Interspecies Signaling Research Articles

Catalyst for exhaust purification uses minimal precious metals

The oral cavity is accessible to microorganisms, and biofilms are present throughout on hard and soft tissues. The shedding of epithelial cell layers is usually effective for controlling biofilm development on soft tissues. Innate immune mechanisms are not so effective against biofilms on tooth surfaces, and oral hygiene measures such as brushing and flossing are required for the periodic removal of dental plaque. Even with good oral hygiene, microbial communities accumulate on teeth in areas that are protected from mechanical abrasion forces. Changes in the composition of these biofilms are associated with oral diseases such as dental caries or periodontitis. Newly formed biofilms and more mature dental plaque each have a level of spatial organization in the horizontal and vertical planes. Communities are shaped by many varied interactions between different species and genera within the biofilm, which include physical cell–cell associations known as coaggregation, interspecies signaling, secretion and turnover of antimicrobial compounds and the sharing of an extracellular matrix. Central to these interactions is the selection for metabolic synergies and it is becoming clear that the ability of communities to extract the maximum energy from the available metabolites is a potent driver for biofilm structure and stratification. This review discusses recent advances in our understanding of intermicrobial interactions in oral biofilms and the roles that they play in determining the spatial organization of biofilm communities.

Interspecies signalling via the Stenotrophomonas maltophilia diffusible signal factor influences biofilm formation and polymyxin tolerance in Pseudomonas aeruginosa

Interspecies signalling through the action of diffusible signal molecules can influence the behaviour of organisms growing in polymicrobial communities. Stenotrophomonas maltophilia and Pseudomonas aeruginosa occur ubiquitously in the environment and can be found together in diverse niches including the rhizosphere of plants and the cystic fibrosis lung. In mixed species biofilms, S. maltophilia substantially influenced the architecture of P. aeruginosa structures, which developed as extended filaments. This effect depended upon the synthesis of the diffusible signal factor (DSF) by S. maltophilia and could be mimicked by the addition of synthetic DSF. This response of P. aeruginosa to DSF required PA1396, a sensor kinase with an input domain of related amino acid sequence to the sensory input domain of RpfC, which is responsible for DSF perception in xanthomonads. Mutation of PA1396 or addition of DSF to P. aeruginosa led to increased levels of a number of proteins with roles in bacterial stress tolerance, including those implicated in resistance to cationic antimicrobial peptides. This effect was associated with increased tolerance to polymyxins. Homologues of PA1396 occur in a number of phytopathogenic and plant-associated pseudomonads, suggesting that modulation of bacterial behaviour through DSF-mediated interspecies signalling with xanthomonads is a phenomenon that occurs widely.

Candida albicans Modulates Host Defense by Biosynthesizing the Pro-Resolving Mediator Resolvin E1

Candida albicans is an opportunistic fungal pathogen of humans that resides commensally on epithelial surfaces, but can cause inflammation when pathogenic. Resolvins are a class of anti-inflammatory lipids derived from omega-3 polyunsaturated fatty acids (PUFA) that attenuate neutrophil migration during the resolution phase of inflammation. In this report we demonstrate that C. albicans biosynthesizes resolvins that are chemically identical to those produced by human cells. In contrast to the trans-cellular biosynthesis of human Resolvin E1 (RvE1), RvE1 biosynthesis in C. albicans occurs in the absence of other cellular partners. C. albicans biosynthesis of RvE1 is sensitive to lipoxygenase and cytochrome P450 monoxygenase inhibitors. We show that 10nM RvE1 reduces neutrophil chemotaxis in response to IL-8; 1nM RvE1 enhanced phagocytosis of Candida by human neutrophils, as well as intracellular ROS generation and killing, while having no direct affect on neutrophil motility. In a mouse model of systemic candidiasis, RvE1 stimulated clearance of the fungus from circulating blood. These results reveal an inter-species chemical signaling system that modulates host immune functions and may play a role in balancing host carriage of commensal and pathogenic C. albicans.

A novel DSF-like signal from Burkholderia cenocepacia interferes with Candida albicans morphological transition

In addition to producing lethal antibiotics, microorganisms may also use a new form of antagonistic mechanism in which signal molecules are exported to influence the gene expression and hence the ecological competence of their competitors. We report here the isolation and characterization of a novel signaling molecule, cis-2-dodecenoic acid (BDSF), from Burkholderia cenocepacia. BDSF is structurally similar to the diffusible signal factor (DSF) that is produced by the RpfF enzyme of Xanthomonas campestris. Deletion analysis demonstrated that Bcam0581, which encodes an RpfF homologue, was essential for BDSF production. The gene is highly conserved and widespread in the Burkholderia cepacia complex. Exogenous addition of BDSF restored the biofilm and extracellular polysaccharide production phenotypes of Xanthomonas campestris pv. campestris DSF-deficient mutants, highlighting its potential role in inter-species signaling. Further analyses showed that Candida albicans germ tube formation was strongly inhibited by either coculture with B. cenocepacia or by exogenous addition of physiological relevant levels of BDSF, whereas deletion of Bcam0581 abrogated the inhibitory ability of the bacterial pathogen. As B. cenocepacia and C. albicans are frequently encountered human pathogens, identification of the BDSF signal and its activity thus provides a new insight into the molecular grounds of their antagonistic interactions whose importance to microbial ecology and pathogenesis is now becoming evident.

Indole is an inter-species biofilm signal mediated by SdiA

BackgroundAs a stationary phase signal, indole is secreted in large quantities into rich medium by Escherichia coli and has been shown to control several genes (e.g., astD, tnaB, gabT), multi-drug exporters, and the pathogenicity island of E. coli; however, its impact on biofilm formation has not been well-studied.ResultsThrough a series of global transcriptome analyses, confocal microscopy, isogenic mutants, and dual-species biofilms, we show here that indole is a non-toxic signal that controls E. coli biofilms by repressing motility, inducing the sensor of the quorum sensing signal autoinducer-1 (SdiA), and influencing acid resistance (e.g., hdeABD, gadABCEX). Isogenic mutants showed these associated proteins are directly related to biofilm formation (e.g., the sdiA mutation increased biofilm formation 50-fold), and SdiA-mediated transcription was shown to be influenced by indole. The reduction in motility due to indole addition results in the biofilm architecture changing from scattered towers to flat colonies. Additionally, there are 12-fold more E. coli cells in dual-species biofilms grown in the presence of Pseudomonas cells engineered to express toluene o-monooxygenase (TOM, which converts indole to an insoluble indigoid) than in biofilms with pseudomonads that do not express TOM due to a 22-fold reduction in extracellular indole. Also, indole stimulates biofilm formation in pseudomonads. Further evidence that the indole effects are mediated by SdiA and homoserine lactone quorum sensing is that the addition of N-butyryl-, N-hexanoyl-, and N-octanoyl-L-homoserine lactones repress E. coli biofilm formation in the wild-type strain but not with the sdiA mutant.ConclusionIndole is an interspecies signal that decreases E. coli biofilms through SdiA and increases those of pseudomonads. Indole may be manipulated to control biofilm formation by oxygenases of bacteria that do not synthesize it in a dual-species biofilm. Furthermore, E. coli changes its biofilm in response to signals it cannot synthesize (homoserine lactones), and pseudomonads respond to signals they do not synthesize (indole).

AI-1 Influences the Kinase Activity but Not the Phosphatase Activity of LuxN of Vibrio harveyi

The Gram-negative bacterium Vibrio harveyi produces and responds to three autoinducers, AI-1, AI-2, and CAI-1 to regulate cell density dependent gene expression by a process referred to as quorum sensing. The concentration of the autoinducers is sensed by three cognate hybrid sensor kinases, and information is channeled via the HPt protein LuxU to the response regulator LuxO. Here, a detailed biochemical study on the enzymatic activities of the membrane-integrated hybrid sensor kinase LuxN, the sensor for N-(d-3-hydroxybutanoyl)homoserine lactone (AI-1), is provided. LuxN was heterologously overproduced as the full-length protein in Escherichia coli. LuxN activities were characterized in vitro and are an autophosphorylation activity with an unusually high ATP turnover rate, stable LuxU phosphorylation, and a slow phosphatase activity with LuxU approximately P as substrate. The presence of AI-1 affected the kinase but not the phosphatase activity of LuxN. The influence of AI-1 on the LuxN--> LuxU signaling step was monitored, and in the presence of AI-1, the kinase activity of LuxN, and hence the amount of LuxU approximately P produced, were significantly reduced. Half-maximal inhibition of kinase activity by AI-1 occurred at 20 mum. Together, these results indicate that AI-1 directly interacts with LuxN to down-regulate its autokinase activity and suggest that the key regulatory step of the AI-1 quorum sensing system of Vibrio harveyi is AI-1-mediated repression of the LuxN kinase activity.

Autoinducer 2: a concentration‐dependent signal for mutualistic bacterial biofilm growth

4,5-Dihydroxy-2,3-pentanedione (DPD), a product of the LuxS enzyme in the catabolism of S-ribosylhomocysteine, spontaneously cyclizes to form autoinducer 2 (AI-2). AI-2 is proposed to be a universal signal molecule mediating interspecies communication among bacteria. We show that mutualistic and abundant biofilm growth in flowing saliva of two human oral commensal bacteria, Actinomyces naeslundii T14V and Streptococcus oralis 34, is dependent upon production of AI-2 by S. oralis 34. A luxS mutant of S. oralis 34 was constructed which did not produce AI-2. Unlike wild-type dual-species biofilms, A. naeslundii T14V and an S. oralis 34 luxS mutant did not exhibit mutualism and generated only sparse biofilms which contained a 10-fold lower biomass of each species. Restoration of AI-2 levels by genetic or chemical (synthetic AI-2 in the form of DPD) complementation re-established the mutualistic growth and high biomass characteristic for the wild-type dual-species biofilm. Furthermore, an optimal concentration of DPD was determined, above and below which biofilm formation was suppressed. The optimal concentration was 100-fold lower than the detection limit of the currently accepted AI-2 assay. Thus, AI-2 acts as an interspecies signal and its concentration is critical for mutualism between two species of oral bacteria grown under conditions that are representative of the human oral cavity.

Evidence for quorum sensing in Clostridium botulinum 56A

Experiments were designed to detect quorum-sensing signals produced by Clostridium botulinum. Clostridium botulinum 56A cell-free supernatants obtained at the end of lag phase, the mid-exponential phase and early stationary phase of growth were assayed for bioluminescence in the Vibrio harveyi quorum-sensing assay system. Twelve and 16-h culture supernatants induced bioluminescence in the auto-inducer 2 (AI-2) but not the auto-inducer 1 (AI-1) assay. Intra-species quorum sensing was also assayed as the ability of the supernatants to promote spore germination and outgrowth in a microtitre plate system. Spore populations exposed to C. botulinum supernatant from the end of lag phase became positive for growth sooner than controls. The influence of cell-free supernatant on ungerminated spores and detection of bioluminescence in the AI-2 assay are evidence for a signalling molecule(s) and provide a first step in characterizing C. botulinum quorum sensing. This study suggests that spores do not behave independently of each other and may explain the inocula size effects observed in challenge studies. Whether AI-2 production in C. botulinum serves as an inter-species signal or as a detoxification mechanism remains to be determined.

Reciprocity, Justice, and Disability

Reciprocity, Justice, and Disability

Interspecies communication in Streptococcus gordonii–Veillonella atypica biofilms: Signaling in flow conditions requires juxtaposition

During the development of human oral biofilm communities, the spatial arrangement of the bacteria is thought to be driven by metabolic interactions between them. Streptococcus gordonii and Veillonella atypica, two early colonizing members of the dental plaque biofilm, have been postulated to participate in metabolic communication; S. gordonii ferments carbohydrates to form lactic acid, which is a preferred fermentation substrate for V. atypica. We found that, during agar-plate coculture of these organisms, a signaling event occurs that results in increased expression of the S. gordonii alpha-amylase-encoding gene amyB. Confocal scanning laser microscopy of coculture flowcell-grown biofilms using human saliva as the sole nutrient showed that V. atypica caused S. gordonii to increase expression of a PamyB-'gfp transcriptional fusion in a spatially resolved fashion. In this open system, only those streptococci in mixed-species microcolonies containing V. atypica expressed GFP; nearby S. gordonii colonies that lacked V. atypica did not express GFP. In a closed system containing S. gordonii and V. atypica, flow cytometric analysis showed that S. gordonii containing the PamyB-'gfp reporter plasmid exhibited mean fluorescence levels 20-fold higher than did S. gordonii that had not been incubated with V. atypica. Thus, in a closed system where a diffusible signal can accumulate above a required threshold, interspecies signaling mediates a change in gene expression. We provide evidence that, in open systems like those that predominate in natural biofilms, diffusible signals between species are designed to function over short distances, on the order of 1 mum.

Salmonella typhimurium Recognizes a Chemically Distinct Form of the Bacterial Quorum-Sensing Signal AI-2

Bacterial populations use cell-cell communication to coordinate community-wide regulation of processes such as biofilm formation, virulence, and bioluminescence. This phenomenon, termed quorum sensing, is mediated by small molecule signals known as autoinducers. While most autoinducers are species specific, autoinducer-2 (AI-2), first identified in the marine bacterium Vibrio harveyi, is produced and detected by many Gram-negative and Gram-positive bacteria. The crystal structure of the V. harveyi AI-2 signaling molecule bound to its receptor protein revealed an unusual furanosyl borate diester. Here, we present the crystal structure of a second AI-2 signal binding protein, LsrB from Salmonella typhimurium. We find that LsrB binds a chemically distinct form of the AI-2 signal, (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), that lacks boron. Our results demonstrate that two different species of bacteria recognize two different forms of the autoinducer signal, both derived from 4,5-dihydroxy-2,3-pentanedione (DPD), and reveal new sophistication in the chemical lexicon used by bacteria in interspecies signaling.

Modulation of Pseudomonas aeruginosa gene expression by host microflora through interspecies communication.

The change in gene expression patterns in response to host environments is a prerequisite for bacterial infection. Bacterial diseases often occur as an outcome of the complex interactions between pathogens and the host. The indigenous, usually non-pathogenic microflora is a ubiquitous constituent of the host. In order to understand the interactions between pathogens and the resident microflora and how they affect the gene expression patterns of the pathogens and contribute to bacterial diseases, the interactions between pathogenic Pseudomonas aeruginosa and avirulent oropharyngeal flora (OF) strains isolated from sputum samples of cystic fibrosis (CF) patients were investigated. Animal experiments using a rat lung infection model indicate that the presence of OF bacteria enhanced lung damage caused by P. aeruginosa. Genome-wide transcriptional analysis with a lux reporter-based promoter library demonstrated that approximately 4% of genes in the genome responded to the presence of OF strains using an in vitro system. Characterization of a subset of the regulated genes indicates that they fall into seven functional classes, and large portions of the upregulated genes are genes important for P. aeruginosa pathogenesis. Autoinducer-2 (AI-2)-mediated quorum sensing, a proposed interspecies signalling system, accounted for some, but not all, of the gene regulation. A substantial amount of AI-2 was detected directly in sputum samples from CF patients and in cultures of most non-pseudomonad bacteria isolated from the sputa. Transcriptional profiling of a set of defined P. aeruginosa virulence factor promoters revealed that OF and exogenous AI-2 could upregulate overlapping subsets of these genes. These results suggest important contributions of the host microflora to P. aeruginosa infection by modulating gene expression via interspecies communications.

Bacterial density dictates virulence in cholera

Intricate systems of message production and reception, termed quorum sensing, have been identified as mechanisms for intercellular bacterial communication. The marine bacterium Vibrio harveyi bioluminesces as a result of high density, and thus has been a useful model organism to study quorum sensing. V. harveyi has been found to contain two pathways that converge on light production: one message is acyl homoserine lactone for intraspecies signaling and the other is furanosyl borate diester for interspecies signaling. The human pathogen Vibrio cholerae is related to V. harveyi, and now Miller et al. have found at least three redundant sensory systems in the pathogen, and that these together control virulence [1].

Physiology and tactic response of the phototrophic consortium "Chlorochromatium aggregatum"

The phototrophic consortium "Chlorochromatium aggregatum" was enriched from sediment samples of a eutrophic freshwater lake and was maintained at high numbers in anoxic sulfide-reduced medium. Growth of intact consortia was observed only in the light and in the presence of 2-oxoglutarate as an organic carbon source. Consortia of "C. aggregatum" reached maximum growth rates at light intensities >/= 5 &mgr;mol quanta m-2 s-1. Of ten compounds tested, sulfide, thiosulfate, 2-oxoglutarate, and citrate served as a chemoattractant for "C. aggregatum". When incubated in the presence of sulfide and in the light, epibionts reduced the fluorochrome 5-cyano-2, 3-di-4-tolyl-tetrazolium chloride (CTC). Reduction of CTC was not observed in the presence of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) or in the dark, indicating that sulfide serves as an electron donor for the phototrophic epibiont. Motile consortia accumulated scotophobically in microcuvettes at a wavelength of 740 nm. Since this wavelength corresponds to the position of the absorption maximum of bacteriochlorophylls c or d, the photosynthetic pigments are most likely the photoreceptors of the scotophobic response. It is concluded that, within the consortia, a rapid interspecies signal transfer occurs between the nonmotile, green-colored epibiont and the motile, colorless central bacterium.

Influence of Pseudomonas aeruginosa exoproducts on virulence factor production in Burkholderia cepacia: evidence of interspecies communication.

The effect of concentrated cell-free extracellular material from stationary-phase cultures of Burkholderia cepacia 10661 and Pseudomonas aeruginosa PAO1 on virulence factor production in B. cepacia was assessed. While increasing concentrations of the B. cepacia exoproduct caused a slight increase in siderophore, lipase, and protease production in the producing organism, a significant in productivity was observed for all three virulence factors with the addition of the PAO1 exoproduct. Moreover, the addition of the exoproduct from a strain of P. aeruginosa producing reduced amounts of autoinducer caused only a slightly greater response than that of the control. Both B. cepacia 10661 and P. aeruginosa PAO1, along with two matched clinical isolates of both organisms obtained from a cystic fibrotic patient, were shown to produce variable amounts of three different types of autoinducer. The potential for interspecies signalling in microbial pathogenicity is discussed.