Diffusible signal factor (DSF) quorum sensing signal and structurally related molecules enhance the antimicrobial efficacy of antibiotics against some bacterial pathogens.

BACKGROUND: Extensive use of antibiotics has fostered the emergence of superbugs that are resistant to multidrugs, which becomes a great healthcare and public concern. Previous studies showed that quorum sensing signal DSF (diffusible signal factor) not only modulates bacterial antibiotic resistance through intraspecies signaling, but also affects bacterial antibiotic tolerance through interspecies communication. These findings motivate us to exploit the possibility of using DSF and its structurally related molecules as adjuvants to influence antibiotic susceptibility of bacterial pathogens. RESULTS: In this study, we have demonstrated that DSF signal and its structurally related molecules could be used to induce bacterial antibiotic susceptibility. Exogenous addition of DSF signal (cis-11-methyl-2-dodecenoic acid) and its structural analogues could significantly increase the antibiotic susceptibility of Bacillus cereus, possibly through reducing drug-resistant activity, biofilm formation and bacterial fitness. The synergistic effect of DSF and its structurally related molecules with antibiotics on B. cereus is dosage-dependent. Combination of DSF with gentamicin showed an obviously synergistic effect on B. cereus pathogenicity in an in vitro model. We also found that DSF could increase the antibiotic susceptibility of other bacterial species, including Bacillus thuringiensis, Staphylococcus aureus, Mycobacterium smegmatis, Neisseria subflava and Pseudomonas aeruginosa. CONCLUSION: The results indicate a promising potential of using DSF and its structurally related molecules as novel adjuvants to conventional antibiotics for treatment of infectious diseases caused by bacterial pathogens.

Cis-2-dodecenoic acid quorum sensing system modulates N-acyl homoserine lactone production through RpfR and cyclic di-GMP turnover in Burkholderia cenocepacia.

BACKGROUND: Burkholderia cenocepacia employs both N-Acyl homoserine lactone (AHL) and cis-2-dodecenoic acid (BDSF) quorum sensing (QS) systems in regulation of bacterial virulence. It was shown recently that disruption of BDSF synthase RpfFBc caused a reduction of AHL signal production in B. cenocepacia. However, how BDSF system influences AHL system is still not clear. RESULTS: We show here that BDSF system controls AHL system through a novel signaling mechanism. Null mutation of either the BDSF synthase, RpfFBc, or the BDSF receptor, RpfR, caused a substantial down-regulation of AHL signal production in B. cenocepacia strain H111. Genetic and biochemical analyses showed that BDSF system controls AHL signal production through the transcriptional regulation of the AHL synthase gene cepI by modulating the intracellular level of second messenger cyclic di-GMP (c-di-GMP). Furthermore, we show that BDSF and AHL systems have a cumulative role in the regulation of various biological functions, including swarming motility, biofilm formation and virulence factor production, and exogenous addition of either BDSF or AHL signal molecules could only partially rescue the changed phenotypes of the double deletion mutant defective in BDSF and AHL signal production. CONCLUSIONS: These results, together with our previous findings, thus depict a molecular mechanism with which BDSF regulates AHL signal production and bacterial virulence through modulating the phosphodiesterase activity of its receptor RpfR to influence the intracellular level of c-di-GMP.

Enhanced vascular activity of a new chimeric promoter containing the full CaMV 35S promoter and the plant XYLOGEN PROTEIN 1 promoter.

To develop a new strategy that controls vascular pathogen infections in economic crops, we examined a possible enhancer of the vascular activity of XYLOGEN PROTEIN 1 promoter (Px). This protein is specifically expressed in the vascular tissues of Arabidopsis thaliana and plays an important role in xylem development. Although Px is predicted as vascular-specific, its activity is hard to detect and highly susceptible to plant and environmental conditions. The cauliflower mosaic virus 35S promoter (35S) is highly active in directing transgene expression. To test if 35S could enhance Px activity, while vascular specificity of the promoter is retained, we examined the expression of the uidA reporter gene, which encodes ß-glucuronidase (GUS), under the control of a chimeric promoter (35S-Px) or Px by generating 35S-Px-GUS and Px-GUS constructs, which were transformed into tobacco seedlings. Both 35S-Px and Px regulated gene expression in vascular tissues. However, GUS expression driven by 35S-Px was not detected in 30- and 60-day-old plants. Quantitative real-time PCR analysis showed that GUS gene expression regulated by 35S-Px was 6.2-14.9-fold higher in vascular tissues than in leaves. Histochemical GUS staining demonstrated that 35S-Px was strongly active in the xylem and phloem. Thus, fusion of 35S and Px might considerably enhance the strength of Px and increase its vascular specificity. In addition to confirming that 35S enhances the activity of a low-level tissue-specific promoter, these findings provide information for further improving the activity of such promoters, which might be useful for engineering new types of resistant genes against vascular infections.

Quorum quenching enzyme activity is widely conserved in the sera of mammalian species.

Acyl-homoserine lactone (AHL) quorum sensing signals play a key role in synchronizing virulence gene expression in Pseudomonas aeruginosa, which could cause fatal bloodstream infections. We showed that AHL inactivation activity, albeit with variable efficiency, was conserved in the serum samples of all the 6 tested mammalian animals. High-performance liquid chromatography and mass spectrometry analyses revealed that mammalian sera had a lactonase-like enzyme(s), which hydrolyzed the lactone ring of AHL to produce acyl homoserine, with enzyme properties reminiscent of paraoxonases (PONs). We further showed that the animal cell lines expressing three mouse PON genes, respectively, displayed strong AHL degradation activities.

Quorum sensing and quorum-quenching enzymes.

To gain maximal benefit in a competitive environment, single-celled bacteria have adopted a community genetic regulatory mechanism, known as quorum sensing (QS). Many bacteria use QS signaling systems to synchronize target gene expression and coordinate biological activities among a local population. N-acylhomoserine lactones (AHLs) are one family of the well-characterized QS signals in Gram-negative bacteria, which regulate a range of important biological functions, including virulence and biofilm formation. Several groups of AHL-degradation enzymes have recently been identified in a range of living organisms, including bacteria and eukaryotes. Expression of these enzymes in AHL-dependent pathogens and transgenic plants efficiently quenches the microbial QS signaling and blocks pathogenic infections. Discovery of these novel quorum quenching enzymes has not only provided a promising means to control bacterial infections, but also presents new challenges to investigate their roles in host organisms and their potential impacts on ecosystems.

Fenpropathrin biodegradation pathway in Bacillus sp. DG-02 and its potential for bioremediation of pyrethroid-contaminated soils.

The widely used insecticide fenpropathrin in agriculture has become a public concern because of its heavy environmental contamination and toxic effects on mammals, yet little is known about the kinetic and metabolic behaviors of this pesticide. This study reports the degradation kinetics and metabolic pathway of fenpropathrin in Bacillus sp. DG-02, previously isolated from the pyrethroid-manufacturing wastewater treatment system. Up to 93.3% of 50 mg L(-1) fenpropathrin was degraded by Bacillus sp. DG-02 within 72 h, and the degradation rate parameters qmax, Ks, and Ki were determined to be 0.05 h(-1), 9.0 mg L(-1), and 694.8 mg L(-1), respectively. Analysis of the degradation products by gas chromatography-mass spectrometry led to identification of seven metabolites of fenpropathrin, which suggest that fenpropathrin could be degraded first by cleavage of its carboxylester linkage and diaryl bond, followed by degradation of the aromatic ring and subsequent metabolism. In addition to degradation of fenpropathrin, this strain was also found to be capable of degrading a wide range of synthetic pyrethroids including deltamethrin, λ-cyhalothrin, ß-cypermethrin, ß-cyfluthrin, bifenthrin, and permethrin, which are also widely used insecticides with environmental contamination problems with the degradation process following the first-order kinetic model. Bioaugmentation of fenpropathrin-contaminated soils with strain DG-02 significantly enhanced the disappearance rate of fenpropathrin, and its half-life was sharply reduced in the soils. Taken together, these results depict the biodegradation mechanisms of fenpropathrin and also highlight the promising potentials of Bacillus sp. DG-02 in bioremediation of pyrethroid-contaminated soils.

The global regulator Crc plays a multifaceted role in modulation of type III secretion system in Pseudomonas aeruginosa.

The opportunistic pathogen Pseudomonas aeruginosa utilizes type III secretion system (T3SS) to translocate effector proteins into eukaryotic host cells that subvert normal host cell functions to the benefit of the pathogen, and results in serious infections. T3SS in P. aeruginosa is controlled by a complex system of regulatory mechanisms and signaling pathways. In this study, we described that Crc, an RNA-binding protein, exerts a positive impact on T3SS in P. aeruginosa, as evidenced by promoter activity assays of several key T3SS genes, transcriptomics, RT-PCR, and immunoblotting in crc mutant. We further demonstrated that the regulatory function of Crc on the T3SS was mediated through the T3SS master regulator ExsA and linked to the Cbr/Crc signaling system. Expression profiling of the crc mutant revealed a downregulation of flagship T3SS genes as well as 16 other genes known to regulate T3SS gene expression in P. aeruginosa. On the basis of these data, we proposed that Crc may exert multifaceted control on the T3SS through various pathways, which may serve to fine-tune this virulence mechanism in response to environmental changes and nutrient sources.

Characterization of a novel cyfluthrin-degrading bacterial strain Brevibacterium aureum and its biochemical degradation pathway.

Brevibacterium aureum DG-12, a new bacterial strain isolated from active sludge, was able to degrade and utilize cyfluthrin as a growth substrate in the mineral medium. Response surface methodology using central composite rotatable design of cultural conditions was successfully employed for optimization resulting in 88.6% degradation of cyfluthrin (50mgL(-1)) within 5days. The bacterium degraded cyfluthrin by cleavage of both the carboxylester linkage and diaryl bond to form 2,2,3,3-tetramethyl-cyclopropanemethanol, 4-fluoro-3-phenexy-benzoic acid, 3,5-dimethoxy phenol, and phenol, and subsequently transformed these compounds with a maximum specific degradation rate, half-saturation constant and inhibition constant of 1.0384day(-1), 20.4967mgL(-1), and 141.9013mgL(-1), respectively. A novel degradation pathway for cyfluthrin was proposed based on analysis of these metabolites. In addition, this strain was found capable of degrading a wide range of synthetic pyrethroid insecticides. Our results suggest that B. aureum DG-12 may be an ideal microorganism for bioremediation of the pyrethroid-contaminated environments.

[Biodegradation characteristics of o-chlorophenol with photosynthetic bacteria PSB-1D].

A strain of photosynthetic bacteria named PSB-1D with degradation of o-chlorophenol (2-CP) was isolated and screened from the shallow substrate sludge in downstream side of the sewage outfall of an insecticide factory. The PSB-1D is identified preliminarily as Rhodopseudomonas sp. according to its colony and cell morphological properties, physiological biochemical characteristics and absorption spectrum analysis of living cells. The experiments results of relationship between PSB-1D growth and o-chlorophenol degradation showed that the degradation rate of o-chlorophenol was up to 57.26% after 7 days cultural time. The main environmental factors including way of illumination and oxygen, initial pH, cultural temperature, illumination intensity had distinctly influenced on the o-chlorophenol degradation with PSB-1D. The results showed that the optimum conditions were as following: an anaerobic light, pH 7.0, temperature 30 degrees C, illumination intensity 4000 lx,initial o-chlorophenol concentration 50 mg/L. Under that cultural condition, the degradation rate of o-chlorophenol could reach to 62.08%. The degradation kinetic data fitted the Andrews model well. In addition, the biodegradation process of o-chlorophenol can be well described by enzymatic reaction of high concentration inhibition, with the maximum substrate utilization rate 0.309 d(-1), Michaelis-Menten constant 2.733 mg/L, inhibitory constant 230.15 mg/L respectively.

A novel two-component system BqsS-BqsR modulates quorum sensing-dependent biofilm decay in Pseudomonas aeruginosa.

Pseudomonas aeruginosa can grow either as planktonic- or biofilm-form in response to environmental changes. Recent studies show that switching from biofilm to planktonic lifestyle requires rhamnolipids. Here we report the identification of a novel twocomponent system BqsS-BqsR that regulates biofilm decay in P. aeruginosa. BqsS is a multidomain sensor kinase and BqsR is an OmpR-like response regulator. Deletion of either bqsS or bqsR in P. aeruginosa mPAO1 resulted in a significant increase in biofilm formation. Time course analysis showed that the bqsS-bqsR mutants were defective in biofilm dispersal and in rhamnolipid production. Mutation of the BqsS-BqsR two-component system did not affect the biosynthesis of long chain quorum sensing (QS) signal N-3-oxo-dodecanoyl-homoserine lactone (3OC12HSL) but resulted in reduced production of the short chain QS signal N-butyryl-L-homoserine lactone (C4HSL) and the Pseudomonas quinolone signal (PQS). Exogenous addition of C4HSL, PQS or rhamnolipids to the bqsS mutant reduced the biofilm formation to the wild-type level. Evidence suggests that the BqsS-BqsR two-component system might promote conversion of anthranilate to PQS. Taken together, these results establish BqsS-BqsR as a novel two-component system that regulates biofilm decay in P. aeruginosa by modulating biosynthesis of QS signals and rhamnolipids.

Xanthomonas campestris cell-cell communication involves a putative nucleotide receptor protein Clp and a hierarchical signalling network.

The bacterial pathogen Xanthomonas campestris pv. campestris co-ordinates virulence factor production and biofilm dispersal through a diffusible signal factor (DSF)-mediated cell-cell communication mechanism. The RpfC/RpfG two-component system plays a key role in DSF signal transduction and appears to modulate downstream DSF regulon by changing intracellular content of cyclic dimeric GMP (c-di-GMP), an unusual nucleotide second messenger. Here we show that Clp, a conserved global regulator showing a strong homology to the cAMP nucleotide receptor protein Crp of Escherichia coli, is essential for DSF regulation of virulence factor production but not for biofilm dispersal. Deletion of clp in Xcc changed the transcriptional expression of 299 genes including a few encoding transcription factors. Further genetic and microarray analysis led to identification of a homologue of the transcriptional regulator Zur, and a novel TetR-type transcription factor FhrR. These two regulatory factors regulated different sets of genes within Clp regulon. These results outline a hierarchical signalling network by which DSF modulates different biological functions, and may also provide a clue on how the novel nucleotide signal can be coupled to its downstream regulatory networks.

Genome scale analysis of diffusible signal factor regulon in Xanthomonas campestris pv. campestris: identification of novel cell-cell communication-dependent genes and functions.

The bacterial pathogen Xanthomonas campestris pv. campestris (Xcc) recruits a diffusible signal factor (DSF), which has recently been structurally characterized as cis-11-methyl-2-dodecenoic acid, as a cell-cell communication signal to synchronize virulence gene expression and biofilm dispersal. In this study, we showed that despite the existance of phenotype variations in different Xcc isolates, the DSF-mediated functions were in general conserved. To investigate the genomic profiles of DSF regulation, we designed and conducted oligomicroarray analysis by comparison of the gene expression patterns of wild-type strain XC1 and its DSF-deficient mutant XC1dF, as well as those of XC1dF in the presence or absence of DSF signals. The analyses led to identification of 165 genes, whose expression was significantly influenced by DSF signals. These genes encode proteins and enzymes belonging to at least 12 functional groups. In addition to those previously known DSF-dependent activities such as production of extracellular enzymes and extracellular polysaccharides, microarray analyses also revealed new functions mediated by DSF, such as flagellum synthesis, resistance to toxins and oxidative stress, and aerobic respiration. Phenotype analyses confirmed that DSF signalling contributed to resistance to toxin acriflavin and hydrogen peroxide, and to the survival of bacterial cells at different temperatures. We conclude that DSF cell-cell signalling is not only essential for co-ordinating the expression of virulence genes but also plays a vital role in keeping up the general competence of the pathogen in ecosystems.

VqsM, a novel AraC-type global regulator of quorum-sensing signalling and virulence in Pseudomonas aeruginosa.

Human pathogen Pseudomonas aeruginosa uses quorum-sensing (QS) signalling systems to synchronize the production of virulence factors. There are two interrelated QS systems, las and rhl, in P. aeruginosa. In addition to this complexity, a number of transcriptional regulators were shown to have complicated interplays with las and rhl central QS components. Here, we describe a novel virulence and QS modulator (VqsM) that positively regulates the QS systems in P. aeruginosa. Mutation in vqsM resulted in much reduced production of N-acylhomoserine lactones (AHLs) and extracellular enzymes. Sequence analysis revealed that vqsM encodes a transcriptional regulator with an AraC-type helix-turn-helix DNA binding domain at the C-terminal of the peptide. Global gene expression profile analysis showed at least a total of 302 genes to be influenced, directly or indirectly, by VqsM. Among the 203 VqsM-promoted genes, 52.2% were known to be QS upregulated. Several genes encoding the key regulators implicated in QS, such as rhlR, rsaL, vqsR, mvfR, pprB and rpoS, and two AHL synthesis genes, lasI and rhlI, were suppressed in the vqsM mutant. Similar to the 'AHL-blind' phenotype of vqsR and pprB mutants, vqsM mutant did not respond to external addition of N-3-oxo-dodecanoyl-homoserine lactone signals. Moreover, overexpression of vqsR in vqsM mutant more or less restored the production of both AHL and virulence factors. The results demonstrate that VqsM, largely through modulation of vqsR expression, plays a vital role in regulation of QS signalling in P. aeruginosa.

The two-component response regulator PprB modulates quorum-sensing signal production and global gene expression in Pseudomonas aeruginosa.

The response regulator PprB and its cognate sensor PprA were recently reported as a two-component regulatory system that controls membrane permeability and antibiotic sensitivity of Pseudomonas aeruginosa. We found that a Tn5 insertion mutation in pprB caused a drastic reduction in virulence factor production and cell motility. A transcriptome analysis revealed that 175 genes were regulated by PprB. Among the 113 PprB-activated genes, 85.5% are known to be activated by N-3-oxo-dodecanoyl-homoserine lactone (OdDHL) and N-butanoyl-homoserine lactone (BHL). In particular, the expression of lasI, rhlI and rhlR, which encode key components of the las and rhl quorum-sensing (QS) systems, were significantly decreased in the pprB mutant. These data suggest that PprB might regulate QS signal production. Measurement of OdDHL and BHL in cultures of the mutant sustained this hypothesis. By using various OdDHL- or BHL-responsive QS reporter systems, including lasB-lacZ, lasI-lacZ and rsaL-lacZ, we found that the mutation in pprB resulted in a large decrease in the sensitivity of P. aeruginosa to exogenous OdDHL. However, there was no difference in sensitivity to BHL. Further analysis showed that the OdDHL influx was significantly reduced in the pprB mutant. We conclude that PprB is a novel QS modulator that positively regulates N-acylhomoserine lactone production probably by affecting the OdDHL signal influx and thereby influences global expression of the QS-dependent genes.

Quorum sensing and signal interference: diverse implications.

Quorum sensing (QS) is a community genetic regulation mechanism that controls microbiological functions of medical, agricultural and industrial importance. Discovery of microbial QS signals and the signalling mechanisms led to identification of numerous enzymatic and non-enzymatic signal interference mechanisms that quench microbial QS signalling. Evidence is accumulating that such signal interference mechanisms can be developed as promising approaches to control microbial infection and biofilm formation. In addition, these mechanisms exist not only in microorganisms but also in the host organisms of bacterial pathogens, highlighting their potential implications in microbial ecology and in host-pathogen interactions. Investigation of QS and signal interference mechanisms might significantly broaden the scope of research in microbiology.

Specificity and enzyme kinetics of the quorum-quenching N-Acyl homoserine lactone lactonase (AHL-lactonase).

N-Acyl homoserine lactone (AHL) quorum-sensing signals are the vital elements of bacterial quorum-sensing systems, which regulate diverse biological functions, including virulence. The AHL-lactonase, a quorumquenching enzyme encoded by aiiA from Bacillus sp., inactivates AHLs by hydrolyzing the lactone bond to produce corresponding N-acyl homoserines. To characterize the enzyme, the recombinant AHL-lactonase and its four variants were purified. Kinetic and substrate specificity analysis showed that AHL-lactonase had no or little residue activity to non-acyl lactones and noncyclic esters, but displayed strong enzyme activity toward all tested AHLs, varying in length and nature of the substitution at the C3 position of the acyl chain. The data also indicate that the amide group and the ketone at the C1 position of the acyl chain of AHLs could be important structural features in enzyme-substrate interaction. Surprisingly, although carrying a (104)HX- HXDH(109) short sequence identical to the zinc-binding motif of several groups of metallohydrolytic enzymes, AHL-lactonase does not contain or require zinc or other metal ions for enzyme activity. Except for the amino acid residue His-104, which was shown previously to not be required for catalysis, kinetic study and conformational analysis using circular dichroism spectrometry showed that substitution of the other key residues in the motif (His-106, Asp-108, and His-109), as well as His-169 with serine, respectively, caused conformational changes and significant loss of enzyme activity. We conclude that AHL-lactonase is a highly specific enzyme and that the (106)HXDH(109) approximately H(169) of AHL-lactonase represents a novel catalytic motif, which does not rely on zinc or other metal ions for activity.

Insecticidal Bacillus thuringiensis silences Erwinia carotovora virulence by a new form of microbial antagonism, signal interference.

It is commonly known that bacteria may produce antibiotics to interfere with the normal biological functions of their competitors in order to gain competitive advantages. Here we report that Bacillus thuringiensis suppressed the quorum-sensing-dependent virulence of plant pathogen Erwinia carotovora through a new form of microbial antagonism, signal interference. E. carotovora produces and responds to acyl-homoserine lactone (AHL) quorum-sensing signals to regulate antibiotic production and expression of virulence genes, whereas B. thuringiensis strains possess AHL-lactonase, which is a potent AHL-degrading enzyme. B. thuringiensis did not seem to interfere with the normal growth of E. carotovora; rather, it abolished the accumulation of AHL signal when they were cocultured. In planta, B. thuringiensis significantly decreased the incidence of E. carotovora infection and symptom development of potato soft rot caused by the pathogen. The biocontrol efficiency is correlated with the ability of bacterial strains to produce AHL-lactonase. While all the seven AHL-lactonase-producing B. thuringiensis strains provided significant protection against E. carotovora infection, Bacillus fusiformis and Escherichia coli strains that do not process AHL-degradation enzyme showed little effect in biocontrol. Mutation of aiiA, the gene encoding AHL-lactonase in B. thuringiensis, resulted in a substantial decrease in biocontrol efficacy. These results suggest that signal interference mechanisms existing in natural ecosystems could be explored as a new version of antagonism for prevention of bacterial infections.

Identification of quorum-quenching N-acyl homoserine lactonases from Bacillus species.

A range of gram-negative bacterial species use N-acyl homoserine lactone (AHL) molecules as quorum-sensing signals to regulate different biological functions, including production of virulence factors. AHL is also known as an autoinducer. An autoinducer inactivation gene, aiiA, coding for an AHL lactonase, was cloned from a bacterial isolate, Bacillus sp. strain 240B1. Here we report identification of more than 20 bacterial isolates capable of enzymatic inactivation of AHLs from different sources. Eight isolates showing strong AHL-inactivating enzyme activity were selected for a preliminary taxonomic analysis. Morphological phenotypes and 16S ribosomal DNA sequence analysis indicated that these isolates probably belong to the species Bacillus thuringiensis. Enzymatic analysis with known Bacillus strains confirmed that all of the strains of B. thuringiensis and the closely related species B. cereus and B. mycoides tested produced AHL-inactivating enzymes but B. fusiformis and B. sphaericus strains did not. Nine genes coding for AHL inactivation were cloned either by functional cloning or by a PCR procedure from selected bacterial isolates and strains. Sequence comparison of the gene products and motif analysis showed that the gene products belong to the same family of AHL lactonases.

A bacterial cell-cell communication signal with cross-kingdom structural analogues.

Extracellular signals are the key components of microbial cell-cell communication systems. This report identified a diffusible signal factor (DSF), which regulates virulence in Xanthomonas campestris pv. campestris, as cis-11-methyl-2-dodecenoic acid, an alpha,beta unsaturated fatty acid. Analysis of DSF derivatives established the double bond at the alpha,beta positions as the most important structural feature for DSF biological activity. A range of bacterial pathogens, including several Mycobacterium species, also displayed DSF-like activity. Furthermore, DSF is structurally and functionally related to farnesoic acid (FA), which regulates morphological transition and virulence by Candida albicans, a fungal pathogen. Similar to FA, which is also an alpha,beta unsaturated fatty acid, DSF inhibits the dimorphic transition of C. albicans at a physiologically relevant concentration. We conclude that alpha,beta unsaturated fatty acids represent a new class of extracellular signals for bacterial and fungal cell-cell communications. As prokaryote-eukaryote interactions are ubiquitous, such cross-kingdom conservation in cell-cell communication systems might have significant ecological and economic importance.