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PubMed2003

The effects of insertional mutations in comQ, comP, srfA, spo0H, spo0A and abrB genes on bacilysin biosynthesis in Bacillus subtilis.

In Bacillus subtilis, two extracellular signaling peptides, ComX pheromone and CSF (competence and sporulation factor), stimulate the development of genetic competence and surfaction biosynthesis in response to high cell density (quorum sensing) by regulating the activity of transcription factor ComA. We recently showed that biosynthesis of dipeptide antibiotic bacilysin is linked to ComA and PhrC(CSF) in a Spo0K(Opp)-dependent manner by constructing phrC-, comA- and oppA-disrupted mutants of B. subtilis. In the present study, another pathway of quorum-sensing global regulation, namely, ComQ/ComX was found to be essential for bacilysin biosynthesis. ComP function in this chain was dispensable, most probably because of the existence of an alternative mean of ComA activation. The disruption of srfA operon in the bacilysin producer resulted with the bacilysin-negative phenotype; thus, our study verified that the srfA operon functions directly in the production of bacilysin. The abrB mutation suppressed the bacilysin-negative phenotype of a spo0A mutant, whereas the same mutation in the wild-type strain resulted in a significant increase in the production of bacilysin. This indicated that abrB gene product negatively controls the transcription of the gene(s) involved in bacilysin formation.

Bacillus subtilis↗
PubMed1998

Current topics in signal transduction in bacteria.

Among the signal transfer systems in bacteria two types predominate: two-component regulatory systems and quorum sensing systems. Both types of system can mediate signal transfer across the bacterial cell envelope; however, the signalling molecule typically is not taken up into the cells in the former type of system, whereas it usually is in the latter. The Two-component systems include the recently described (eukaryotic) phosphorelay systems; quorum sensing systems can be based upon autoinducers of the N-acylated homoserine lactones, and on autoinducers of a peptidic nature. A single bacterial cell contains many signalling modules that primarily operate in parallel. This may give rise to neural-network behaviour. Recently, however, for both types of basic signal transfer modules, it has been demonstrated that they also can be organised in series (i.e. in a hierarchical order). Besides their hierarchical position in the signal transduction network of the cell, the spatial distribution of individual signalling modules may also be an important factor in their efficiency in signal transfer. Many challenges lie hidden in future work to understand these signal transfer processes in more detail. These are discussed here, with emphasis on the mutual interactions between different signal transfer processes. Successful contributions to this work will require rigorous mathematical modelling of the performance of signal transduction components, and -networks, as well as studies on light-sensing signal transduction systems, because of the unsurpassed time resolution obtainable in those latter systems, the opportunity to apply repeated reproducible stimuli, etc. The increased understanding of bacterial behaviour that already has resulted--and may further result--from these studies, can be used to fine-tune the beneficial activities of bacteria and/or more efficiently inhibit their deleterious ones.

Bacteria↗
PubMed2000

Extraction of violacein from Chromobacterium violaceum provides a new quantitative bioassay for N-acyl homoserine lactone autoinducers.

Fatty acyl homoserine lactones (AHLs) are used as extracellular quorum sensing signals by a variety of gram-negative bacteria. By activating proteins belonging to the LuxR family of transcriptional regulators, these signal metabolites allow population density-dependent gene regulation within a species, as well as interspecies communication among different bacteria. The experimental detection of AHLs is important in the identification of quorum sensing capabilities in bacteria. Chromobacterium violaceum is a gram-negative bacterium that produces the purple pigment violacein in response to the presence of the AHL N-hexanoyl homoserine lactone (C6HSL). The mini-Tn5 mutant strain C. violaceum CV0blu is deficient in the production of this signal molecule but retains the ability to synthesize violacein in response to the presence of C6HSL and a variety of other short-chain AHLs. We have developed a quantitative bioassay that measures the amount of violacein produced by this strain in response to the presence of different concentrations of various AHL molecules. This new assay provides a means of quantifying the amount of a given AHL present in a bacterial culture and can be used to measure differences in AHL production among different strains or different batch cultures of a given species.

4-Butyrolactone↗
PubMed1997

Evidence of autoinducer activity in naturally occurring biofilms.

N-Acyl homoserine lactone (AHL) molecules have been shown to act as mediators of population density-dependent (quorum-sensing) gene expression in numerous Gram-negative bacteria. Functions associated with AHL include light production in Vibrio fischeri, expression of virulence factors in Pseudomonas aeruginosa, and conjugation in Agrobacterium tumefaciens. In nature, bacteria often grow as surface-adherent biofilm communities. As biofilms typically contain high concentrations of cells, AHL activity and quorum-sensing gene expression have been proposed as essential components of biofilm physiology. However, proof of AHL production within biofilms has heretofore been lacking. In this study we have employed a cross-feeding assay, using A, tumefaciens A136 (traI::lacZ) as an AHL-responsive reporter strain, to show the presence of naturally occurring AHL production in aquatic biofilms growing on submerged stones. AHL was detected in living biofilms and biofilm extracts, but was not present in rocks lacking a biofilm. This represents the first report of AHL activity in naturally occurring biofilms.

4-Butyrolactone↗
PubMed2003

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

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

Animals↗
PubMed2006

Production and properties of the native Chromobacterium violaceum fucose-binding lectin (CV-IIL) compared to homologous lectins of Pseudomonas aeruginosa (PA-IIL) and Ralstonia solanacearum (RS-IIL).

Chromobacterium violaceum is a versatile, violet pigment (violacein)-producing beta-proteobacterium, confined to tropical and subtropical regions, dwelling in soil and water, like Pseudomonas aeruginosa and Ralstonia solanacearum. These three bacteria are saprophytes that occasionally become aggressive opportunistic pathogens virulently attacking animals (the first two) and plants (the third). The recent availability of their genome sequences enabled identification in the C. violaceum genome of an ORF (locus no. 1744) that is similar to those of P. aeruginosa and R. solanacearum lectins, PA-IIL and RS-IIL, respectively. A recombinant protein, CV-IIL, encoded by that ORF exhibited fucose>mannose-specific lectin activity resembling PA-IIL. This paper describes production and properties of the native CV-IIL, which, like PA-IIL and RS-IIL, is probably also a quorum-sensing-driven secondary metabolite, appearing concomitantly with violacein. Its formation is repressed in the CV026 mutant of C. violaceum, which lacks endogenous N-acylhomoserine lactone. The upstream extragenic sequence of its ORF contains a 20 bp sequence (5'-101-120) with partial similarities to the luxI-box and the related P. aeruginosa and R. solanacearum promoter boxes of quorum-sensing-controlled genes. The lectin level is augmented by addition of trehalose to the medium. The subunit size of CV-IIL (around 11.86 kDa) is similar to those of PA-IIL (11.73 kDa) and RS-IIL (11.60 kDa). Like PA-IIL, in the tetrameric form CV-IIL preferentially agglutinates alpha1-2 fucosylated H-positive human erythrocytes (regardless of their A, B or O type), as opposed to the O(h) Bombay type, but differs from it in having no interaction with rabbit erythrocytes and in displaying stronger affinity to l-galactose than to l-fucose. The greater similarity of CV-IIL to PA-IIL than to RS-IIL might be related to the selective adaptation of both C. violaceum and P. aeruginosa to animal tissues versus the preferential homing of R. solanacearum to plants.

Bacterial Proteins↗
PubMed2006

Chemical communication--do we have a quorum?

There are two types of bacterial communication systems, those in which the signal produced by bacteria is directed only at other organisms, and those where the signal is detected by others and self. The latter is involved in adaptation to the environment. The adaptation signals are autoinducers, the response is population density-dependent and has been termed "quorum sensing". Our current knowledge of bacterial signaling systems indicates that Gram positive bacteria use small peptides for both types of signaling, whereas Gram negative organisms use homoserine lactones as autoinducers. Gram- negative bacteria internalize the signals which act upon an intracellular receptor. Gram-positive bacteria use the signals as ligands for an extracellular receptor of a two-component signaling system. Inhibitors of quorum sensing compounds for both Gram positive and Gram negative bacteria are being explored. Signal inhibitors could be potentially effective in impeding biofilm formation, which might prolong the utility of the currently available antibiotics in this era of antibiotic resistance. In this review, we will explore both bacteria-host and bacteria-bacteria communication systems, with an emphasis on inhibitors of these systems both natural and synthetic.

Bacterial Physiological Phenomena↗
PubMed2001

The Agrobacterium tumefaciens rnd homolog is required for TraR-mediated quorum-dependent activation of Ti plasmid tra gene expression.

Conjugal transfer of Agrobacterium tumefaciens Ti plasmids is regulated by quorum sensing via TraR and its cognate autoinducer, N-(3-oxo-octanoyl)-L-homoserine lactone. We isolated four Tn5-induced mutants of A. tumefaciens C58 deficient in TraR-mediated activation of tra genes on pTiC58DeltaaccR. These mutations also affected the growth of the bacterium but had no detectable influence on the expression of two tester gene systems that are not regulated by quorum sensing. In all four mutants Tn5 was inserted in a chromosomal open reading frame (ORF) coding for a product showing high similarity to RNase D, coded for by rnd of Escherichia coli, an RNase known to be involved in tRNA processing. The wild-type allele of the rnd homolog cloned from C58 restored the two phenotypes to each mutant. Several ORFs, including a homolog of cya2, surround A. tumefaciens rnd, but none of these genes exerted a detectable effect on the expression of the tra reporter. In the mutant, traR was expressed from the Ti plasmid at a level about twofold lower than that in NT1. The expression of tra, but not the growth rate, was partially restored by increasing the copy number of traR or by disrupting traM, a Ti plasmid gene coding for an antiactivator specific for TraR. The mutation in rnd also slightly reduced expression of two tested vir genes but had no detectable effect on tumor induction by this mutant. Our data suggest that the defect in tra gene induction in the mutants results from lowered levels of TraR. In turn, production of sufficient amounts of TraR apparently is sensitive to a cellular function requiring RNase D.

Agrobacterium tumefaciens↗
PubMed2007

Pseudomonas aeruginosa chronic colonization in cystic fibrosis patients.

PURPOSE OF REVIEW: Chronic infection with Pseudomonas aeruginosa is a leading cause of morbidity and mortality in individuals with cystic fibrosis despite the aggressive use of antibiotics. P. aeruginosa employs a number of strategies that promote chronic pulmonary colonization instead of acute infection. These include biofilm formation, evasion of the host immune system, and conversion to a mucoid phenotype. This review discusses recent advances regarding P. aeruginosa pathogenesis and biofilm behavior in the setting of chronic pulmonary disease. RECENT FINDINGS: Biofilm formation in the cystic fibrosis lung likely occurs under anaerobic conditions, is controlled by bacterial quorum-sensing mechanisms, and is enhanced by environmental components in the cystic fibrosis airway. P. aeruginosa possesses regulatory pathways that recognize environmental cues to favor either acute infection or chronic colonization. P. aeruginosa that inhabit the respiratory tract accumulate mutations favoring chronic colonization. Azithromycin, a macrolide with clinical benefit in cystic fibrosis, alters P. aeruginosa biofilm formation. Promising new therapies that target biofilm formation include molecules that disrupt quorum sensing. SUMMARY: Eradication of P. aeruginosa in cystic fibrosis patients remains problematic. As more information emerges about P. aeruginosa behavior in vivo, potential therapeutics directed against biofilms and mucoid P. aeruginosa are being developed.

Anti-Bacterial Agents↗
PubMed2004

Identification of a novel virulence factor in Burkholderia cenocepacia H111 required for efficient slow killing of Caenorhabditis elegans.

Burkholderia cenocepacia H111, which was isolated from a cystic fibrosis patient, employs a quorum-sensing (QS) system, encoded by cep, to control the expression of virulence factors as well as the formation of biofilms. The QS system is thought to ensure that pathogenic traits are expressed only when the bacterial population density is high enough to overwhelm the host before it is able to mount an efficient response. While the wild-type strain effectively kills the nematode Caenorhabditis elegans, the pathogenicity of mutants with defective quorum sensing is attenuated. To date, very little is known about the cep-regulated virulence factors required for nematode killing. Here we report the identification of a cep-regulated gene, whose predicted amino acid sequence is highly similar to the QS-regulated protein AidA of the plant pathogen Ralstonia solanacearum. By use of polyclonal antibodies directed against AidA, it is demonstrated that the protein is expressed in the late-exponential phase and accumulates during growth arrest. We show that B. cenocepacia H111 AidA is essential for slow killing of C. elegans but has little effect on fast killing, suggesting that the protein plays a role in the accumulation of the strain in the nematode gut. Thus, AidA appears to be required for establishing an infection-like process rather than acting as a toxin. Furthermore, evidence is provided that AidA is produced not only by B. cenocepacia but also by many other strains of the Burkholderia cepacia complex.

Amino Acid Sequence↗
PubMed2003

Inactivation of gacS does not affect the competitiveness of Pseudomonas chlororaphis in the Arabidopsis thaliana rhizosphere.

Quorum-sensing-controlled processes are considered to be important for the competitiveness of microorganisms in the rhizosphere. They affect cell-cell communication, biofilm formation, and antibiotic production, and the GacS-GacA two-component system plays a role as a key regulator. In spite of the importance of this system for the regulation of various processes, strains with a Gac(-) phenotype are readily recovered from natural habitats. To analyze the influence of quorum sensing and the influence of the production of the antibiotic phenazine-1-carboxamide on rhizosphere colonization by Pseudomonas chlororaphis, a gnotobiotic system based on Arabidopsis thaliana seedlings in soil was investigated. Transposon insertion mutants of P. chlororaphis isolate SPR044 carrying insertions in different genes required for the production of N-acyl-homoserine lactones and phenazine-1-carboxamide were generated. Analysis of solitary rhizosphere colonization revealed that after prolonged growth, the population of the wild type was significantly larger than that of the homoserine lactone-negative gacS mutant and that of a phenazine-1-carboxamide-overproducing strain. In cocultivation experiments, however, the population size of the gacS mutant was similar to that of the wild type after extended growth in the rhizosphere. A detailed analysis of growth kinetics was performed to explain this phenomenon. After cells grown to the stationary phase were transferred to fresh medium, the gacS mutant had a reduced lag phase, and production of the stationary-phase-specific sigma factor RpoS was strongly reduced. This may provide a relative competitive advantage in cocultures with other bacteria, because it permits faster reinitiation of growth after a change to nutrient-rich conditions. In addition, delayed entry into the stationary phase may allow more efficient nutrient utilization. Thus, GacS-GacA-regulated processes are not absolutely required for efficient rhizosphere colonization in populations containing the wild type and Gac(-) mutants.

4-Butyrolactone↗
PubMed2003

The competitiveness of Pseudomonas chlororaphis carrying pJP4 is reduced in the Arabidopsis thaliana rhizosphere.

The effect of the large catabolic IncP plasmid pJP4 on the competitiveness of Pseudomonas chlororaphis SPR044 and on its derivatives SPR244 (GacS deficient), SPR344 (phenazine-1-carboxamide overproducer), and SPR644 (phenazine-1-carboxamide deficient) in the Arabidopsis thaliana rhizosphere was assessed. Solitary rhizosphere colonization by the wild type, SPR244, and SPR644 was not affected by the plasmid. The size of the population of SPR344 carrying pJP4, however, was significantly reduced compared to the size of the population of the plasmid-free derivative. The abiotic stress caused by phenazine-1-carboxamide overproduction probably resulted in a selective disadvantage for cells carrying pJP4. Next, the effect of biotic stress caused by coinoculation of other bacteria was analyzed. Cells carrying pJP4 had a selective disadvantage compared to plasmid-free cells in the presence of the efficient colonizer Pseudomonas fluorescens WCS417r. This effect was not observed after coinoculation with a variety of other bacteria, and it was independent of quorum sensing and phenazine-1-carboxamide production. Thus, the presence of large catabolic plasmids imposes a detectable metabolic burden in the presence of biotic stress. Plasmid transfer in the A. thaliana rhizosphere from P. chlororaphis and its derivatives to Ralstonia eutropha was determined by using culture-dependent and culture-independent techniques. With the cultivation-independent technique we detected a significantly higher portion of exconjugants, but pJP4 transfer was independent of the quorum-sensing system and of phenazine-1-carboxamide production.

Arabidopsis↗
PubMed2003

Biofilms, bacterial signaling, and their ties to marine biology.

Much of what is know about quorum sensing has come from the study of marine biology. The original description of the phenomenon was based on the study of marine bacteria and the luminescent pathway. More recently, aquatic organisms have been found to inhibit bacterial fouling of surfaces by blocking signaling pathways in the bacteria. These signaling effects have, over the last 5 years, been linked to biofilms. However, this correlation is not as straight forward as originally believed. Here, a brief overview of quorum sensing, and background on biofilms is provided, followed by a discussion of more recent work looking at the effects that environment may have on signal expression.

Bacteria↗
PubMed2002

Pseudomonas aeruginosa displays multiple phenotypes during development as a biofilm.

Complementary approaches were employed to characterize transitional episodes in Pseudomonas aeruginosa biofilm development using direct observation and whole-cell protein analysis. Microscopy and in situ reporter gene analysis were used to directly observe changes in biofilm physiology and to act as signposts to standardize protein collection for two-dimensional electrophoretic analysis and protein identification in chemostat and continuous-culture biofilm-grown populations. Using these approaches, we characterized five stages of biofilm development: (i) reversible attachment, (ii) irreversible attachment, (iii) maturation-1, (iv) maturation-2, and (v) dispersion. Biofilm cells were shown to change regulation of motility, alginate production, and quorum sensing during the process of development. The average difference in detectable protein regulation between each of the five stages of development was 35% (approximately 525 proteins). When planktonic cells were compared with maturation-2 stage biofilm cells, more than 800 proteins were shown to have a sixfold or greater change in expression level (over 50% of the proteome). This difference was higher than when planktonic P. aeruginosa were compared with planktonic cultures of Pseudomonas putida. Las quorum sensing was shown to play no role in early biofilm development but was important in later stages. Biofilm cells in the dispersion stage were more similar to planktonic bacteria than to maturation-2 stage bacteria. These results demonstrate that P. aeruginosa displays multiple phenotypes during biofilm development and that knowledge of stage-specific physiology may be important in detecting and controlling biofilm growth.

Amino Acids↗
PubMed2003

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.

Bacillus subtilis↗
PubMed2007

Temporal gene-expression in Escherichia coli K-12 biofilms.

Analysis of the temporal development of Escherichia coli K-12 biofilms in complex medium indicates the greatest differential gene expression between biofilm and suspension cells occurred in young biofilms at 4 and 7 h (versus 15 and 24 h). The main classes of genes differentially expressed (biofilm versus biofilm and biofilm versus suspension cells) include 42 related to stress response (e.g. cspABFGI), 66 related to quorum sensing (e.g. ydgG, gadABC, hdeABD), 20 related to motility (e.g. flgBCEFH, fliLMQR, motB), 13 related to fimbriae (e.g. sfmCHM, fimZ, csgC), 24 related to sulfur and tryptophan metabolism (e.g. trpLBA, tnaLA, cysDNCJH), 80 related to transport (e.g. gatABC, agaBC, ycjJ, ydfJ, phoU, phnCJKM), and six related to extracellular matrix (e.g. wcaBDEC). Of the 93 mutants identified and studied, 76 showed altered biofilm formation. Biofilm architecture changed from thin and dense to globular and dispersed to dense and smooth. The quorum-sensing signal AI-2 controls gene expression most clearly in mature biofilms (24 h) when intracellular AI-2 levels are highest. Sulfate transport and metabolism genes (cysAUWDN) and genes with unknown functions (ymgABCZ) were repressed in young (4, 7 h) biofilms, induced in developed biofilms (15 h), and repressed in mature (24 h) biofilms. Genes related to both motility and fimbriae were induced in biofilms at all sampling time points and colanic acid genes were induced in mature biofilms (24 h). Genes related to dihydroxyacetone phosphate synthesis from galactitol and galactosamine (e.g. gatZABCDR, agaBCY) were highly regulated in biofilms. Genes involved in the biosynthesis of indole and sulfide (tnaLA) are repressed in biofilms after 7 h (corroborated by decreasing intracellular indole concentrations in biofilms). Cold-shock protein transcriptional regulators (cspABFGI) appear to be positive biofilm regulators, and deletions in respiratory genes (e.g. hyaACD, hyfCG, appC, narG) increased biofilm formation sevenfold.

Biofilms↗
PubMed2005

Influence of fusaric acid on phenazine-1-carboxamide synthesis and gene expression of Pseudomonas chlororaphis strain PCL1391.

Production of the antifungal metabolite phenazine-1-carboxamide (PCN) by Pseudomonas chlororaphis strain PCL1391 is essential for the suppression of tomato foot and root rot caused by the soil-borne fungus F. oxysporum f. sp. radicis-lycopersici. The authors have shown previously that fusaric acid (FA), a phytotoxin produced by Fusarium oxysporum, represses the production of PCN and of the quorum-sensing signal N-hexanoyl-l-homoserine lactone (C(6)-HSL). Here they report that PCN repression by FA is maintained even during PCN-stimulating environmental conditions such as additional phenylalanine, additional ferric iron and a low Mg(2+) concentration. Constitutive expression of phzI or phzR increases the production of C(6)-HSL and abolishes the repression of PCN production by FA. Transcriptome analysis using P. chlororaphis PCL1391 microarrays showed that FA represses expression of the phenazine biosynthetic operon (phzABCDEFGH) and of the quorum-sensing regulatory genes phzI and phzR. FA does not alter expression of the PCN regulators gacS, rpoS and psrA. In conclusion, reduction of PCN levels by FA is due to direct or indirect repression of phzR and phzI. Microarray analyses identified genes of which the expression is strongly influenced by FA. Genes highly upregulated by FA are also upregulated by iron starvation in Pseudomonas aeruginosa. This remarkable overlap in the expression profile suggests an overlapping stress response to FA and iron starvation.

Fusaric Acid↗
PubMed2004

Environmental signals and regulatory pathways that influence biofilm formation.

In nature, bacteria often exist as biofilms. Here, we discuss the environmental signals and regulatory proteins that affect both the initiation of bacterial biofilm formation and the nature of the mature biofilm structure. Current research suggests that the environmental signals regulating whether bacterial cells will initiate a biofilm differ from one bacterial species to another. This may allow each bacterial species to colonize its preferred environment efficiently. In contrast, some of the environmental signals that have currently been identified to regulate the structure of a mature biofilm are nutrient availability and quorum sensing, and are not species specific. These environmental signals evoke changes in the nature of the mature biofilm that may ensure optimal nutrient acquisition. Nutrient availability regulates the depth of the biofilm in such a way that the maximal number of cells in a biofilm appears to occur at suboptimal nutrient concentrations. At either extreme, nutrient-rich or very nutrient-poor conditions, greater numbers of cells are in the planktonic phase where they have greater access to the local nutrients or can be distributed to a new environment. Similarly, quorum-sensing control of the formation of channels and pillar-like structures may ensure efficient nutrient delivery to cells in a biofilm.

Bacterial Adhesion↗