LuxS: its role in central metabolism and the in vitro synthesis of 4-hydroxy-5-methyl-3(2H)-furanone

Sulfur amino acid metabolism and its control in Lactococcus lactis IL1403

Cysteine and methionine availability influences many processes in the cell. In bacteria, transcription of the specific genes involved in the synthesis of these two amino acids is usually regulated by different mechanisms or regulators. Pathways for the synthesis of cysteine and methionine and their interconversion were experimentally determined for Lactococcus lactis, a lactic acid bacterium commonly found in food. A new gene, yhcE, was shown to be involved in methionine recycling to cysteine. Surprisingly, 18 genes, representing almost all genes of these pathways, are under the control of a LysR-type activator, FhuR, also named CmbR. DNA microarray experiments showed that FhuR targets are restricted to this set of 18 genes clustered in seven transcriptional units, while cysteine starvation modifies the transcription level of several other genes potentially involved in oxidoreduction processes. Purified FhuR binds a 13-bp box centered 46 to 53 bp upstream of the transcriptional starts from the seven regulated promoters, while a second box with the same consensus is present upstream of the first binding box, separated by 8 to 10 bp. O-Acetyl serine increases FhuR binding affinity to its binding boxes. The overall view of sulfur amino acid metabolism and its regulation in L. lactis indicates that CysE could be a master enzyme controlling the activity of FhuR by providing its effector, while other controls at the enzymatic level appear to be necessary to compensate the absence of differential regulation of the genes involved in the interconversion of methionine and cysteine and other biosynthesis genes.

Synthesis of autoinducer 2 by the lyme disease spirochete, Borrelia burgdorferi

Defining the metabolic capabilities and regulatory mechanisms controlling gene expression is a valuable step in understanding the pathogenic properties of infectious agents such as Borrelia burgdorferi. The present studies demonstrated that B. burgdorferi encodes functional Pfs and LuxS enzymes for the breakdown of toxic products of methylation reactions. Consistent with those observations, B. burgdorferi was shown to synthesize the end product 4,5-dihydroxy-2,3-pentanedione (DPD) during laboratory cultivation. DPD undergoes spontaneous rearrangements to produce a class of pheromones collectively named autoinducer 2 (AI-2). Addition of in vitro-synthesized DPD to cultured B. burgdorferi resulted in differential expression of a distinct subset of proteins, including the outer surface lipoprotein VlsE. Although many bacteria can utilize the other LuxS product, homocysteine, for regeneration of methionine, B. burgdorferi was found to lack such ability. It is hypothesized that B. burgdorferi produces LuxS for the express purpose of synthesizing DPD and utilizes a form of that molecule as an AI-2 pheromone to control gene expression.

Events at the host-microbial interface of the gastrointestinal tract III. Cell-to-cell signaling among microbial flora, host, and pathogens: there is a whole lot of talking going on

Humans have an important association with their intestinal microbial flora. The microbial flora helps to shape the mammalian innate immune system, absorbs nutrients, and plays an intricate role on intestinal development. Microbes and mammals communicate with each other through an array of hormone and hormonelike chemical compounds. These "signals," however, are hijacked by bacterial pathogens, such as enterohemorrhagic Eschrichia coli (EHEC), to activate its virulence genes, colonize the host, and start the disease process. This review explores the cell-to-cell signaling events in the gastrointestinal tract that lead EHEC to regulate its virulence genes in a coordinate manner.

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.

Cyclic AMP (cAMP) and cAMP receptor protein influence both synthesis and uptake of extracellular autoinducer 2 in Escherichia coli

Bacterial autoinducer 2 (AI-2) is proposed to be an interspecies mediator of cell-cell communication that enables cells to operate at the multicellular level. Many environmental stimuli have been shown to affect the extracellular AI-2 levels, carbon sources being among the most important. In this report, we show that both AI-2 synthesis and uptake in Escherichia coli are subject to catabolite repression through the cyclic AMP (cAMP)-CRP complex, which directly stimulates transcription of the lsr (for "luxS regulated") operon and indirectly represses luxS expression. Specifically, cAMP-CRP is shown to bind to a CRP binding site located in the upstream region of the lsr promoter and works with the LsrR repressor to regulate AI-2 uptake. The functions of the lsr operon and its regulators, LsrR and LsrK, previously reported in Salmonella enterica serovar Typhimurium, are confirmed here for E. coli. The elucidation of cAMP-CRP involvement in E. coli autoinduction impacts many areas, including the growth of E. coli in fermentation processes.

Chemical synthesis of (S)-4,5-dihydroxy-2,3-pentanedione, a bacterial signal molecule precursor, and validation of its activity in Salmonella typhimurium

We describe an original, short, and convenient chemical synthesis of enantiopure (S)-4,5-dihydroxy-2,3-pentanedione (DPD), starting from commercial methyl (S)-(-)-2,2-dimethyl-1,3-dioxolane-4-carboxylate. DPD is the precursor of autoinducer (AI)-2, the proposed signal for bacterial interspecies communication. AI-2 is synthesized by many bacterial species in three enzymatic steps. The last step, a LuxS-catalyzed reaction, leads to the formation of DPD, which spontaneously cyclizes into AI-2. AI-2-like activity of the synthesized molecule was ascertained by the Vibrio harveyi bioassay. To further validate the biological activity of synthetic DPD and to explore its potential in studying DPD (AI-2)-mediated signaling, a Salmonella typhimurium luxS mutant was constructed. Expression of the AI-2 regulated lsr operon can be rescued in this luxS mutant by addition of synthetic DPD or genetic complementation. Biofilm formation by S. typhimurium has been reported to be defective in a luxS mutant, and this was confirmed in this study to test DPD for chemical complementation. However, biofilm formation of the luxS mutant cannot be restored by addition of DPD. In contrast, introduction of luxS under control of its own promoter complemented biofilm formation. Further results demonstrated that biofilm formation of the luxS mutant cannot be restored with luxS under control of the strong nptII promoter. This indicates that altering the intrinsic promoter activity of luxS affects Salmonella biofilm formation. Conclusively, we synthesized biologically active DPD. Using this chemical compound in combination with genetic approaches opens new avenues in studying AI-2-mediated signaling.

Femtomolar transition state analogue inhibitors of 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase from Escherichia coli

Escherichia coli 5'-methylthioadenosine/S-adenosyl-homocysteine nucleosidase (MTAN) hydrolyzes its substrates to form adenine and 5-methylthioribose (MTR) or S-ribosylhomocysteine (SRH). 5'-Methylthioadenosine (MTA) is a by-product of polyamine synthesis and SRH is a precursor to the biosynthesis of one or more quorum sensing autoinducer molecules. MTAN is therefore involved in quorum sensing, recycling MTA from the polyamine pathway via adenine phosphoribosyltransferase and recycling MTR to methionine. Hydrolysis of MTA by E. coli MTAN involves a highly dissociative transition state with ribooxacarbenium ion character. Iminoribitol mimics of MTA at the transition state of MTAN were synthesized and tested as inhibitors. 5'-Methylthio-Immucillin-A (MT-ImmA) is a slow-onset tight-binding inhibitor giving a dissociation constant (K(i)(*)) of 77 pm. Substitution of the methylthio group with a p-Cl-phenylthio group gives a more powerful inhibitor with a dissociation constant of 2 pm. DADMe-Immucillins are better inhibitors of E. coli MTAN, since they are more closely related to the highly dissociative nature of the transition state. MT-DADMe-Immucillin-A binds with a K(i)(*) value of 2 pm. Replacing the 5'-methyl group with other hydrophobic groups gave 17 transition state analogue inhibitors with dissociation constants from 10(-12) to 10(-14) m. The most powerful inhibitor was 5'-p-Cl-phenylthio-DADMe-Immucillin-A (pClPhT-DADMe-ImmA) with a K(i)(*) value of 47 fm (47 x 10(-15) m). These are among the most powerful non-covalent inhibitors reported for any enzyme, binding 9-91 million times tighter than the MTA and SAH substrates, respectively. The inhibitory potential of these transition state analogue inhibitors supports a transition state structure closely resembling a fully dissociated ribooxacarbenium ion. Powerful inhibitors of MTAN are candidates to disrupt key bacterial pathways including methylation, polyamine synthesis, methionine salvage, and quorum sensing. The accompanying article reports crystal structures of MTAN with these analogues.

An Expeditious Synthesis of DPD and Boron Binding Studies

A practical synthesis has been developed for DPD (4,5-dihydroxypentane-2,3-dione), an unstable small molecule that is proposed to be the source of universal signaling agents for quorum sensing in bacteria. The synthesis allows preparation of isotopically labeled DPD and ent-DPD as well as detailed studies of spontaneous binding to borate to give the unusual borate complex 6, the signal for marine bacteria such as Vibrio harveyi. [reaction: see text]

Quorum sensing in Clostridium difficile: analysis of a luxS-type signalling system

The increasing incidence of Clostridium difficile-associated disease, and the problems associated with its control, highlight the need for additional countermeasures. The attenuation of virulence through the blockade of bacterial cell-to-cell communication (quorum sensing) is one potential therapeutic target. Preliminary studies have shown that C. difficile produces at least one potential signalling molecule. Through the molecule's ability to induce bioluminescence in a Vibrio harveyi luxS reporter strain, it has been shown to correspond to autoinducer 2 (AI-2). In keeping with this observation, a homologue of luxS has been identified in the genome of C. difficile. Adjacent to luxS(Cd) a potential transcriptional regulator and sensor kinase, rolA and rolB, have been located. RT-PCR has been used to confirm the genetic organization of the luxS(Cd) locus. While AI-2 production has not been blocked so far using antisense technology, AI-2 levels could be modulated by controlling expression of the putative transcriptional regulator rolA. RolA, therefore, acts as a negative regulator of AI-2 production. Finally, it has been shown that the exogenous addition of AI-2 or 4-hydroxy-5-methyl-3(2H) furanone has no discernible effect on the production of toxins by C. difficile.

Regulation of uptake and processing of the quorum-sensing autoinducer AI-2 in Escherichia coli

AI-2 is a quorum-sensing signaling molecule proposed to be involved in interspecies communication. In Escherichia coli and Salmonella enterica serovar Typhimurium, extracellular AI-2 accumulates in exponential phase, but the amount decreases drastically upon entry into stationary phase. In S. enterica serovar Typhimurium, the reduction in activity is due to import and processing of AI-2 by the Lsr transporter. We show that the Lsr transporter is functional in E. coli, and screening for mutants defective in AI-2 internalization revealed lsrK and glpD. Unlike the wild type, lsrK and glpD mutants do not activate transcription of the lsr operon in response to AI-2. lsrK encodes the AI-2 kinase, and the lsrK mutant fails to activate lsr expression because it cannot produce phospho-AI-2, which is the lsr operon inducer. glpD encodes the glycerol-3-phosphate (G3P) dehydrogenase, which is involved in glycerol and G3P metabolism. G3P accumulates in the glpD mutant and represses lsr transcription by preventing cyclic AMP (cAMP)-catabolite activator protein (CAP)-dependent activation. Dihydroxyacetone phosphate (DHAP) also accumulates in the glpD mutant, and DHAP represses lsr transcription by a cAMP-CAP-independent mechanism involving LsrR, the lsr operon repressor. The requirement for cAMP-CAP in lsr activation explains why AI-2 persists in culture fluids of bacteria grown in media containing sugars that cause catabolite repression. These findings show that, depending on the prevailing growth conditions, the amount of time that the AI-2 signal is present and, in turn, the time that a given community of bacteria remains exposed to this signal can vary greatly.

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

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

Chemical communication in proteobacteria: biochemical and structural studies of signal synthases and receptors required for intercellular signalling

Cell-cell communication via the production and detection of chemical signal molecules has been the focus of a great deal of research over the past decade. One class of chemical signals widely used by proteobacteria consists of N-acyl-homoserine lactones, which are synthesized by proteins related to LuxI of Vibrio fischeri and are detected by proteins related to the V. fischeri LuxR protein. A related marine bacterium, Vibrio harveyi, communicates using two chemical signals, one of which, autoinducer-2 (AI-2), is a furanone borate diester that is synthesized by the LuxS protein and detected by a periplasmic protein called LuxP. Evidence from a number of laboratories suggests that AI-2 may be used as a signal by diverse groups of bacteria, and might permit intergeneric signalling. These two families of signalling systems have been studied from the perspectives of physiology, ecology, biochemistry, and more recently, structural biology. Here, we review the biochemistry and structural biology of both acyl-homoserine-lactone-dependent and AI-2-dependent signalling systems.

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.

Growth phase regulation of flaA expression in Helicobacter pylori is luxS dependent

LuxS plays a role in the synthesis of an extracellular signaling molecule, autoinducer 2 (AI-2). To analyze a possible role of AI-2 in regulating Helicobacter pylori gene expression, we constructed a panel of transcriptional reporter strains. We show that the expression of H. pylori flaA is growth phase dependent and that flaA transcription increases in association with increased culture density. Mutating the luxS gene eliminates growth-phase-dependent control of flaA, and this growth phase dependence is restored when the luxS mutant strain is complemented with the wild-type luxS gene.

Chemical signaling among bacteria and its inhibition

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

Regulation of Vibrio anguillarum empA metalloprotease expression and its role in virulence

Atlantic salmon (Salmo salar) were challenged with Vibrio anguillarum strains M93Sm and NB10 and empA null mutants M99 and NB12. Both wild types were virulent when administered by intraperitoneal (i.p.) injection or anal intubation. NB12 was avirulent via either route of infection. M99 virulence was attenuated when delivered by intubation, but fully virulent by i.p. injection. Northern blot analysis revealed empA expression in M93Sm and NB10 cells incubated in mucus, while incubation in Luria-Bertani broth plus 2% NaCl (LB20) induced empA expression only in NB10. Nucleotide differences between M93Sm and NB10 empA sequences were found in regions located 207 and 229 bp upstream of the empA translational start. Reverse transcription-PCR and 5' rapid amplification of cDNA ends revealed the empA transcriptional start site 85 bp upstream of the translational start for both strains. A putative sigma(S)-dependent promoter was identified upstream of the transcriptional start in both strains. Site-directed mutagenesis was used to create rpoS mutants of M93Sm and NB10. Neither rpoS mutant exhibited protease activity. Since empA is expressed during stationary phase, the effects of conditioned medium on protease activity were examined. M99 conditioned LB20 supernatants stimulated protease activity in NB10 while allowing M93Sm to produce protease in LB20. Neither acyl homoserine lactones nor AI-2 induced protease activity. Conditioned LB20 supernatant from a V. anguillarum luxS mutant caused a more rapid induction of protease activity in wild-type cells. Our data show that expression of empA is differentially regulated in V. anguillarum strains NB10 and M93Sm and requires sigma(S), quorum-sensing molecules, and gastrointestinal mucus.

The luxS gene is not required for Borrelia burgdorferi tick colonization, transmission to a mammalian host, or induction of disease

luxS mutants of Borrelia burgdorferi strain 297 naturally colonized their arthropod (Ixodes scapularis) vector, were maintained in ticks throughout the molting process (larvae to nymphs), were tick transmitted to uninfected mice, and elicited histopathology in mice indistinguishable from that induced by wild-type B. burgdorferi.

Autoinducer-2-like activity associated with foods and its interaction with food additives

The autoinducer-2 (AI-2) molecule produced by bacteria as part of quorum sensing is considered to be a universal inducer signal in bacteria because it reportedly influences gene expression in a variety of both gram-negative and gram-positive bacteria. The objective of this study was to determine whether selected fresh produce and processed foods have AI-2-like activity and whether specific food additives can act as AI-2 mimics and result in AI-2-like activity. The luminescence-based response of the reporter strain Vibrio harveyi BB170 was used as the basis for determining AI-2 activity in the selected foods and food ingredients. Maximum AI-2 activity was seen on the frozen fish sample (203-fold, compared with the negative control) followed by tomato, cantaloupe, carrots, tofu, and milk samples. Interestingly, some samples were capable of inhibiting AI-2 activity. Turkey patties showed the highest inhibition (99.8% compared with the positive control) followed by chicken breast (97.5%), homemade cheeses (93.7%), beef steak (90.6%), and beef patties (84.4%). AI-2 activity was almost totally inhibited by sodium propionate, whereas sodium benzoate caused 93.3% inhibition, compared with 75% inhibition by sodium acetate. Sodium nitrate did not have any appreciable effect, even at 200 ppm. Understanding the relationships that exist between AI-2 activity on foods and the ecology of pathogens and food spoilage bacteria on foods could yield clues about factors controlling food spoilage and pathogen virulence.

LuxS is required for persistent pneumococcal carriage and expression of virulence and biosynthesis genes

Streptococcus pneumoniae causes several diseases, including otitis media, pneumonia, and meningitis. Although little is known about the regulation of or how individual pneumococcal factors contribute to these disease states, there is evidence suggesting that some factors are regulated by a cell-density-dependent mechanism (quorum sensing). Quorum sensing allows bacteria to couple transcription with changes in cell density; bacteria achieve this by sensing and responding to small diffusible signaling molecules. We investigated how the LuxS signaling system impacts the biology of S. pneumoniae. An analysis of the transcriptional profiles of a serotype 2 strain and an isogenic luxS deletion strain utilizing an S. pneumoniae-specific microarray indicated that LuxS regulates gene expression involved in discrete cellular processes, including pneumolysin expression. Contrary to the paradigm for quorum sensing, we observed pronounced effects on transcription in early log phase, where gene expression was repressed in the mutant. Assessing the mutant for its ability to infect and cause disease in animals revealed a profound defect in ability to persist in the nasopharyngeal tissues. Our analysis of an S. pneumoniae transcriptome revealed a function for LuxS in gene regulation that is not dependent upon high cell density and is likely involved in the maintenance of pneumococcal load in susceptible hosts.

Specific growth rate and not cell density controls the general stress response in Escherichia coli

In batch cultures ofEscherichia coli, the intracellular concentration of the general stress response sigma factor RpoS typically increases during the transition from the exponential to the stationary growth phase. However, because this transition is accompanied by complex physico-chemical and biological changes, which signals predominantly elicit this induction is still the subject of debate. Careful design of the growth environment in chemostat and batch cultures allowed the separate study of individual factors affecting RpoS. Specific growth rate, and not cell density or the nature of the growth-limiting nutrient, controlled RpoS expression and RpoS-dependent hydroperoxidase activity. Furthermore, it was demonstrated that the standardE. coliminimal medium A (MMA) is not suitable for high-cell-density cultivation because it lacks trace elements. Previously reported cell-density effects in chemostat cultures ofE. colican be explained by a hidden, secondary nutrient limitation, which points to the importance of medium design and appropriate experimental set-up for studying cell-density effects.

luxS mutants of Serratia defective in autoinducer-2-dependent ‘quorum sensing’ show strain-dependent impacts on virulence and production of carbapenem and prodigiosin

The enzyme LuxS is responsible for the production of autoinducer-2 (AI-2), a molecule that has been implicated in quorum sensing in many bacterial species. This study investigated whether there is aluxS-dependent signalling system in the Gram-negative bacteriaSerratiaspp.Serratia marcescensis a broad-host-range pathogen and an important cause of nosocomial infections. Production of AI-2 activity was detected inS. marcescensATCC 274 andSerratiaATCC 39006 and theirluxSgenes were sequenced.luxSmutants were constructed in these strains and were analysed to determine which phenotypes are regulated byluxSand therefore, potentially, by AI-2. The phenotypes of theluxSmutants included decreased carbapenem antibiotic production inSerratiaATCC 39006 and decreased prodigiosin and secreted haemolysin production inS. marcescensATCC 274. TheluxSmutant ofS. marcescensATCC 274 was also found to exhibit modestly reduced virulence in aCaenorhabditis elegansmodel. Finally, it was shown that the culture supernatant of a wild-type strain contains a signal, presumably AI-2, capable of complementing the prodigiosin defect of theluxSmutant of another strain, even when substantially diluted. It is concluded thatluxSmodulates virulence and antibiotic production inSerratia, in a strain-dependent manner, and that, for at least one phenotype, this regulation is via extracellular signalling.

Quorum sensing: a primer for food microbiologists

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

Lsr-mediated transport and processing of AI-2 in Salmonella typhimurium

The LuxS-dependent autoinducer AI-2 is proposed to function in interspecies cell-cell communication in bacteria. In Salmonella typhimurium, AI-2 is produced and released during exponential growth and is subsequently imported into the bacteria via the Lsr (luxS regulated) ATP binding cassette (ABC) transporter. AI-2 induces transcription of the lsrACDBFGE operon, the first four genes of which encode the Lsr transport apparatus. In this report, we identify and characterize LsrK, a new protein that is required for the regulation of the lsr operon and the AI-2 uptake process. LsrK is a kinase that phosphorylates AI-2 upon entry into the cell. Our data indicate that phosphorylation of AI-2 results in its sequestration in the cytoplasm. We suggest that phospho-AI-2 is the inducer responsible for inactivation of LsrR, the repressor of the lsr operon. We also show that two previously uncharacterized members of the lsr operon, LsrF and LsrG, are necessary for the further processing of phospho-AI-2. Transport and processing of AI-2 could be required for removing the quorum-sensing signal, conveying the signal to an internal detector and/or scavenging boron.

Restricted distribution of the butyrate kinase pathway among butyrate-producing bacteria from the human colon

The final steps in butyrate synthesis by anaerobic bacteria can occur via butyrate kinase and phosphotransbutyrylase or via butyryl-coenzyme A (CoA):acetate CoA-transferase. Degenerate PCR and enzymatic assays were used to assess the presence of butyrate kinase among 38 anaerobic butyrate-producing bacterial isolates from human feces that represent three different clostridial clusters (IV, XIVa, and XVI). Only four strains were found to possess detectable butyrate kinase activity. These were also the only strains to give PCR products (verifiable by sequencing) with degenerate primer pairs designed within the butyrate kinase gene or between the linked butyrate kinase/phosphotransbutyrylase genes. Further analysis of the butyrate kinase/phosphotransbutyrylase genes of one isolate, L2-50, revealed similar organization to that described previously from different groups of clostridia, along with differences in flanking sequences and phylogenetic relationships. Butyryl-CoA:acetate CoA-transferase activity was detected in all 38 strains examined, suggesting that it, rather than butyrate kinase, provides the dominant route for butyrate formation in the human colonic ecosystem that contains a constantly high concentration of acetate.

A novel type of two-component regulatory system affecting gingipains in Porphyromonas gingivalis

We surveyed the Porphyromonas gingivalis W83 genome database for homologues of FimS, the first two-component sensor histidine kinase, which could possibly control virulence factors. Including fimS, we found six putative sensor kinase genes in the genome. The gene encoding one of the homologues was cloned from a P. gingivalis plasmid library, sequenced, and analyzed using its mutants. Two gene-disruption mutants were created in strain ATCC 33277 by introducing a drug cassette into the gene. The mutants formed nonpigmented colonies, indicating that they might be defective in proteinase production, a characteristic of this organism. Proteinase activities, measured as arginine- and lysine-specific (Rgp and Kgp gingipains, respectively) activities, of the mutants were almost half those of the parent strain. Unlike the parent and wildtype strains, most of the gingipain activities were detected in the culture supernatant, not in cells, of the mutants. Abnormal production of gingipains was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western blot analyses. These results strongly suggest that this newly-discovered two-component sensor kinase is involved in maturation and proper localization of gingipains to the outer membrane through an unknown mechanism. The gene encoding the sensor histidine kinase was designated gppX, which represents regulation (X) of gingipains and black pigmentation in P. gingivalis.

Bacterial variations on the methionine salvage pathway

BACKGROUND

The thiomethyl group of S-adenosylmethionine is often recycled as methionine from methylthioadenosine. The corresponding pathway has been unravelled in Bacillus subtilis. However methylthioadenosine is subjected to alternative degradative pathways depending on the organism.

RESULTS

This work uses genome in silico analysis to propose methionine salvage pathways for Klebsiella pneumoniae, Leptospira interrogans, Thermoanaerobacter tengcongensis and Xylella fastidiosa. Experiments performed with mutants of B. subtilis and Pseudomonas aeruginosa substantiate the hypotheses proposed. The enzymes that catalyze the reactions are recruited from a variety of origins. The first, ubiquitous, enzyme of the pathway, MtnA (methylthioribose-1-phosphate isomerase), belongs to a family of proteins related to eukaryotic intiation factor 2B alpha. mtnB codes for a methylthioribulose-1-phosphate dehydratase. Two reactions follow, that of an enolase and that of a phosphatase. While in B. subtilis this is performed by two distinct polypeptides, in the other organisms analyzed here an enolase-phosphatase yields 1,2-dihydroxy-3-keto-5-methylthiopentene. In the presence of dioxygen an aci-reductone dioxygenase yields the immediate precursor of methionine, ketomethylthiobutyrate. Under some conditions this enzyme produces carbon monoxide in B. subtilis, suggesting a route for a new gaseous mediator in bacteria. Ketomethylthiobutyrate is finally transaminated by an aminotransferase that exists usually as a broad specificity enzyme (often able to transaminate aromatic aminoacid keto-acid precursors or histidinol-phosphate).

CONCLUSION

A functional methionine salvage pathway was experimentally demonstrated, for the first time, in P. aeruginosa. Apparently, methionine salvage pathways are frequent in Bacteria (and in Eukarya), with recruitment of different polypeptides to perform the needed reactions (an ancestor of a translation initiation factor and RuBisCO, as an enolase, in some Firmicutes). Many are highly dependent on the presence of oxygen, suggesting that the ecological niche may play an important role for the existence and/or metabolic steps of the pathway, even in phylogenetically related bacteria. Further work is needed to uncover the corresponding steps when dioxygen is scarce or absent (this is important to explore the presence of the pathway in Archaea). The thermophile T. tengcongensis, that thrives in the absence of oxygen, appears to possess the pathway. It will be an interesting link to uncover the missing reactions in anaerobic environments.

The Synthesis of N-Ribosyl Transferase Inhibitors Based on a Transition State Blueprint

A quarter of a century ago transition state analysis and transition state analogue design promised the prospect of extraordinarily potent enzyme inhibitors. The present overview describes the transition state analysis of a variety of N-ribosyl transferases, the design and synthesis of extremely powerful transition state analogue inhibitors of these nucleoside processing enzymes, and their current therapeutic uses and potentials.

Differential gene expression shows natural brominated furanones interfere with the autoinducer-2 bacterial signaling system ofEscherichia coli

The quorum sensing disrupter (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone (furanone) of the alga Delisea pulchra was previously found by us (Environ Microbiol 3:731-736, 2001) to inhibit quorum sensing in Escherichia coli via autoinducer-2 (AI-2, produced by LuxS). In this study, DNA microarrays were used to study the genetic basis of this natural furanone inhibition of AI-2 signaling (significant values with p < 0.05 are reported). Using DNA microarrays, the AI-2 mutant Escherichia coli DH5alpha was compared with the AI-2 wild-type strain, E. coli K12, to determine how AI-2 influenced gene expression. Escherichia coli K12 was also grown with 0 and 60 microg/mL furanone to study the inhibition of quorum sensing gene expression. It was found that 166 genes were differentially expressed by AI-2 (67 were induced and 99 were repressed) and 90 genes were differentially expressed by furanone (34 were induced and 56 were repressed). Importantly, 79% (44 out of 56) of the genes repressed by furanone were induced by AI-2, which indicated that furanone inhibited AI-2 signaling and influenced the same suite of genes as a regulon. Most of these genes have functions of chemotaxis, motility, and flagellar synthesis. Interestingly, the aerotaxis genes aer and tsr were discovered to be induced by AI-2 and repressed by furanone. Representative microarray results were confirmed by RNA dot blotting. Furthermore, the E. coli air-liquid interface biofilm formation was repressed by furanone, supporting the results that taxis and flagellar genes were repressed by furanone. The autoinducer bioassay indicated that 100 microg/mL furanone decreased the extracellular concentration of AI-2 2-fold, yet luxS and pfs transcription were not significantly altered. Hence, furanone appeared to alter AI-2 signaling post-transcriptionally.

Pharmacological inhibition of quorum sensing for the treatment of chronic bacterial infections

Traditional treatment of infectious diseases is based on compounds that aim to kill or inhibit bacterial growth. A major concern with this approach is the frequently observed development of resistance to antimicrobial compounds. The discovery of bacterial-communication systems (quorum-sensing systems), which orchestrate important temporal events during the infection process, has afforded a novel opportunity to ameliorate bacterial infection by means other than growth inhibition. Compounds able to override bacterial signaling are present in nature. Herein we discuss the known signaling mechanisms and potential antipathogenic drugs that specifically target quorum-sensing systems in a manner unlikely to pose a selective pressure for the development of resistant mutants.

Chemical synthesis of S-ribosyl-l-homocysteine and activity assay as a LuxS substrate

Bacterial quorum sensing is mediated by autoinducers, small signaling molecules generated by bacteria. It has been proposed that the LuxS enzyme converts S-ribosyl-L-homocysteine to 4,5-dihydroxy-2,3-pentanedione, the precursor of autoinducer 2 (AI-2). We report here a chemical synthesis of S-ribosyl-L-homocysteine and its analogue using Mitsunobu coupling. Chemically synthesized ribosylhomocysteine has been confirmed as a substrate for LuxS in both an enzyme assay and a whole cell quorum sensing assay. The chemical entities of products from the LuxS reaction were also established. Several ribosylhomocysteine analogues have been tested as LuxS inhibitors.

Alternative pathway for the formation of 4,5-dihydroxy-2,3-pentanedione, the proposed precursor of 4-hydroxy-5-methyl-3(2H)-furanone as well as autoinducer-2, and its detection as natural constituent of tomato fruit

The formation of 4-hydroxy-5-methyl-3(2H)-furanone (HMF, norfuraneol) by spinach ribosephosphate isomerase was reinvestigated. Incubation experiments using D-ribose-5-phosphate and D-ribulose-5-phosphate clearly revealed a spontaneous nonenzymatic formation of the hydroxy-furanone from the ketose-phosphate under physiological conditions at 35 degrees C and pH 7.5, whereupon up to 1.3% of D-ribulose-5-phosphate was transformed to HMF within 15 h. 4,5-Dihydroxy-2,3-pentanedione was deduced as ultimate precursor of HMF, since addition of o-phenylenediamine to the incubation mixture led to lower amounts of HMF and to the formation of 3-(1,2-dihydroxyethyl)-2-methylquinoxaline, which was identified by means of high pressure liquid chromatography with diode array detection (HPLC-DAD), HPLC-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) and NMR spectroscopy. Additionally, the spontaneous formation of 4,5-dihydroxy-2,3-pentanedione was demontrated by its conversion to the respective alditol acetate using either NaBH(4) or NaBD(4) for the reduction. Comparative gas chromatography-mass spectrometry (GC-MS) analysis revealed the incorporation of two deuterium atoms and confirmed the dicarbonyl structure. Application of 1-13C-D-ribulose-5-phosphate as well as 5-13C-D-ribulose-5-phosphate and analysis of the derived quinoxaline derivatives by HPLC-ESI-MS/MS demonstrated the formation of the methyl-group at C-5 of the carbohydrate phosphate in consequence of a nonenzymatic phosphate elimination. Application of o-phenylenediamine into ripe tomatoes led to the detection of 3-(1,2-dihydroxyethyl)-2-methylquinoxaline by means of HPLC-MS/MS analysis implying the genuine occurrence of 4,5-dihydroxy-2,3-pentanedione in this fruit.

Enzymatic transition state poise and transition state analogues

The development of kinetic isotope effect methods for enzymatic reactions has resulted in the systematic determination of enzymatic transition state structure for several distinct chemical reaction mechanisms. Although it is early in the experimental development of the method, examples of concerted nucleophilic displacements (A(N)D(N) or S(N)2), aromatic nucleophilic displacements (A(N)D(N) or S(N)Ar), and both concerted and stepwise dissociative nucleophilic displacements (D(N)A(N) and D(N)A(N); S(N)1 reactions) have been exemplified. The transition state for each reaction exhibits a characteristic extent of bond-breaking and bond-making, defined here as transition state poise. Thus, concerted nucleophilic displacements (S(N)2 or D(N)A(N)) exhibit various extents of residual bond order to the leaving group and to the attacking nucleophile at the transition state. Aromatic nucleophilic displacements reach their rate-limiting transition states before or after formation of the tetrahedral intermediate. Several concerted, symmetric nucleophilic displacements at carbon have been described. Enzymatic transition state poise is summarized in a single diagram of bond orders using the terminology of Jencks. The analysis reveals enzymatic contributions to transition state poise, provides precedent for assignment of reaction types, and summarizes the current status of the experimental characterization of enzymatic transition states. Binding strengths of transition state analogues are readily correlated with transition state poise.

Autoinducer 2 production by Streptococcus gordonii DL1 and the biofilm phenotype of a luxS mutant are influenced by nutritional conditions

The luxS gene, present in many bacterial genera, encodes the autoinducer 2 (AI-2) synthase. AI-2 has been implicated in bacterial signaling, and this study investigated its role in biofilm formation by Streptococcus gordonii, an organism that colonizes human tooth enamel within the first few hours after professional cleaning. Northern blotting and primer extension analyses revealed that S. gordonii luxS is monocistronic. AI-2 production was dependent on nutritional conditions, and maximum AI-2 induction was detected when S. gordonii was grown in the presence of serum and carbonate. In planktonic cultures, AI-2 production rose sharply during the transition from exponential to stationary phase, and the AI-2 concentration peaked approximately 4 h into stationary phase. An S. gordonii luxS mutant that did not produce AI-2 was constructed by homologous recombination. Complementation of the mutant by insertion of an intact luxS gene into the chromosome in tandem with the disrupted gene restored AI-2 production to a level similar to that of the wild-type strain. In planktonic culture, no growth differences were observed between the mutant and wild-type strains when five different media were used. However, when grown for 4 h as biofilms in 25% human saliva under flow, the luxS mutant formed tall microcolonies that differed from those formed by the wild-type and complemented mutant strains. Biofilms of the luxS mutant exhibited finger-like projections of cells that extended into the flow cell lumen. Thus, the inability to produce AI-2 is associated with altered microcolony architecture within S. gordonii biofilms formed in saliva during a time frame consistent with initial colonization of freshly cleaned enamel surfaces.

Signal-mediated cross-talk regulates stress adaptation in Vibrio species

Quorum sensing systems serve as a means of 'census taking' of conspecific and non-conspecific bacteria in the near vicinity. The acylated homoserine lactone (AHL) quorum sensing system has been proposed to be primarily an intra-specific communication system, while the AI-2 autoinducer signalling system is proposed to be an interspecific communication system. Here it is shown that AI-2-like signalling in two marine Vibrio species, Vibrio vulnificus and 'Vibrio angustum' S14, induces the core response phenotypes of starvation adaptation and stress resistance, and that a signal antagonist can competitively inhibit these phenotypes. Furthermore, the signals produced by a range of Vibrio species have the ability to induce these phenotypes in V. vulnificus and 'V. angustum' S14, indicating that, at least in Vibrio species, AI-2-like signalling systems function as interspecies communication systems capable of 'cross-talk' and of regulating environmentally relevant phenotypes.

Virulence of enteropathogenic Escherichia coli, a global pathogen

Enteropathogenic Escherichia coli (EPEC) remains an important cause of diarrheal disease worldwide. Research into EPEC is intense and provides a good virulence model of other E. coli infections as well as other pathogenic bacteria. Although the virulence mechanisms are now better understood, they are extremely complex and much remains to be learnt. The pathogenesis of EPEC depends on the formation of an ultrastructural lesion in which the bacteria make intimate contact with the host apical enterocyte membrane. The formation of this lesion is a consequence of the ability of EPEC to adhere in a localized manner to the host cell, aided by bundle-forming pili. Tyrosine phosphorylation and signal transduction events occur within the host cell at the lesion site, leading to a disruption of the host cell mechanisms and, consequently, to diarrhea. These result from the action of highly regulated EPEC secreted proteins which are released via a type III secretion system, many genes of which are located within a pathogenicity island known as the locus of enterocyte effacement. Over the last few years, dramatic increases in our knowledge of EPEC virulence have taken place. This review therefore aims to provide a broad overview of and update to the virulence aspects of EPEC.

Detection of a luxS-signaling molecule in Bacillus anthracis

Quorum-sensing regulation of density-dependent genes has been described for numerous bacterial species. The partially annotated genome sequence of Bacillus anthracis contains an open reading frame (BA5047) predicted to encode an ortholog of luxS, required for synthesis of the quorum-sensing signaling molecule autoinducer-2 (AI-2). To determine whether B. anthracis produces AI-2, the Vibrio harveyi luminescence bioassay was used. Cell-free conditioned media from vaccine (Sterne) strain 34F(2) induced luminescence in V. harveyi reporter strain BB170, indicating its production of AI-2. Cloned BA5047, expressed in Escherichia coli DH5 alpha cells, restored AI-2 activity to these cells. To evaluate whether BA5047 is essential for AI-2 synthesis, it was deleted through allelic exchange with marker rescue; the resulting mutant had no functional luxS activity and had reduced growth in vitro. In the wild-type strain, AI-2 activity was greatest during the exponential phase of growth. In total, these data indicate that BA5047 is a functional luxS ortholog in B. anthracis necessary for growth-phase-specific AI-2 expression. Thus, B. anthracis may utilize extracellular signaling molecules to regulate density-dependent gene expression.

Microbial dinner-party conversations: the role of LuxS in interspecies communication

Bacteria have a tendency to be gregarious by nature. Whether on abiotic surfaces in the environment or on the mucosal surfaces of humans, bacteria accumulate in complex multi-species communities. In these dynamic accretions, bacteria can be densely packed and often depend on each other for the provision of metabolic substrates. Under these circumstances, it will be advantageous for bacteria to be able to detect the presence of their neighbours, to communicate with them and to co-ordinate various physiological activities. Such cell-cell sensing and communication systems can be established through the release and detection of chemical signalling molecules. While originally considered a feature characteristic of eukaryotes, the exchange of chemical signals has now been demonstrated in many bacterial species and ecosystems. Indeed, it has even been suggested that assemblages of bacterial species can be considered as proto-multicellular organisms, whereby biological processes are controlled for the benefit of the entire community. Regardless of the extent to which bacterial communication represents a step on the road to multicellularity, it is becoming increasingly apparent that the signalling systems devised by bacteria are essential for successful relationships with other bacteria and with eukaryotic hosts.

Identification of Actinobacillus pleuropneumoniae genes important for survival during infection in its natural host

Actinobacillus pleuropneumoniae is a strict respiratory tract pathogen of swine and is the causative agent of porcine pleuropneumonia. We have used signature-tagged mutagenesis (STM) to identify genes required for survival of the organism within the pig. A total of 2,064 signature-tagged Tn10 transposon mutants were assembled into pools of 48 each, and used to inoculate pigs by the endotracheal route. Out of 105 mutants that were consistently attenuated in vivo, only 11 mutants showed a >2-fold reduction in growth in vitro compared to the wild type, whereas 8 of 14 mutants tested showed significant levels of attenuation in pig as evidenced from competitive index experiments. Inverse PCR was used to generate DNA sequence of the chromosomal domains flanking each transposon insertion. Only one sibling pair of mutants was identified, but three apparent transposon insertion hot spots were found--an anticipated consequence of the use of a Tn10-based system. Transposon insertions were found within 55 different loci, and similarity (BLAST) searching identified possible analogues or homologues for all but four of these. Matches included proteins putatively involved in metabolism and transport of various nutrients or unknown substances, in stress responses, in gene regulation, and in the production of cell surface components. Ten of the sequences have homology with genes involved in lipopolysaccharide and capsule production. The results highlight the importance of genes involved in energy metabolism, nutrient uptake and stress responses for the survival of A. pleuropneumoniae in its natural host: the pig.

Glucan-binding proteins of the oral streptococci

The synthesis of extracellular glucan is an integral component of the sucrose-dependent colonization of tooth surfaces by species of the mutans streptococci. In investigators' attempts to understand the mechanisms of plaque biofilm development, several glucan-binding proteins (GBPs) have been discovered. Some of these, the glucosyltransferases, catalyze the synthesis of glucan, whereas others, designated only as glucan-binding proteins, have affinities for different forms of glucan and contribute to aspects of the biology of their host organisms. The functions of these latter glucan-binding proteins include dextran-dependent aggregation, dextranase inhibition, plaque cohesion, and perhaps cell wall synthesis. In some instances, their glucan-binding domains share common features, whereas in others the mechanism for glucan binding remains unknown. Recent studies indicate that at least some of the glucan-binding proteins modulate virulence and some can act as protective immunogens within animal models. Overall, the multiplicity of GBPs and their aforementioned properties are testimonies to their importance. Future studies will greatly advance the understanding of the distribution, function, and regulation of the GBPs and place into perspective the facets of their contributions to the biology of the oral streptococci.

Mutation of luxS of Streptococcus pneumoniae affects virulence in a mouse model

The LuxS protein is required for the biosynthesis of the type 2 autoinducer (AI-2), which is involved in quorum sensing in a wide range of bacterial species. We have determined the effects of a defined luxS mutation on the virulence of Streptococcus pneumoniae. Although the luxS mutant displayed reduced virulence relative to its wild-type parent, the type 2 strain D39, it was by no means avirulent in a mouse model. After intranasal administration, the luxS mutant was able to colonize the nasopharynx of the mouse as efficiently as the wild type. However, it was less able to spread from the nasopharynx to the lungs or the blood. Intraperitoneal coadministration studies indicated that the luxS mutant was less fit and was readily outcompeted by wild-type D39. However, when administered on its own by this route, the mutant was able to proliferate and cause fatal systemic disease, albeit at a lower rate than the wild type. Western blot analysis of whole-cell lysates of the mutant and its parent did not reveal any differences in the levels of several well-characterized virulence proteins. However, analysis of Coomassie blue-stained protein profiles after separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that mutation of luxS had pleiotropic effects on protein expression in all cellular compartments. This is consistent with the product of luxS having a regulatory role in S. pneumoniae. This is the first report of a direct role for luxS (and by inference, AI-2) in the virulence of a gram-positive pathogen. However, the fact that mutagenesis of luxS does not completely attenuate S. pneumoniae has implications for the possible use of AI-2 antagonists for treatment of pneumococcal infections.

Expression of a luxS gene is not required for Borrelia burgdorferi infection of mice via needle inoculation

The luxS gene product is an integral component of LuxS/autoinducer-2 (AI-2) quorum-sensing systems in bacteria. A putative luxS gene was expressed at comparable levels by Borrelia burgdorferi strain 297 cultivated either in vitro or in dialysis membrane chambers implanted in rat peritoneal cavities. Although the borrelial luxS gene functionally complemented a LuxS deficiency in Escherichia coli DH5 alpha, AI-2-like activity could not be detected within B. burgdorferi culture supernatants or concentrated cell lysates. Finally, a luxS-deficient mutant of B. burgdorferi was infectious at wild-type levels when it was intradermally needle inoculated into mice, indicating that expression of luxS probably is not required for infectivity but, at the very least, is not essential for mammalian host adaptation. Our findings also challenge the notion that a LuxS/AI-2 quorum-sensing system is operative in B. burgdorferi.

Quorum sensing and its relevance to infectious diseases

Quorum sensing allows bacteria to detect the density of their own species and alter their metabolism to take advantage of this density. Quorum sensing is used by a wide variety of bacteria including human pathogens. Quorum sensing genes are important for the pathogenic potential of Pseudomonas aeruginosa and Staphylococcus aureus, as well as other invasive bacteria. An understanding of quorum sensing may lead to new therapeutic strategies.

LuxS quorum sensing: more than just a numbers game

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

Autoinducer 2 activity in Escherichia coli culture supernatants can be actively reduced despite maintenance of an active synthase, LuxS

Production of the signalling molecule (autoinducer-2) synthesized by LuxS has been proposed to be pivotal to a universal mechanism of inter-species bacterial cell-cell communication (quorum sensing); however recently the function of LuxS has been noted to be integral to central metabolism since it contributes to the activated methyl cycle. This paper shows that when Helicobacter pylori LuxS is overproduced in Escherichia coli, it forms cross-linkable multimers. These multimers persist at comparable levels after 24 h of growth if glucose is omitted from the growth medium; however, the levels of extracellular autoinducer-2 decline (Glucose Retention of AI-2 Levels: GRAIL). Glycerol, maltose, galactose, ribose and L-arabinose could substitute for glucose, but lactose, D-arabinose, acetate, citrate and pyruvate could not. Mutations in (i). metabolic pathways (glycolytic enzymes eno, pgk, pgm; galactose epimerase; the Pta-AckA pathway), (ii). sugar transport (pts components, rbs operon, mgl, trg), and (iii). regulators involved in conventional catabolic repression (crp, cya), cAMP-independent catabolite repression (creC, fruR, rpoS,) the stringent response (relA, spoT) and the global carbon storage regulator (csrA) did not prevent GRAIL. Although the basis of GRAIL remains uncertain, it is clear that the mechanism is distinct from conventional catabolite repression. Moreover, GRAIL is not due to inactivation of the enzymic activity of LuxS, since in E. coli, LuxS contained within stationary-phase cells grown in the absence of glucose maintains its activity in vitro.

Formation of 5-methyl-4-hydroxy-3[2H]-furanone in cytosolic extracts obtained from Zygosaccharomyces rouxii

Formation of the flavor compound and precursor 4-hydroxy-5-methyl-3[2H]-furanone (HMF, norfuraneol) was demonstrated in cytosolic protein extracts obtained from Zygosaccharomyces rouxii after incubation with a number of carbohydrate phosphates. 4-Hydroxy-5-methyl-3[2H]-furanone was produced from d-fructose-1,6-diphosphate, d-fructose-6-phosphate, d-glucose-6-phosphate, 6-phosphogluconate, d-ribose-5-phosphate, and d-ribulose-1,5-diphosphate. Enzyme assays revealed d-fructose-1,6-diphosphatase, phosphohexose isomerase, d-glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase activity in the cytosolic extracts. Model studies showed the spontaneous formation of HMF from d-ribulose-5-phosphate. It is assumed that d-ribulose-5-phosphate is generated in cytosolic extracts by the action of the investigated enzymes from the carbohydrate phosphates and is then chemically transformed to HMF. The hypothesis was proven by the production of HMF in solutions containing commercially available enzymes and [6-(13)C]-d-glucose-6-phosphate.

luxS and arcB control aerobic growth of Actinobacillus actinomycetemcomitans under iron limitation

LuxS is responsible for the production of autoinducer 2 (AI-2), which functions in Vibrio harveyi as a quorum-sensing signal that controls the cell density-dependent expression of the lux operon. In nonluminescent organisms, the physiologic role of AI-2 is not clear. We report that inactivation of luxS in Actinobacillus actinomycetemcomitans JP2 results in reduced growth of the mutant, but not the wild-type organism, under aerobic, iron-limited conditions. Stunted cultures of the luxS mutant A. actinomycetemcomitans JP2-12 grew to high cell density when subcultured under iron-replete conditions. In addition, the mutant strain grew to high cell density under iron limitation after transformation with a plasmid containing a functional copy of luxS. Results of real-time PCR showed that A. actinomycetemcomitans JP2-12 exhibited significantly reduced expression of afuA (eightfold), fecBCDE (10-fold), and ftnAB (>50-fold), which encode a periplasmic ferric transport protein, a putative ferric citrate transporter, and ferritin, respectively. The expressions of putative receptors for transferrin, hemoglobin, and hemophore binding protein were also reduced at more modest levels (two- to threefold). In contrast, expressions of sidD and frpB (encoding putative siderophore receptors) were increased 10- and 3-fold, respectively, in the luxS mutant. To better understand the mechanism of the AI-2 response, the A. actinomycetemcomitans genome was searched for homologs of the V. harveyi signal transduction proteins, LuxP, LuxQ, LuxU, and LuxO. Interestingly, ArcB was found to be most similar to LuxQ sensor/kinase. To determine whether arcB plays a role in the response of A. actinomycetemcomitans to AI-2, an arcB-deficient mutant was constructed. The isogenic arcB mutant grew poorly under anaerobic conditions but grew normally under aerobic iron-replete conditions. However, the arcB mutant failed to grow aerobically under iron limitation, and reverse transcriptase PCR showed that inactivation of arcB resulted in decreased expression of afuA and ftnAB. Thus, isogenic luxS and arcB mutants of A. actinomycetemcomitans exhibit similar phenotypes when cultured aerobically under iron limitation, and both mutants exhibit reduced expression of a common set of genes involved in the transport and storage of iron. These results suggest that LuxS and ArcB may act in concert to control the adaptation of A. actinomycetemcomitans to iron-limiting conditions and its growth under such conditions.