid compounds. The addition of chitin and lipo-chitin oligomers to Bradyrhizobium japonicum cultures resulted in a significant reduction in the expression of a nod-lacZ fusion. Intracellular expression of NodC, encoding a chitin synthase, also reduced nod gene expression. In contrast, expression of the ChiB chitinase increased nod gene expression. The chain length of the oligosaccharide was important in feedback regulation, with chitotetraose molecules the best modulators of nod gene expression. Feedback regulation is mediated by the induction of nolA by chitin, resulting in elevated levels of the repressor protein, NodD2.

Quorum-sensing in Gram-negative bacteria

It has become increasingly and widely recognised that bacteria do not exist as solitary cells, but are colonial organisms that exploit elaborate systems of intercellular communication to facilitate their adaptation to changing environmental conditions. The languages by which bacteria communicate take the form of chemical signals, excreted from the cells, which can elicit profound physiological changes. Many types of signalling molecules, which regulate diverse phenotypes across distant genera, have been described. The most common signalling molecules found in Gram-negative bacteria are N-acyl derivatives of homoserine lactone (acyl HSLs). Modulation of the physiological processes controlled by acyl HSLs (and, indeed, many of the non-acyl HSL-mediated systems) occurs in a cell density- and growth phase-dependent manner. Therefore, the term 'quorum-sensing' has been coined to describe this ability of bacteria to monitor cell density before expressing a phenotype. In this paper, we review the current state of research concerning acyl HSL-mediated quorum-sensing. We also describe two non-acyl HSL-based systems utilised by the phytopathogens Ralstonia solanacearum and Xanthomonas campestris.

Methods for detecting acylated homoserine lactones produced by Gram-negative bacteria and their application in studies of AHL-production kinetics

In the process of evaluating the role of acylated homoserine lactones (AHLs) in food-spoiling Gram-negative bacteria, we have combined a range of bacterial AHL monitor systems to determine the AHL-profile and the kinetics of AHL-production. AHL production from 148 strains of Enterobacteriaceae isolated from foods was tested using Escherichia coli pSB403 (LuxR), Agrobacterium tumefaciens A136 (TraR) and both induction and inhibition of Chromobacterium violaceum CV026 (CviR). All strains except one was found to produce AHL(s). In no case could a single monitor system identify more than 64% of the Enterobacteriaceae as AHL-producers, showing that the simultaneous use of monitor strains is required in the process of screening bacterial populations for AHL-production. AHLs from 20 selected strains were profiled by thin layer chromatography. Most strains produced more than one AHL with 3-N-oxo-hexanoyl homoserine lactone being the most prominent. It was found that the simultaneous use of monitor strains in the top-layer was necessary for the detection of (presumably) all the AHLs. An agar well-diffusion assay based on A. tumefaciens pDZLR4 was used for quantifying AHLs from bacterial supernatants and enabled an assessment of the kinetics of AHL-production of 3 strains (Serratia proteamaculans strain B5a, Erwinia carotovora ATCC 39048 and V. fischeri strain MJ-1). As expected, the production of AHL (OHHL) and luminescence in Vibrio fischeri strain MJ-1 increased faster than growth indicating up-regulation of the AHL regulated phenotype and auto-induction of AHL production. In contrast, production kinetics of AHL (OHHL) in the two Enterobacteriaceae indicated lack of auto-induction.

Intracellular Ca(2+) mobilization and kinase activity during acylated homoserine lactone-dependent quorum sensing in Serratia liquefaciens

Quorum sensing in Gram-negative bacteria involves acylated homoserine lactones (AHLs) and a transcription factor, activated by the AHLs. In this study, a possible involvement of intracellular Ca(2+) as second messenger and/or protein kinase activity during signal transduction is analyzed. When N-hexanoyl-l-homoserine lactone was added to a suspension of Fura-2-loaded Serratia liquefaciens, there was a decline in [Ca(2+)](i), measured as a decrease in the Fura-2 fluorescence ratio. As controls, the addition of the signal molecule N-3-oxohexanoyl-l-homoserine lactone, which is not produced by S. liquefaciens, did not induce changes in [Ca(2+)](i). Using a protein kinase activity assay on AHL-stimulated cells, an increase in kinase activity after N-butanoyl-l-homoserine lactone stimulation of S. liquefaciens cells was detected, whereas the kinase activity induced by N-3-oxohexanoyl-l-homoserine lactone was not statistically significant. The conclusion from this study is that changes in [Ca(2+)](i) are involved in quorum sensing signal transduction in the Gram-negative bacteria S. liquefaciens. We also conclude that kinase activity is induced in S. liquefaciens upon AHL stimulation. We suggest that the transient intracellular [Ca(2+)] changes and kinase activity, activated by the AHL signal, are critical for the quorum-sensing signal transduction.

Vibrio fischeri lux genes play an important role in colonization and development of the host light organ

The bioluminescent bacterium Vibrio fischeri and juveniles of the squid Euprymna scolopes specifically recognize and respond to one another during the formation of a persistent colonization within the host's nascent light-emitting organ. The resulting fully developed light organ contains brightly luminescing bacteria and has undergone a bacterium-induced program of tissue differentiation, one component of which is a swelling of the epithelial cells that line the symbiont-containing crypts. While the luminescence (lux) genes of symbiotic V. fischeri have been shown to be highly induced within the crypts, the role of these genes in the initiation and persistence of the symbiosis has not been rigorously examined. We have constructed and examined three mutants (luxA, luxI, and luxR), defective in either luciferase enzymatic or regulatory proteins. All three are unable to induce normal luminescence levels in the host and, 2 days after initiating the association, had a three- to fourfold defect in the extent of colonization. Surprisingly, these lux mutants also were unable to induce swelling in the crypt epithelial cells. Complementing, in trans, the defect in light emission restored both normal colonization capability and induction of swelling. We hypothesize that a diminished level of oxygen consumption by a luciferase-deficient symbiotic population is responsible for the reduced fitness of lux mutants in the light organ crypts. This study is the first to show that the capacity for bioluminescence is critical for normal cell-cell interactions between a bacterium and its animal host and presents the first examples of V. fischeri genes that affect normal host tissue development.

Control of bioluminescence in Vibrio fischeri by the LuxO signal response regulator

Bioluminescence in the marine bacterium Vibrio fischeri is controlled by the excretion of a N-acyl homoserine lactone (HSL) autoinducer which interacts with a regulator, LuxR, and activates transcription of the lux operon at high-cell density. This system has become the prototype for quorum sensing in many bacteria. Although light emission in Vibrio harveyi is also regulated by a N-acyl-HSL inducer, in sharp contrast, a completely different and more complex system is involved in quorum sensing which is mediated via LuxO, the response regulator of a phosphorelay signal transduction system. In the present work, luxO and the overlapping luxU gene, also involved in the phosphorelay system in V. harveyi, have been discovered in V. fischeri. By gene replacement technology, a V. fischeri luxO- mutant was generated whose phenotype was similar to that of V. harveyi luxO- showing that LuxO is involved in control of luminescence in V. fischeri. This mutant could be complemented with luxO from either V. fischeri or V. harveyi resulting in the restoration of the dependence of luminescence intensity on cell density. In contrast to V. harveyi luxO-, light emission of V. fischeri luxO- was stimulated by the N-acyl-HSL autoinducer indicating that luxO is part of a second signal transduction system controlling luminescence in this species. The presence of a luxO-based phosphorelay regulatory system as well as the luxR-based system in V. fischeri suggests that the former system, originally discovered in V. harveyi, may be a general regulatory mechanism in luminescent bacteria.

LuxR- and acyl-homoserine-lactone-controlled non-lux genes define a quorum-sensing regulon in Vibrio fischeri

The luminescence (lux) operon (luxICDABEG) of the symbiotic bacterium Vibrio fischeri is regulated by the transcriptional activator LuxR and two acyl-homoserine lactone (acyl-HSL) autoinducers (the luxI-dependent 3-oxo-hexanoyl-HSL [3-oxo-C6-HSL] and the ainS-dependent octanoyl-HSL [C8-HSL]) in a population density-responsive manner called quorum sensing. To identify quorum-sensing-regulated (QSR) proteins different from those encoded by lux genes, we examined the protein patterns of V. fischeri quorum-sensing mutants defective in luxI, ainS, and luxR by two-dimensional polyacrylamide gel electrophoresis. Five non-Lux QSR proteins, QsrP, RibB, AcfA, QsrV, and QSR 7, were identified; their production occurred preferentially at high population density, required both LuxR and 3-oxo-C6-HSL, and was inhibited by C8-HSL at low population density. The genes encoding two of the QSR proteins were characterized: qsrP directs cells to synthesize an apparently novel periplasmic protein, and ribB is a homolog of the Escherichia coli gene for 3,4-dihydroxy-2-butanone 4-phosphate synthase, a key enzyme for riboflavin synthesis. The qsrP and ribB promoter regions each contained a sequence similar to the lux operon lux box, a 20-bp region of dyad symmetry necessary for LuxR/3-oxo-C6-HSL-dependent activation of lux operon transcription. V. fischeri qsrP and ribB mutants exhibited no distinct phenotype in culture. However, a qsrP mutant, in competition with its parent strain, was less successful in colonizing Euprymna scolopes, the symbiotic host of V. fischeri. The newly identified QSR genes, together with the lux operon, define a LuxR/acyl-HSL-responsive quorum-sensing regulon in V. fischeri.

Quorum sensing and starvation: signals for entry into stationary phase

Quorum sensing occurs at high cell density in many microorganisms. It regulates specialized processes such as genetic competence, bioluminescence, virulence, and sporulation. However, recent evidence suggests that quorum-sensing may play a more central role in the physiology of bacteria, where quorum-sensing pathways converge with starvation-sensing pathways to regulate cell entry into stationary phase.

Luminescence control in the marine bacterium Vibrio fischeri: An analysis of the dynamics of lux regulation

A mathematical model has been developed based on the fundamental properties of the control system formed by the lux genes and their products in Vibrio fischeri. The model clearly demonstrates how the components of this system work together to create two, stable metabolic states corresponding to the expression of the luminescent and non-luminescent phenotypes. It is demonstrated how the cell can "switch" between these steady states due to changes in parameters describing metabolic processes and the extracellular concentration of the signal molecule N-3-oxohexanoyl-l-homoserine lactone. In addition, it is shown how these parameters influence how sensitive the switch mechanism is to cellular LuxR and N-3-oxohexanoyl-l-homoserine lactone and complex concentration. While these properties could lead to the collective phenomenon known as quorum sensing, the model also predicts that under certain metabolic circumstances, basal expression of the lux genes could cause a cell to luminesce in the absence of extracellular signal molecule. Finally, the model developed in this study provides a basis for analysing the impact of other levels of control upon lux regulation.

Novel synthetic analogs of the Pseudomonas autoinducer

Release of virulence factors in Pseudomonas aeruginosa is regulated by two N-acylhomoserine lactones, PAI-1 and PAI-2, that activate the respective transcription factors LasR and RhlR. With the goal of developing novel therapeutic agents, we synthesized constrained analogs of PAI-1 and evaluated them in P. aeruginosa. Two of the novel analogs bound to LasR and showed agonist activity in LasR stimulation of a lasI-lacZ reporter construct.