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PubMed2004

Is autoinducer-2 a universal signal for interspecies communication: a comparative genomic and phylogenetic analysis of the synthesis and signal transduction pathways.

BACKGROUND: Quorum sensing is a process of bacterial cell-to-cell communication involving the production and detection of extracellular signaling molecules called autoinducers. Recently, it has been proposed that autoinducer-2 (AI-2), a furanosyl borate diester derived from the recycling of S-adenosyl-homocysteine (SAH) to homocysteine, serves as a universal signal for interspecies communication. RESULTS: In this study, 138 completed genomes were examined for the genes involved in the synthesis and detection of AI-2. Except for some symbionts and parasites, all organisms have a pathway to recycle SAH, either using a two-step enzymatic conversion by the Pfs and LuxS enzymes or a one-step conversion using SAH-hydrolase (SahH). 51 organisms including most Gamma-, Beta-, and Epsilonproteobacteria, and Firmicutes possess the Pfs-LuxS pathway, while Archaea, Eukarya, Alphaproteobacteria, Actinobacteria and Cyanobacteria prefer the SahH pathway. In all 138 organisms, only the three Vibrio strains had strong, bidirectional matches to the periplasmic AI-2 binding protein LuxP and the central signal relay protein LuxU. The initial two-component sensor kinase protein LuxQ, and the terminal response regulator luxO are found in most Proteobacteria, as well as in some Firmicutes, often in several copies. CONCLUSIONS: The genomic analysis indicates that the LuxS enzyme required for AI-2 synthesis is widespread in bacteria, while the periplasmic binding protein LuxP is only present in Vibrio strains. Thus, other organisms may either use components different from the AI-2 signal transduction system of Vibrio strains to sense the signal of AI-2, or they do not have such a quorum sensing system at all.

Adenosylhomocysteinase↗
PubMed1998

[Virulence and its relationship to antibiotic resistance].

PATHOGENIC ISLANDS: Certain DNA blocks inserted into the chromosome of most Gram negative bacteria originated in pathogens found in plants. VIRULENCE-ANTIBIOTIC INTERACTIONS: During the invasive phase, the bacterial cell covers itself with adhesins which facilitate its adherence to tissues. The bacterial cell produces a fibronectin which protects its defense systems. Antibiotics favor bacterial resistance by increasing the expression of surface adhesins and fibronectin production. PENICILLIN RESISTANT PNEUMOCOCCI: Experimental models have demonstrated that mortality in mice and host resistance to pneumococcal infection are related to the type of capsule and not to antibiotic resistance. QUORUM SENSING: The bacterial inoculum regulates the production of virulence factors in vivo via quorum sensing. This regulation can play an important role in Pseudomonas aeruginosa infections. ACINETOBACTER BAUMANNI VIRULENCE: Long poorly understood, factors favoring A. baumanni virulence appear to result from bacterial production of IROMPs in the extracellular growth medium in response to iron depletion during the exponential growth phase.

Acinetobacter↗
PubMed2004

Reversible acyl-homoserine lactone binding to purified Vibrio fischeri LuxR protein.

The Vibrio fischeri LuxR protein is the founding member of a family of acyl-homoserine lactone-responsive quorum-sensing transcription factors. Previous genetic evidence indicates that in the presence of its quorum-sensing signal, N-(3-oxohexanoyl) homoserine lactone (3OC6-HSL), LuxR binds to lux box DNA within the promoter region of the luxI gene and activates transcription of the luxICDABEG luminescence operon. We have purified LuxR from recombinant Escherichia coli. Purified LuxR binds specifically and with high affinity to DNA containing a lux box. This binding requires addition of 3OC6-HSL to the assay reactions, presumably forming a LuxR-3OC6-HSL complex. When bound to the lux box at the luxI promoter in vitro, LuxR-3OC6-HSL enables E. coli RNA polymerase to initiate transcription from the luxI promoter. Unlike the well-characterized LuxR homolog TraR in complex with its signal (3-oxo-octanoyl-HSL), the LuxR-30C6-HSL complex can be reversibly inactivated by dilution, suggesting that 3OC6-HSL in the complex is not tightly bound and is in equilibrium with the bulk solvent. Thus, although LuxR and TraR both bind 3-oxoacyl-HSLs, the binding is qualitatively different. The differences have implications for the ways in which these proteins respond to decreases in signal concentrations or rapid drops in population density.

4-Butyrolactone↗
PubMed2006

Pheromone-induced expression of recombinant proteins in Streptococcus thermophilus.

A locus encoding proteins with high homology to the pneumococcal BlpABCHR quorum-sensing system was identified in Streptococcus thermophilus LMG 18311. The BlpABCHR system regulates bacteriocin production in Streptococcus pneumoniae by monitoring the extracellular concentration of a peptide-pheromone encoded by blpC. The homologous system in S. thermophilus, termed StbABCHR, contains a corresponding gene (stbC) encoding a possible peptide-pheromone (STP) that presumably controls bacteriocin production in S. thermophilus. We synthesized this peptide and found that it activates transcription of a gusA reporter gene placed behind the promoter of the bacteriocin-like gene stbD. Furthermore, deletion mapping and mutational analysis of the stbD promoter region were used to identify a degenerated direct repeat motif required for STP induced GusA expression. Our findings provide strong evidence that STP regulates bacteriocin production in S. thermophilus LMG 18311, and show that the StbABCHR quorum-sensing system can be exploited for inducible expression of recombinant proteins in this bacterial species.

Bacteriocins↗
PubMed2000

Regulation of virulence factors of enterohemorrhagic Escherichia coli O157:H7 by self-produced extracellular factors.

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 causes serious diarrhea and hemolytic uremic syndrome in humans. The expressions of EspD and intimin by O157:H7 have now been shown to be down-regulated by medium conditioned by O157:H7 grown at stationary phase. Preparation of conditioned medium showing the effect on the amount of EspD was not dependent on temperature or growth medium, but was dependent on growth phase. Inhibition of EspD and intimin expression was also induced by medium conditioned by E. coli K-12 strains and homoserine lactone, a signal molecule of the quorum-sensing system in gram-negative bacteria. These results suggest the possibility that the quorum-sensing system mediated by self-produced extracellular factors plays an important role in control of colonization of EHEC O157:H7.

Adhesins, Bacterial↗
PubMed2006

Synthesis of N-acyl homoserine lactone analogues reveals strong activators of SdiA, the Salmonella enterica serovar Typhimurium LuxR homologue.

N-Acyl homoserine lactones (AHLs) are molecules that are synthesized and detected by many gram-negative bacteria to monitor the population density, a phenomenon known as quorum sensing. Salmonella enterica serovar Typhimurium is an exceptional species since it does not synthesize its own AHLs, while it does encode a LuxR homologue, SdiA, which enables this bacterium to detect AHLs that are produced by other species. To obtain more information about the specificity of the ligand binding by SdiA, we synthesized and screened a limited library of AHL analogues. We identified two classes of analogues that are strong activators of SdiA: the N-(3-oxo-acyl)-homocysteine thiolactones (3O-AHTLs) and the N-(3-oxo-acyl)-trans-2-aminocyclohexanols. To our knowledge, this is the first report of compounds (the 3O-AHTLs) that are able to activate a LuxR homologue at concentrations that are lower than the concentrations of the most active AHLs. SdiA responds with greatest sensitivity to AHTLs that have a keto modification at the third carbon atom and an acyl chain that is seven or eight carbon atoms long. The N-(3-oxo-acyl)-trans-2-aminocyclohexanols were found to be less sensitive to deactivation by lactonase and alkaline pH than the 3O-AHTLs and the AHLs are. We also examined the activity of our library with LuxR of Vibrio fischeri and identified three new inhibitors of LuxR. Finally, we performed preliminary binding experiments which suggested that SdiA binds its activators reversibly. These results increase our understanding of the specificity of the SdiA-ligand interaction, which could have uses in the development of anti-quorum-sensing-based antimicrobials.

4-Butyrolactone↗
PubMed2006

Comprehensive profiling of N-acylhomoserine lactones produced by Yersinia pseudotuberculosis using liquid chromatography coupled to hybrid quadrupole-linear ion trap mass spectrometry.

A method for the comprehensive profiling of the N-acylhomoserine lactone (AHL) family of bacterial quorum-sensing molecules is presented using liquid chromatography (LC) coupled to hybrid quadrupole-linear ion trap (QqQLIT) mass spectrometry. Information-dependent acquisition (IDA), using triggered combinations of triple-quadrupole and linear ion trap modes in the same LC-MS/MS run, was used to simultaneously screen, quantify and identify multiple AHLs in a single sample. This MS method uses common AHL fragment ions attributed to the homoserine moiety and the 3-oxo-, 3-hydroxy- or unsubstituted acyl side chains, to identify unknown AHLs in cell-free culture supernatants in an unbiased manner. This LC-MS technique was applied to determine the relative molar ratios of AHLs produced by Yersinia pseudotuberculosis and the consequences of inactivating by mutation either or both of the AHL synthase genes (ypsI and ytbI) on AHL profile and concentration. The Y. pseudotuberculosis wild type but not the ypsI ytbI double mutant produced at least 24 different AHLs with acyl chains ranging from C4 to C15 with or without 3-oxo or 3-hydroxy substituents. YtbI, in contrast to YpsI, could direct the synthesis of all of the AHLs identified. The most abundant and hence most biologically relevant Y. pseudotuberculosis AHLs were found to be the 3-oxo-substituted C6, C7 and C8 AHLs and the unsubstituted C6 and C8 compounds. The LC-QqQLIT methodology is broadly applicable to quorum-sensing signal molecule analysis and can provide comprehensive AHL profiles and concentrations from a single sample and simultaneously collect confirmatory spectra for each AHL identified.

4-Butyrolactone↗
PubMed2005

Colonial architecture in mixed species assemblages affects AHL mediated gene expression.

Many bacterial species produce metabolites that accumulate in the extracellular environment and induce specific transcriptional responses in producing cells. This phenomenon, most often referred to as quorum sensing, is thought to constitute a self-cell-density sensing mechanism allowing bacterial populations to alter gene expression in response to increases in their own density. Quorum sensing systems involving N-acyl-L-homoserine lactone (AHL) production and response are the most intensively investigated example. In this study we have employed a novel technique, known as dielectrophoresis, to investigate the impact of colonial architecture on the induction of AHL mediated gene expression. Using dielectrophoresis, we constructed artificial mixed species microcolonies with specific architectures. In this way, we were able to show that approximately 1000 Escherichia coli cells layered over an immobilised cluster of approximately 500 AHL responsive cells alters the response of this cluster to AHLs supplied either exogenously or endogenously. These findings lend credence to the hypothesis that the accumulation of extracellular metabolites signifies generic crowding in mixed species assemblages.

4-Butyrolactone↗
PubMed2005

Directed evolution of Vibrio fischeri LuxR for increased sensitivity to a broad spectrum of acyl-homoserine lactones.

LuxR-type transcriptional regulators play key roles in quorum-sensing systems that employ acyl-homoserine lactones (acyl-HSLs) as signal molecules. These proteins mediate quorum control by changing their interactions with RNA polymerase and DNA in response to binding their cognate acyl-HSL. The evolutionarily related LuxR-type proteins exhibit considerable diversity in primary sequence and in their response to acyl-HSLs having acyl groups of differing length and composition. Little is known about which residues determine acyl-HSL specificity, and less about the evolutionary time scales required to forge new ones. To begin to examine such issues, we have focused on the LuxR protein from Vibrio fischeri, which activates gene transcription in response to binding its cognate quorum signal, 3-oxohexanoyl-homoserine lactone (3OC6HSL). Libraries of luxR mutants were screened for variants exhibiting increased gene activation in response to octanoyl-HSL (C8HSL), with which wild-type LuxR interacts only weakly. Eight LuxR variants were identified that showed a 100-fold increase in sensitivity to C8HSL; these variants also displayed increased sensitivities to pentanoyl-HSL and tetradecanoyl-HSL, while maintaining a wild-type or greater response to 3OC6HSL. The most sensitive variants activated gene transcription as strongly with C8HSL as the wild type did with 3OC6HSL. With one exception, the amino acid residues involved were restricted to the N-terminal, 'signal-binding' domain of LuxR. These residue positions differed from critical positions previously identified via 'loss-of-function' mutagenesis. We have demonstrated that acyl-HSL-dependent quorum-sensing systems can evolve rapidly to respond to new acyl-HSLs, suggesting that there may be an evolutionary advantage to maintaining such plasticity.

4-Butyrolactone↗
PubMed2006

Capsular polysaccharide phase variation in Vibrio vulnificus.

Commonly found in raw oysters, Vibrio vulnificus poses a serious health threat to immunocompromised individuals and those with serum iron overload, with a fatality rate of approximately 50%. An essential virulence factor is its capsular polysaccharide (CPS), which is responsible for a significant increase in virulence compared to nonencapsulated strains. However, this bacterium is known to vary the amount of CPS expressed on the cell surface, converting from an opaque (Op) colony phenotype to a translucent (Tr) colony phenotype. In this study, the consistency of CPS conversion was determined for four strains of V. vulnificus. Environmental conditions including variations in aeration, temperature, incubation time, oxidative stress, and media (heart infusion or modified maintenance medium agar) were investigated to determine their influence on CPS conversion. All conditions, with the exception of variations in media and oxidative stress, significantly affected the conversion of the population, with high ranges of CPS expression found even within cells from a single colony. The global quorum-sensing regulators RpoS and AI-2 were also examined. While RpoS was found to significantly mediate phenotypic conversion, quorum sensing was not. Finally, 12 strains that comprise the recently found clinical (C) and environmental (E) genotypes of V. vulnificus were examined to determine their rates of population conversion. C-genotype strains, which are most often associated with infection, had a significantly lower rate of population conversion from Op to Tr phenotypes than did E-genotype strains (ca. 38% versus ca. 14%, respectively). Biofilm capabilities of these strains, however, were not correlated with increased population conversion.

Animals↗
PubMed2005

Heterogeneity of biofilms formed by nonmucoid Pseudomonas aeruginosa isolates from patients with cystic fibrosis.

Biofilms are thought to play a key role in the occurrence of lung infections by Pseudomonas aeruginosa in patients with cystic fibrosis (CF). In this study, 20 nonmucoid P. aeruginosa isolates collected during different periods of chronic infection from eight CF patients were assessed with respect to phenotypic changes and in vitro biofilm formation. The physiological alterations were associated with a loss of motility (35% were nonmotile) and with decreased production of virulence factors (pyocyanin, proteases) and quorum-sensing molecules (45% of the isolates were unable to produce 3-O-C(12)-homoserine lactone quorum-sensing molecules). Compared with wild-type strain PAO 1, most P. aeruginosa isolates demonstrated different degrees of reduction of adherence on polystyrene surfaces. The in vitro biofilm formation of isolates was investigated in a hydrodynamic flow system. Confocal laser scanning microscope analysis showed that the biofilm structures of the P. aeruginosa isolates were highly variable in biomass and morphology. Biofilm development of six genotypically identical sequential isolates recovered from a particular patient at different time points of chronic infection (20 years) and after lung transplantation demonstrated significant changes in biofilm architectures. P. aeruginosa biofilm formation followed a trend of decreased adherence with progression of the chronic lung infection. The results suggest that the adherent characteristic of in vitro biofilm development was not essential for the longitudinal survival of nonmucoid P. aeruginosa during chronic lung colonization.

Biofilms↗
PubMed2005

Making 'sense' of metabolism: autoinducer-2, LuxS and pathogenic bacteria.

Bacteria exploit many mechanisms to communicate with each other and their surroundings. Mechanisms using small diffusible signals to coordinate behaviour with cell density (quorum sensing) frequently contribute to pathogenicity. However, pathogens must also be able to acquire nutrients and replicate to successfully invade their host. One quorum-sensing system, based on the possession of LuxS, bears the unique feature of contributing directly to metabolism, and therefore has the potential to influence both gene regulation and bacterial fitness. Here, we discuss the influence that LuxS and its product, autoinducer-2, have on virulence, relating the current evidence to the preferred niche of the pathogen and the underlying mechanisms involved.

Animals↗
PubMed2006

N-acylhomoserine lactonase producing Rhodococcus spp. with different AHL-degrading activities.

N-acylhomoserine lactones (AHLs) are conserved signal molecules that control diverse biological activities in quorum sensing system of Gram-negative bacteria. Recently, several soil bacteria were found to degrade AHLs, thereby interfering with the quorum sensing system. Previously, Rhodococcus erythropolis W2 was reported to degrade AHLs by both oxido-reductase and AHL-acylase. In the present study, two AHL-utilizing bacteria, strains LS31 and PI33, were isolated and identified as the genus Rhodococcus. They exhibited different AHL-utilization abilities: Rhodococcus sp. strain LS31 rapidly degraded a wide range of AHLs, including N-3-oxo-hexanoyl-l-homoserine lactone (OHHL), whereas Rhodococcus sp. strain PI33 showed relatively less activity towards 3-oxo substituents. Coculture of strain LS31 with Erwinia carotovora effectively reduced the amount of OHHL and pectate lyase activity, compared with coculture of strain PI33 with E. carotovora. A mass spectrometry analysis indicated that both strains hydrolyzed the lactone ring of AHL to generate acylhomoserine, suggesting that AHL-lactonases (AHLases) from the two Rhodococcus strains are involved in the degradation of AHL, in contrast to R. erythropolis W2. To the best of our knowledge, this is the first report on AHLases of Rhodococcus spp.

4-Butyrolactone↗
PubMed2001

Production of N-acyl homoserine lactones by gram-negative bacteria isolated from contact lens wearers.

The purpose of this study was to investigate the production of N-acyl-homoserine lactone (AHL) signal molecules in ocular gram-negative bacteria. A total of 91 ocular strains isolated from contact lens adverse response patients and asymptomatic subjects were used in the study. These included Acinetobacter, Aeromonas hydrophila, Escherichia coli, Haemophilus influenzae, Klebsiella oxytoca, Pseudomonas aeruginosa, Serratia liquefaciens, Serratia marcescens, and Stenotrophomonas maltophilia. The biosensor strains Chromobacterium violaceum mutant CV026 and Agrobacterium tumefaciens A136 were used for detection of AHL signal molecules. The majority of A. hydrophila, P. aeruginosa, and S. liquefaciens strains produced more than one AHL molecule. Serratia marcescens strains were AHL positive only under detection of A136. The rest of the test species did not show any AHL production under the current detection system. These findings indicate that AHL-mediated quorum-sensing systems are present in some of the ocular bacteria, and the different signal molecules may be involved with the quorum-sensing pathway in the other bacterial species.

4-Butyrolactone↗
PubMed2004

THE BIOFILM CONCEPT: CONSEQUENCES FOR FUTURE PROPHYLAXIS OF ORAL DISEASES?

Biofilm control is fundamental to oral health. Existing oral prophylactic measures, however, are insufficient. The main reason is probably because the micro-organisms involved organize into complex biofilm communities with features that differ from those of planktonic cells. Micro-organisms have traditionally been studied in the planktonic state. Conclusions drawn from many of these studies, therefore, need to be revalidated. Recent global approaches to the study of microbial gene expression and regulation in non-oral micro-organisms have shed light on two-component and quorum-sensing systems for the transduction of stimuli that allow for coordinated gene expression. We suggest interference with two-component and quorum-sensing systems as potential novel strategies for the prevention of oral diseases through control of oral biofilms. Information is still lacking, however, on the genetic regulation of oral biofilm formation. A better understanding of these processes is of considerable importance.

Journal Article↗
PubMed2004

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.

Bacterial Proteins↗
PubMed2006

Communication in bacteria: an ecological and evolutionary perspective.

Individual bacteria can alter their behaviour through chemical interactions between organisms in microbial communities - this is generally referred to as quorum sensing. Frequently, these interactions are interpreted in terms of communication to mediate coordinated, multicellular behaviour. We show that the nature of interactions through quorum-sensing chemicals does not simply involve cooperative signals, but entails other interactions such as cues and chemical manipulations. These signals might have a role in conflicts within and between species. The nature of the chemical interaction is important to take into account when studying why and how bacteria react to the chemical substances that are produced by other bacteria.

Bacteria↗
PubMed2001

The LuxS-dependent autoinducer AI-2 controls the expression of an ABC transporter that functions in AI-2 uptake in Salmonella typhimurium.

In a process called quorum sensing, bacteria communicate with one another using secreted chemical signalling molecules termed autoinducers. A novel autoinducer called AI-2, originally discovered in the quorum-sensing bacterium Vibrio harveyi, is made by many species of Gram-negative and Gram-positive bacteria. In every case, production of AI-2 is dependent on the LuxS autoinducer synthase. The genes regulated by AI-2 in most of these luxS-containing species of bacteria are not known. Here, we describe the identification and characterization of AI-2-regulated genes in Salmonella typhimurium. We find that LuxS and AI-2 regulate the expression of a previously unidentified operon encoding an ATP binding cassette (ABC)-type transporter. We have named this operon the lsr (luxS regulated) operon. The Lsr transporter has homology to the ribose transporter of Escherichia coli and S. typhimurium. A gene encoding a DNA-binding protein that is located adjacent to the Lsr transporter structural operon is required to link AI-2 detection to operon expression. This gene, which we have named lsrR, encodes a protein that represses lsr operon expression in the absence of AI-2. Mutations in the lsr operon render S. typhimurium unable to eliminate AI-2 from the extracellular environment, suggesting that the role of the Lsr apparatus is to transport AI-2 into the cells. It is intriguing that an operon regulated by AI-2 encodes functions resembling the ribose transporter, given recent findings that AI-2 is derived from the ribosyl moiety of S-ribosylhomocysteine.

ATP-Binding Cassette Transporters↗