Extracellular fatty acids facilitate flagella-independent translocation by Stenotrophomonas maltophilia

Stenotrophomonas maltophilia virulence: a current view

Stenotrophomonas maltophilia is an opportunistic pathogen intrinsically resistant to multiple and broad-spectrum antibiotics. Although the bacterium is considered a low-virulence pathogen, it can cause various severe diseases and contributes significantly to the pathogenesis of multibacterial infections. During the COVID-19 pandemic, S. maltophilia has been recognized as one of the most common causative agents of respiratory co-infections and bacteremia in critically ill COVID-19 patients. The high ability to adapt to unfavorable environments and new habitat niches, as well as the sophisticated switching of metabolic pathways, are unique mechanisms that attract the attention of clinical researchers and experts studying the fundamental basis of virulence. In this review, we have summarized the current knowledge on the molecular aspects of S. maltophilia virulence and putative virulence factors, partially touched on interspecific bacterial interactions and iron uptake systems in the context of virulence, and have not addressed antibiotic resistance.

Synthesis and evaluation of aromatic BDSF bioisosteres on biofilm formation and colistin sensitivity in pathogenic bacteria

The diffusible signal factor family (DSF) of molecules play an important role in regulating intercellular communication, or quorum sensing, in several disease-causing bacteria. These messenger molecules, which are comprised of cis-unsaturated fatty acids, are involved in the regulation of biofilm formation, antibiotic tolerance, virulence and the control of bacterial resistance. We have previously demonstrated how olefinic N-acyl sulfonamide bioisosteric analogues of diffusible signal factor can reduce biofilm formation or enhance antibiotic sensitivity in a number of bacterial strains. This work describes the design and synthesis of a second generation of aromatic N-acyl sulfonamide bioisosteres. The impact of these compounds on biofilm production in Acinetobacter baumannii, Escherichia coli, Burkholderia multivorans, Burkholderia cepacia, Burkholderia cenocepacia, Pseudomonas aeruginosa and Stenotrophomonas maltophilia is evaluated, in addition to their effects on antibiotic tolerance. The ability of these molecules to increase survival rates on co-administration with colistin is also investigated using the Galleria infection model.

Quorum sensing improves the plant growth-promoting ability of Stenotrophomonas rhizophila under saline-alkaline stress by enhancing its environmental adaptability

Quorum sensing (QS) system has an essential function in plant growth-promoting rhizobacteria (PGPR) response to environmental stress and PGPR induction of plant tolerance to saline-alkaline stress. Nevertheless, there is a lack of understanding about how QS influences the growth-promoting effects of PGPR on plants. Stenotrophomonas rhizophila DSM14405^T is a PGPR with a QS system, which can secrete diffusible signal factor (DSF), one of the QS signal molecules. In this study, we used the S. rhizophila wild type (WT) and an incompetent DSF production rpfF-knockout mutant strain to explore whether DSF-QS could affect the growth-promoting ability of PGPR in Brassica napus L. By measuring the seed germination rate, plant fresh weight, biomass, the total antioxidant capacity (T-AOC) level, and the content of chlorophyll in leaves, we found that DSF was unable to enhance the growth-promoting capacity of ΔrpfF and did not directly assist the plants in tolerating saline-alkaline stress. However, DSF aided S. rhizophila ΔrpfF in resisting stress during its effective period, and QS represents a continuous and precise regulatory mechanism. Altogether, our results show that DSF is helpful to improve the environmental adaptability and survival rate of S. rhizophila, thus indirectly improving the germination rate of seeds and helping plants grow under saline-alkaline stress. In this study, the mechanism of QS enhancing the environmental adaptability of PGPR was studied, which provided a theoretical basis for improving the application of PGPR to help plants better cope with saline-alkaline stress.

Diffusible signal factor enhances the saline-alkaline resistance and rhizosphere colonization of Stenotrophomonas rhizophila by coordinating optimal metabolism

Quorum sensing (QS) regulates various physiological processes in a cell density-dependent mode via cell-cell communication. Stenotrophomonas rhizophila DSM14405^T having the diffusible signal factor (DSF)-QS system, is a plant growth-promoting rhizobacteria (PGPR) that enables host plants to tolerate saline-alkaline stress. However, the regulatory mechanism of DSF-QS in S. rhizophila is not fully understood. In this study, we used S. rhizophila DSM14405^T wild-type (WT) and an incompetent DSF production rpfF-knockout mutant to explore the regulatory role of QS in S. rhizophila growth, stress responses, biofilm formation, and colonization under saline-alkaline stress. We found that a lack of DSF-QS reduces the tolerance of S. rhizosphere ΔrpfF to saline-alkaline stress, with a nearly 25-fold reduction in the ΔrpfF population compared with WT at 24 h under stress. Transcriptome analysis revealed that QS helps S. rhizophila WT respond to saline-alkaline stress by enhancing metabolism associated with the cell wall and membrane, oxidative stress response, cell adhesion, secretion systems, efflux pumps, and TonB systems. These metabolic systems enhance penetration defense, Na⁺ efflux, iron uptake, and reactive oxygen species scavenging. Additionally, the absence of DSF-QS causes overexpression of biofilm-associated genes under the regulation of sigma 54 and other transcriptional regulators. However, greater biofilm formation capacity confers no advantage on S. rhizosphere ΔrpfF in rhizosphere colonization. Altogether, our results show the importance of QS in PGPR growth and colonization; QS gives PGPR a collective adaptive advantage in harsh natural environments.

Taxonomic position, antibiotic resistance and virulence factor production by Stenotrophomonas isolates from patients with cystic fibrosis and other chronic respiratory infections

BACKGROUND

The potential pathogenic role of Stenotrophomonas maltophilia in lung disease and in particular in cystic fibrosis is unclear. To develop further understanding of the biology of this taxa, the taxonomic position, antibiotic resistance and virulence factors of S. maltophilia isolates from patients with chronic lung disease were studied.

RESULTS

A total of 111 isolates recovered between 2003 and 2016 from respiratory samples from patients in five different countries were included. Based on a cut-off of 95%, analysis of average nucleotide identity by BLAST (ANIb) showed that the 111 isolates identified as S. maltophilia by Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/MS) belonged to S. maltophilia (n = 65), S. pavanii (n = 6) and 13 putative novel species (n = 40), which each included 1-5 isolates; these groupings coincided with the results of the 16S rDNA analysis, and the L1 and L2 ß-lactamase Neighbor-Joining phylogeny. Chromosomally encoded aminoglycoside resistance was identified in all S. maltophilia and S. pavani isolates, while acquired antibiotic resistance genes were present in only a few isolates. Nevertheless, phenotypic resistance levels against commonly used antibiotics, determined by standard broth microbroth dilution, were high. Although putative virulence genes were present in all isolates, the percentage of positive isolates varied. The Xps II secretion system responsible for the secretion of the StmPr1-3 proteases was mainly limited to isolates identified as S. maltophilia based on ANIb, but no correlation with phenotypic expression of protease activity was found. The RPF two-component quorum sensing system involved in virulence and antibiotic resistance expression has two main variants with one variant lacking 190 amino acids in the sensing region.

CONCLUSIONS

The putative novel Stenotrophomonas species recovered from patient samples and identified by MALDI-TOF/MS as S. maltophilia, differed from S. maltophilia in resistance and virulence genes, and therefore possibly in pathogenicity. Revision of the Stenotrophomonas taxonomy is needed in order to reliably identify strains within the genus and elucidate the role of the different species in disease.

The Potential of Phage Therapy against the Emerging Opportunistic Pathogen Stenotrophomonas maltophilia

The isolation and characterization of bacteriophages for the treatment of infections caused by the multidrug resistant pathogen Stenotrophomonas maltophilia is imperative as nosocomial and community-acquired infections are rapidly increasing in prevalence. This increase is largely due to the numerous virulence factors and antimicrobial resistance genes encoded by this bacterium. Research on S. maltophilia phages to date has focused on the isolation and in vitro characterization of novel phages, often including genomic characterization, from the environment or by induction from bacterial strains. This review summarizes the clinical significance, virulence factors, and antimicrobial resistance mechanisms of S. maltophilia, as well as all phages isolated and characterized to date and strategies for their use. We further address the limited in vivo phage therapy studies conducted against this bacterium and discuss the future research needed to spearhead phages as an alternative treatment option against multidrug resistant S. maltophilia.

A putative enoyl-CoA hydratase contributes to biofilm formation and the antibiotic tolerance of Achromobacter xylosoxidans

Achromobacter xylosoxidans has attracted increasing attention as an emerging pathogen in patients with cystic fibrosis. Intrinsic resistance to several classes of antimicrobials and the ability to form robust biofilms in vivo contribute to the clinical manifestations of persistent A. xylosoxidans infection. Still, much of A. xylosoxidans biofilm formation remains uncharacterized due to the scarcity of existing genetic tools. Here we demonstrate a promising genetic system for use in A. xylosoxidans; generating a transposon mutant library which was then used to identify genes involved in biofilm development in vitro. We further described the effects of one of the genes found in the mutagenesis screen, encoding a putative enoyl-CoA hydratase, on biofilm structure and tolerance to antimicrobials. Through additional analysis, we find that a fatty acid signaling compound is essential to A. xylosoxidans biofilm ultrastructure and maintenance. This work describes methods for the genetic manipulation of A. xylosoxidans and demonstrated their use to improve our understanding of A. xylosoxidans pathophysiology.

Novel Stenotrophomonas maltophilia temperate phage DLP4 is capable of lysogenic conversion

BACKGROUND

Temperate bacteriophages are capable of lysogenic conversion of new bacterial hosts. This phenomenon is often ascribed to "moron" elements that are acquired horizontally and transcribed independently from the rest of the phage genes. Whereas some bacterial species exhibit relatively little prophage-dependent phenotypic changes, other bacterial species such as Stenotrophomonas maltophilia appear to commonly adopt prophage genetic contributions.

RESULTS

The novel S. maltophilia bacteriophage DLP4 was isolated from soil using the highly antibiotic-resistant S. maltophilia strain D1585. Genome sequence analysis and functionality testing showed that DLP4 is a temperate phage capable of lysogenizing D1585. Two moron genes of interest, folA (BIT20_024) and ybiA (BIT20_065), were identified and investigated for their putative activities using complementation testing and phenotypic and transcriptomic changes between wild-type D1585 and the D1585::DLP4 lysogen. The gp24 / folA gene encodes dihydrofolate reductase (DHFR: FolA), an enzyme responsible for resistance to the antibiotic trimethoprim. I-TASSER analysis of DLP4 FolA predicted structural similarity to Bacillus anthracis DHFR and minimum inhibitory concentration experiments demonstrated that lysogenic conversion of D1585 by DLP4 provided the host cell with an increase in trimethoprim resistance. The gp65 / ybiA gene encodes N-glycosidase YbiA, which in E. coli BW25113 is required for its swarming motility phenotype. Expressing DLP4 ybiA in strain ybiA770(del)::kan restored its swarming motility activity to wildtype levels. Reverse transcription-PCR confirmed the expression of both of these genes during DLP4 lysogeny.

CONCLUSIONS

S. maltophilia temperate phage DLP4 contributes to the antibiotic resistance exhibited by its lysogenized host strain. Genomic analyses can greatly assist in the identification of phage moron genes potentially involved in lysogenic conversion. Further research is required to fully understand the specific contributions temperate phage moron genes provide with respect to the antibiotic resistance and virulence of S. maltophilia host cells.

Structural characterization of a Δ3, Δ2-enoyl-CoA isomerase from Pseudomonas aeruginosa: implications for its involvement in unsaturated fatty acid metabolism

Gene PA4980 from Pseudomonas aeruginosa encodes a putative enoyl-coenzyme A hydratase/isomerase that is associated with the function of the biofilm dispersion-inducing signal molecule cis-2-decenoic acid. To elucidate the role of PA4980 in cis-2-decenoic acid biosynthesis, we reported the crystal structure of its protein product at 2.39 Å. The structural analysis and substrate binding prediction suggest that it acts as a monofunctional enoyl-coenzyme A isomerase, implicating an alternative pathway of the cis-2-decenoic acid synthesis.

Quorum Sensing Signaling and Quenching in the Multidrug-Resistant Pathogen Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is an opportunistic Gram-negative pathogen with increasing incidence in clinical settings. The most critical aspect of S. maltophilia is its frequent resistance to a majority of the antibiotics of clinical use. Quorum Sensing (QS) systems coordinate bacterial populations and act as major regulatory mechanisms of pathogenesis in both pure cultures and poly-microbial communities. Disruption of QS systems, a phenomenon known as Quorum Quenching (QQ), represents a new promising paradigm for the design of novel antimicrobial strategies. In this context, we review the main advances in the field of QS in S. maltophilia by paying special attention to Diffusible Signal Factor (DSF) signaling, Acyl Homoserine Lactone (AHL) responses and the controversial Ax21 system. Advances in the DSF system include regulatory aspects of DSF synthesis and perception by both rpf-1 and rpf-2 variant systems, as well as their reciprocal communication. Interaction via DSF of S. maltophilia with unrelated organisms including bacteria, yeast and plants is also considered. Finally, an overview of the different QQ mechanisms involving S. maltophilia as quencher and as object of quenching is presented, revealing the potential of this species for use in QQ applications. This review provides a comprehensive snapshot of the interconnected QS network that S. maltophilia uses to sense and respond to its surrounding biotic or abiotic environment. Understanding such cooperative and competitive communication mechanisms is essential for the design of effective anti QS strategies.

Structural and functional studies on Pseudomonas aeruginosa DspI: implications for its role in DSF biosynthesis

DspI, a putative enoyl-coenzyme A (CoA) hydratase/isomerase, was proposed to be involved in the synthesis of cis-2-decenoic acid (CDA), a quorum sensing (QS) signal molecule in the pathogen Pseudomonas aeruginosa (P. aeruginosa). The present study provided a structural basis for the dehydration reaction mechanism of DspI during CDA synthesis. Structural analysis reveals that Glu126, Glu146, Cys127, Cys131 and Cys154 are important for its enzymatic function. Moreover, we show that the deletion of dspI results in a remarkable decreased in the pyoverdine production, flagella-dependent swarming motility, and biofilm dispersion as well as attenuated virulence in P. aeruginosa PA14. This study thus unravels the mechanism of DspI in diffusible signal factor (DSF) CDA biosynthesis, providing vital information for developing inhibitors that interfere with DSF associated pathogenicity in P. aeruginosa.

Fatty acid DSF binds and allosterically activates histidine kinase RpfC of phytopathogenic bacterium Xanthomonas campestris pv. campestris to regulate quorum-sensing and virulence

As well as their importance to nutrition, fatty acids (FA) represent a unique group of quorum sensing chemicals that modulate the behavior of bacterial population in virulence. However, the way in which full-length, membrane-bound receptors biochemically detect FA remains unclear. Here, we provide genetic, enzymological and biophysical evidences to demonstrate that in the phytopathogenic bacterium Xanthomonas campestris pv. campestris, a medium-chain FA diffusible signal factor (DSF) binds directly to the N-terminal, 22 amino acid-length sensor region of a receptor histidine kinase (HK), RpfC. The binding event remarkably activates RpfC autokinase activity by causing an allosteric change associated with the dimerization and histidine phosphotransfer (DHp) and catalytic ATP-binding (CA) domains. Six residues were found essential for sensing DSF, especially those located in the region adjoining to the inner membrane of cells. Disrupting direct DSF-RpfC interaction caused deficiency in bacterial virulence and biofilm development. In addition, two amino acids within the juxtamembrane domain of RpfC, Leu¹⁷² and Ala¹⁷⁸, are involved in the autoinhibition of the RpfC kinase activity. Replacements of them caused constitutive activation of RpfC-mediated signaling regardless of DSF stimulation. Therefore, our results revealed a biochemical mechanism whereby FA activates bacterial HK in an allosteric manner, which will assist in future studies on the specificity of FA-HK recognition during bacterial virulence regulation and cell-cell communication.

Besides roles in nutrition, lipids also function as important signals in the regulation of prokaryotic and eukaryotic cells. In bacteria, fatty acids are part of the language of cell-cell communication known as quorum sensing for a decade. However, how bacteria detect these signals and regulate virulence remains elusive. Here, we provide multiple evidences to show that a full-length receptor histidine kinase, RpfC, directly binds to a fatty acid-based signal factor using a short sensor region. This binding event stimulates RpfC autokinase activity by triggering conformational change in its catalytic region, which is critical in regulating bacterial quorum sensing and virulence. Our results confirm a long-outstanding assumption in cell signaling of phytobacteria, and provide a technical pipeline to analyze fatty acid-receptor interactions.

Stenotrophomonas maltophilia outer membrane vesicles elicit a potent inflammatory response in vitro and in vivo

Stenotrophomonas maltophilia has become one of the most prevalent opportunistic pathogens in hospitalized patients. This microorganism secretes outer membrane vesicles (OMVs), but the pathogenesis of S. maltophilia as it relates to OMVs has not been characterized. This study investigated the cytotoxic activity of S. maltophilia OMVs and their ability to induce inflammatory responses both in vitro and in vivo Stenotrophomonas maltophilia ATCC 13637 and two clinical isolates were found to secrete spherical OMVs during in vitro culture. OMVs from S. maltophilia ATCC 13637 were cytotoxic to human lung epithelial A549 cells. Stenotrophomonas maltophilia OMVs stimulated the expression of proinflammatory cytokine and chemokine genes, including interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor-α and monocyte chemoattractant protein-1, in A549 cells. Early inflammatory responses such as congestion and neutrophilic infiltrations and profound expression of proinflammatory cytokine and chemokine genes were observed in the lungs of mice injected with S. maltophilia OMVs, and were similar to responses elicited by the bacteria. Our data demonstrate that S. maltophilia OMVs are important secretory nanocomplexes that elicit a potent inflammatory response that might contribute to S. maltophilia pathogenesis during infection.

Promiscuous Diffusible Signal Factor Production and Responsiveness of the Xylella fastidiosa Rpf System

Cell density-dependent regulation of gene expression in Xylella fastidiosa that is crucial to its switching between plant hosts and insect vectors is dependent on RpfF and its production of 2-enoic acids known as diffusible signal factor (DSF). We show that X. fastidiosa produces a particularly large variety of similar, relatively long-chain-length 2-enoic acids that are active in modulating gene expression. Both X. fastidiosa itself and a Pantoea agglomerans surrogate host harboring X. fastidiosa RpfF (XfRpfF) is capable of producing a variety of both saturated and unsaturated free fatty acids. However, only 2-cis unsaturated acids were found to be biologically active in X. fastidiosa X. fastidiosa produces, and is particularly responsive to, a novel DSF species, 2-cis-hexadecanoic acid that we term XfDSF2. It is also responsive to other, even longer 2-enoic acids to which other taxa such as Xanthomonas campestris are unresponsive. The 2-enoic acids that are produced by X. fastidiosa are strongly affected by the cellular growth environment, with XfDSF2 not detected in culture media in which 2-tetradecenoic acid (XfDSF1) had previously been found. X. fastidiosa is responsive to much lower concentrations of XfDSF2 than XfDSF1. Apparently competitive interactions can occur between various saturated and unsaturated fatty acids that block the function of those agonistic 2-enoic fatty acids. By altering the particular 2-enoic acids produced and the relative balance of free enoic and saturated fatty acids, X. fastidiosa might modulate the extent of DSF-mediated quorum sensing.

IMPORTANCE

X. fastidiosa, having a complicated lifestyle in which it moves and multiplies within plants but also must be vectored by insects, utilizes DSF-based quorum sensing to partition the expression of traits needed for these two processes within different cells in this population based on local cellular density. The finding that it can produce a variety of DSF species in a strongly environmentally context-dependent manner provides insight into how it coordinates the many genes under the control of DSF signaling to successfully associate with its two hosts. Since the new DSF variant XfDSF2 described here is much more active than the previously recognized DSF species, it should contribute to plant disease control, given that the susceptibility of plants can be greatly reduced by artificially elevating the levels of DSF in plants, creating "pathogen confusion," resulting in lower virulence.

Diffusible signal factor family signals provide a fitness advantage to Xanthomonas campestris pv. campestris in interspecies competition

Diffusible signal factor (DSF) represents a new class of widely conserved quorum sensing signals, which regulates various biological functions through intra- or interspecies signaling. The previous studies identified that there is an antagonistic interaction between Xanthomonas and Bacillus species bacteria in natural ecosystem, but the detailed molecular mechanism of interspecies competition is not clear. This study showed that Xanthomonas campestris pv. campestris (Xcc) interfered with morphological transition and sporulation of Bacillus thuringiensis in mixed cultures, whereas abrogation of the DSF synthase RpfF reduced the interference. DSF inhibited B. thuringiensis cell division and sporulation through modulation of ftsZ, which encodes an important cell division protein in bacterial cells. In addition, RpfF is essential for production of six DSF-family signals in Xcc, which employ the same signaling pathways to regulate biological functions in Xcc and play similar effects on reduction of cell division, sporulation and antibiotic resistance of B. thuringiensis. Furthermore, abrogation of RpfF decreased the competitive capability of Xcc against B. thuringiensis on the surface of Chinese cabbage leaves. Our findings provide new insights into the role of DSF-family signals in interspecies competition and depict molecular mechanisms with which Xcc competes with B. thuringiensis.

Going beyond the Control of Quorum-Sensing to Combat Biofilm Infections

Most bacteria attach to surfaces where they form a biofilm, cells embedded in a complex matrix of polymers. Cells in biofilms are much better protected against noxious agents than free-living cells. As a consequence it is very difficult to control pathogens with antibiotics in biofilm infections and novel targets are urgently needed. One approach aims at the communication between cells to form and to maintain a biofilm, a process called quorum-sensing. Water soluble small-sized molecules mediate this process and a number of antagonists of these compounds have been found. In this review natural compounds and synthetic drugs which do not interfere with the classical quorum-sensing compounds are discussed. For some of these compounds the targets are still not known, but others interfere with the formation of exopolysaccharides, virulence factors, or cell wall synthesis or they start an internal program of biofilm dispersal. Some of their targets are more conserved among pathogens than the receptors for quorum sensing autoinducers mediating quorum-sensing, enabling a broader application of the drug. The broad spectrum of mechanisms, the diversity of bioactive compounds, their activity against several targets, and the conservation of some targets among bacterial pathogens are promising aspects for several clinical applications of this type of biofilm-controlling compound in the future.

Control of Biofilms with the Fatty Acid Signaling Molecule cis-2-Decenoic Acid

Biofilms are complex communities of microorganisms in organized structures attached to surfaces. Importantly, biofilms are a major cause of bacterial infections in humans, and remain one of the most significant challenges to modern medical practice. Unfortunately, conventional therapies have shown to be inadequate in the treatment of most chronic biofilm infections based on the extraordinary innate tolerance of biofilms to antibiotics. Antagonists of quorum sensing signaling molecules have been used as means to control biofilms. QS and other cell-cell communication molecules are able to revert biofilm tolerance, prevent biofilm formation and disrupt fully developed biofilms, albeit with restricted effectiveness. Recently however, it has been demonstrated that Pseudomonas aeruginosa produces a small messenger molecule cis-2-decenoic acid (cis-DA) that shows significant promise as an effective adjunctive to antimicrobial treatment of biofilms. This molecule is responsible for induction of the native biofilm dispersion response in a range of Gram-negative and Gram-positive bacteria and in yeast, and has been shown to reverse persistence, increase microbial metabolic activity and significantly enhance the cidal effects of conventional antimicrobial agents. In this manuscript, the use of cis-2-decenoic acid as a novel agent for biofilm control is discussed. Stimulating the biofilm dispersion response as a novel antimicrobial strategy holds significant promise for enhanced treatment of infections and in the prevention of biofilm formation.

Decoding the genetic and functional diversity of the DSF quorum-sensing system in Stenotrophomonas maltophilia

Stenotrophomonas maltophilia uses the Diffusible Signal Factor (DSF) quorum sensing (QS) system to mediate intra- and inter-specific signaling and regulate virulence-related processes. The components of this system are encoded by the rpf cluster, with genes rpfF and rpfC encoding for the DSF synthase RpfF and sensor RpfC, respectively. Recently, we have shown that there exist two variants of the rpf cluster (rpf-1 and rpf-2), distinguishing two groups of S. maltophilia strains. Surprisingly, only rpf-1 strains produce detectable DSF, correlating with their ability to control biofilm formation, swarming motility and virulence. The evolutive advantage of acquiring two different rpf clusters, the phylogenetic time point and mechanism of this acquisition and the conditions that activate DSF production in rpf-2 strains, are however not known. Examination of this cluster in various species suggests that its variability originated most probably by genetic exchange between rhizosphere bacteria. We propose that rpf-2 variant strains make use of a strategy recently termed as "social cheating." Analysis of cellular and extracellular fatty acids (FAs) of strains E77 (rpf-1) and M30 (rpf-2) suggests that their RpfFs have also a thioesterase activity that facilitates the release of unspecific FAs to the medium in addition to DSF. Production of DSF in rpf-1 strains appears in fact to be modulated by some of these extracellular FAs in addition to other factors such as temperature and nutrients, while in rpf-2 strains DSF biosynthesis is derepressed only upon detection of DSF itself, suggesting that they require cohabitation with DSF-producer bacteria to activate their DSF regulatory machinery. Finally, we show that the mixed rpf-1/rpf-2 population presents synergism in DSF production and virulence capacity in an in vivo infection model. Recovery and quantification of DSF from co-infected animals correlates with the observed mortality rate.

Stenotrophomonas maltophilia responds to exogenous AHL signals through the LuxR solo SmoR (Smlt1839)

Quorum Sensing (QS) mediated by Acyl Homoserine Lactone (AHL) molecules are probably the most widespread and studied among Gram-negative bacteria. Canonical AHL systems are composed by a synthase (LuxI family) and a regulator element (LuxR family), whose genes are usually adjacent in the genome. However, incomplete AHL-QS machinery lacking the synthase LuxI is frequently observed in Proteobacteria, and the regulator element is then referred as LuxR solo. It has been shown that certain LuxR solos participate in interspecific communication by detecting signals produced by different organisms. In the case of Stenotrophomonas maltophilia, a preliminary genome sequence analysis revealed numerous putative luxR genes, none of them associated to a luxI gene. From these, the hypothetical LuxR solo Smlt1839, here designated SmoR, presents a conserved AHL binding domain and a helix-turn-helix DNA binding motif. Its genomic organization—adjacent to hchA gene—indicate that SmoR belongs to the new family “LuxR regulator chaperone HchA-associated.” AHL-binding assays revealed that SmoR binds to AHLs in-vitro, at least to oxo-C8-homoserine lactone, and it regulates operon transcription, likely by recognizing a conserved palindromic regulatory box in the hchA upstream region. Supplementation with concentrated supernatants from Pseudomonas aeruginosa, which contain significant amounts of AHLs, promoted swarming motility in S. maltophilia. Contrarily, no swarming stimulation was observed when the P. aeruginosa supernatant was treated with the lactonase AiiA from Bacillus subtilis, confirming that AHL contributes to enhance the swarming ability of S. maltophilia. Finally, mutation of smoR resulted in a swarming alteration and an apparent insensitivity to the exogenous AHLs provided by P. aeruginosa. In conclusion, our results demonstrate that S. maltophilia senses AHLs produced by neighboring bacteria through the LuxR solo SmoR, regulating population behaviors such as swarming motility.

The effect of imipenem and diffusible signaling factors on the secretion of outer membrane vesicles and associated Ax21 proteins in Stenotrophomonas maltophilia

Outer membrane vesicles (OMVs) are small nanoscale structures that are secreted by bacteria and that can carry nucleic acids, proteins, and small metabolites. They can mediate intracellular communication and play a role in virulence. In this study, we show that treatment with the β-lactam antibiotic imipenem leads to a dramatic increase in the secretion of outer membrane vesicles in the nosocomial pathogen Stenotrophomonas maltophilia. Proteomic analysis of their protein content demonstrated that the OMVs contain the chromosomal encoded L1 metallo-β-lactamase and L2 serine-β-lactamase. Moreover, the secreted OMVs contain large amounts of two Ax21 homologs, i.e., outer membrane proteins known to be involved in virulence and biofilm formation. We show that OMV secretion and the levels of Ax21 in the OMVs are dependent on the quorum sensing diffusible signal system (DSF). More specific, we demonstrate that the S. maltophilia DSF cis-Δ2-11-methyl-dodecenoic acid and, to a lesser extent, the Burkholderia cenocepacia DSF cis-Δ2-dodecenoic acid, stimulate OMV secretion. By a targeted proteomic analysis, we confirmed that DSF-induced OMVs contain large amounts of the Ax21 homologs, but not the β-lactamases. This work illustrates that both quorum sensing and disturbance of the peptidoglycan biosynthesis provoke the release of OMVs and that OMV content is context dependent.

The host plant metabolite glucose is the precursor of diffusible signal factor (DSF) family signals in Xanthomonas campestris

Plant pathogen Xanthomonas campestris pv. campestris produces cis-11-methyl-2-dodecenoic acid (diffusible signal factor [DSF]) as a cell-cell communication signal to regulate biofilm dispersal and virulence factor production. Previous studies have demonstrated that DSF biosynthesis is dependent on the presence of RpfF, an enoyl-coenzyme A (CoA) hydratase, but the DSF synthetic mechanism and the influence of the host plant on DSF biosynthesis are still not clear. We show here that exogenous addition of host plant juice or ethanol extract to the growth medium of X. campestris pv. campestris could significantly boost DSF family signal production. It was subsequently revealed that X. campestris pv. campestris produces not only DSF but also BDSF (cis-2-dodecenoic acid) and another novel DSF family signal, which was designated DSF-II. BDSF was originally identified in Burkholderia cenocepacia to be involved in regulation of motility, biofilm formation, and virulence in B. cenocepacia. Functional analysis suggested that DSF-II plays a role equal to that of DSF in regulation of biofilm dispersion and virulence factor production in X. campestris pv. campestris. Furthermore, chromatographic separation led to identification of glucose as a specific molecule stimulating DSF family signal biosynthesis in X. campestris pv. campestris. (13)C-labeling experiments demonstrated that glucose acts as a substrate to provide a carbon element for DSF biosynthesis. The results of this study indicate that X. campestris pv. campestris could utilize a common metabolite of the host plant to enhance DSF family signal synthesis and therefore promote virulence.

Phytohormone-mediated interkingdom signaling shapes the outcome of rice-Xanthomonas oryzae pv. oryzae interactions

BACKGROUND

Small-molecule hormones are well known to play key roles in the plant immune signaling network that is activated upon pathogen perception. In contrast, little is known about whether phytohormones also directly influence microbial virulence, similar to what has been reported in animal systems.

RESULTS

In this paper, we tested the hypothesis that hormones fulfill dual roles in plant-microbe interactions by orchestrating host immune responses, on the one hand, and modulating microbial virulence traits, on the other. Employing the rice-Xanthomonas oryzae pv. oryzae (Xoo) interaction as a model system, we show that Xoo uses the classic immune hormone salicylic acid (SA) as a trigger to activate its virulence-associated quorum sensing (QS) machinery. Despite repressing swimming motility, sodium salicylate (NaSA) induced production of the Diffusible Signal Factor (DSF) and Diffusible Factor (DF) QS signals, with resultant accumulation of xanthomonadin and extracellular polysaccharides. In contrast, abscisic acid (ABA), which favors infection by Xoo, had little impact on DF- and DSF-mediated QS, but promoted bacterial swimming via the LuxR solo protein OryR. Moreover, we found both DF and DSF to influence SA- and ABA-responsive gene expression in planta.

CONCLUSIONS

Together our findings indicate that the rice SA and ABA signaling pathways cross-communicate with the Xoo DF and DSF QS systems and underscore the importance of bidirectional interkingdom signaling in molding plant-microbe interactions.

Identification of a small molecule signaling factor that regulates the biosynthesis of the antifungal polycyclic tetramate macrolactam HSAF in Lysobacter enzymogenes

Lysobacter species are emerging as new sources of antibiotics. The regulation of these antibiotics is not well understood. Here, we identified a small molecule metabolite (LeDSF3) that regulates the biosynthesis of the antifungal antibiotic heat-stable antifungal factor (HSAF), a polycyclic tetramate macrolactam with a structure and mode of action distinct from the existing antifungal drugs. LeDSF3 was isolated from the culture broth of Lysobacter enzymogenes, and its chemical structure was established by NMR and MS. The purified compound induced green fluorescence in a reporter strain of Xanthomonas campestris, which contained a gfp gene under the control of a diffusible signaling factor (DSF)-inducible promoter. Exogenous addition of LeDSF3 in L. enzymogenes cultures significantly increased the HSAF yield, the transcription of HSAF biosynthetic genes, and the antifungal activity of the organism. The LeDSF3-regulated HSAF production is dependent on the two-component regulatory system RpfC/RpfG. Moreover, LeDSF3 upregulated the expression of the global regulator cAMP receptor-like protein (Clp). The disruption of clp led to no HSAF production. Together, the results show that LeDSF3 is a fatty acid-derived, diffusible signaling factor positively regulating HSAF biosynthesis and that the signaling is mediated by the RfpC/RpfG-Clp pathway. These findings may facilitate the antibiotic production through applied genetics and molecular biotechnology in Lysobacter, a group of ubiquitous yet underexplored microorganisms.

Xanthomonas campestris RpfB is a fatty Acyl-CoA ligase required to counteract the thioesterase activity of the RpfF diffusible signal factor (DSF) synthase

In Xanthomonas campestris pv. campestris (Xcc), the proteins encoded by the rpf (regulator of pathogenicity factor) gene cluster produce and sense a fatty acid signal molecule called diffusible signalling factor (DSF, 2(Z)-11-methyldodecenoic acid). RpfB was reported to be involved in DSF processing and was predicted to encode an acyl-CoA ligase. We report that RpfB activates a wide range of fatty acids to their CoA esters in vitro. Moreover, RpfB can functionally replace the paradigm bacterial acyl-CoA ligase, Escherichia coli FadD, in the E. coli ß-oxidation pathway and deletion of RpfB from the Xcc genome results in a strain unable to utilize fatty acids as carbon sources. An essential RpfB function in the pathogenicity factor pathway was demonstrated by the properties of a strain deleted for both the rpfB and rpfC genes. The ΔrpfB ΔrpfC strain grew poorly and lysed upon entering stationary phase. Deletion of rpfF, the gene encoding the DSF synthetic enzyme, restored normal growth to this strain. RpfF is a dual function enzyme that synthesizes DSF by dehydration of a 3-hydroxyacyl-acyl carrier protein (ACP) fatty acid synthetic intermediate and also cleaves the thioester bond linking DSF to ACP. However, the RpfF thioesterase activity is of broad specificity and upon elimination of its RpfC inhibitor RpfF attains maximal activity and its thioesterase activity proceeds to block membrane lipid synthesis by cleavage of acyl-ACP intermediates. This resulted in release of the nascent acyl chains to the medium as free fatty acids. This lack of acyl chains for phospholipid synthesis results in cell lysis unless RpfB is present to counteract the RpfF thioesterase activity by catalysing uptake and activation of the free fatty acids to give acyl-CoAs that can be utilized to restore membrane lipid synthesis. Heterologous expression of a different fatty acid activating enzyme, the Vibrio harveyi acyl-ACP synthetase, replaced RpfB in counteracting the effects of high level RpfF thioesterase activity indicating that the essential role of RpfB is uptake and activation of free fatty acids.

Two different rpf clusters distributed among a population of Stenotrophomonas maltophilia clinical strains display differential diffusible signal factor production and virulence regulation

The quorum-sensing (QS) system present in the emerging nosocomial pathogen Stenotrophomonas maltophilia is based on the signaling molecule diffusible signal factor (DSF). Production and detection of DSF are governed by the rpf cluster, which encodes the synthase RpfF and the sensor RpfC, among other components. Despite a well-studied system, little is known about its implication in virulence regulation in S. maltophilia. Here, we have analyzed the rpfF gene from 82 S. maltophilia clinical isolates. Although rpfF was found to be present in all of the strains, it showed substantial variation, with two populations (rpfF-1 and rpfF-2) clearly distinguishable by the N-terminal region of the protein. Analysis of rpfC in seven complete genome sequences revealed a corresponding variability in the N-terminal transmembrane domain of its product, suggesting that each RpfF variant has an associated RpfC variant. We show that only RpfC-RpfF-1 variant strains display detectable DSF production. Heterologous rpfF complementation of ΔrpfF mutants of a representative strain of each variant suggests that RpfF-2 is, however, functional and that the observed DSF-deficient phenotype of RpfC-RpfF-2 variant strains is due to permanent repression of RpfF-2 by RpfC-2. This is corroborated by the ΔrpfC mutant of the RpfC-RpfF-2 representative strain. In line with this observations, deletion of rpfF from the RpfC-RpfF-1 strain leads to an increase in biofilm formation, a decrease in swarming motility, and relative attenuation in the Caenorhabditis elegans and zebrafish infection models, whereas deletion of the same gene from the representative RpfC-RpfF-2 strain has no significant effect on these virulence-related phenotypes.

The putative enoyl-coenzyme A hydratase DspI is required for production of the Pseudomonas aeruginosa biofilm dispersion autoinducer cis-2-decenoic acid

In the present study, we report the identification of a putative enoyl-coenzyme A (CoA) hydratase/isomerase that is required for synthesis of the biofilm dispersion autoinducer cis-2-decenoic acid in the human pathogen Pseudomonas aeruginosa. The protein is encoded by PA14_54640 (PA0745), named dspI for dispersion inducer. The gene sequence for this protein shows significant homology to RpfF in Xanthomonas campestris. Inactivation of dspI was shown to abolish biofilm dispersion autoinduction in continuous cultures of P. aeruginosa and resulted in biofilms that were significantly greater in thickness and biomass than those of the parental wild-type strain. Dispersion was shown to be inducible in dspI mutants by the exogenous addition of synthetic cis-2-decenoic acid or by complementation of ΔdspI in trans under the control of an arabinose-inducible promoter. Mutation of dspI was also shown to abolish cis-2-decenoic acid production, as revealed by gas chromatography-mass spectrometry (GC-MS) analysis of cell-free spent culture medium. The transcript abundance of dspI correlated with cell density, as determined by quantitative reverse transcriptase (RT) PCR. This regulation is consistent with the characterization of cis-2-decenoic acid as a cell-to-cell communication molecule that regulates biofilm dispersion in a cell density-dependent manner.

Stenotrophomonas maltophilia: an emerging global opportunistic pathogen

Stenotrophomonas maltophilia is an emerging multidrug-resistant global opportunistic pathogen. The increasing incidence of nosocomial and community-acquired S. maltophilia infections is of particular concern for immunocompromised individuals, as this bacterial pathogen is associated with a significant fatality/case ratio. S. maltophilia is an environmental bacterium found in aqueous habitats, including plant rhizospheres, animals, foods, and water sources. Infections of S. maltophilia can occur in a range of organs and tissues; the organism is commonly found in respiratory tract infections. This review summarizes the current literature and presents S. maltophilia as an organism with various molecular mechanisms used for colonization and infection. S. maltophilia can be recovered from polymicrobial infections, most notably from the respiratory tract of cystic fibrosis patients, as a cocolonizer with Pseudomonas aeruginosa. Recent evidence of cell-cell communication between these pathogens has implications for the development of novel pharmacological therapies. Animal models of S. maltophilia infection have provided useful information about the type of host immune response induced by this opportunistic pathogen. Current and emerging treatments for patients infected with S. maltophilia are discussed.

The surfactant of Legionella pneumophila Is secreted in a TolC-dependent manner and is antagonistic toward other Legionella species

When Legionella pneumophila grows on agar plates, it secretes a surfactant that promotes flagellum- and pilus-independent "sliding" motility. We isolated three mutants that were defective for surfactant. The first two had mutations in genes predicted to encode cytoplasmic enzymes involved in lipid metabolism. These genes mapped to two adjacent operons that we designated bbcABCDEF and bbcGHIJK. Backcrossing and complementation confirmed the importance of the bbc genes and suggested that the Legionella surfactant is lipid containing. The third mutant had an insertion in tolC. TolC is the outer membrane part of various trimolecular complexes involved in multidrug efflux and type I protein secretion. Complementation of the tolC mutant restored sliding motility. Mutants defective for an inner membrane partner of TolC also lacked a surfactant, confirming that TolC promotes surfactant secretion. L. pneumophila (lspF) mutants lacking type II protein secretion (T2S) are also impaired for a surfactant. When the tolC and lspF mutants were grown next to each other, the lsp mutant secreted surfactant, suggesting that TolC and T2S conjoin to mediate surfactant secretion, with one being the conduit for surfactant export and the other the exporter of a molecule that is required for induction or maturation of surfactant synthesis/secretion. Although the surfactant was not required for the extracellular growth, intracellular infection, and intrapulmonary survival of L. pneumophila, it exhibited antimicrobial activity toward seven other species of Legionella but not toward various non-Legionella species. These data suggest that the surfactant provides L. pneumophila with a selective advantage over other legionellae in the natural environment.

Rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae produces multiple DSF-family signals in regulation of virulence factor production

Background

Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of rice bacterial blight disease. Xoo produces a range of virulence factors, including EPS, extracellular enzyme, iron-chelating siderophores, and type III-secretion dependent effectors, which are collectively essential for virulence. Genetic and genomics evidence suggest that Xoo might use the diffusible signal factor (DSF) type quorum sensing (QS) system to regulate the virulence factor production. However, little is known about the chemical structure of the DSF-like signal(s) produced by Xoo and the factors influencing the signal production.

Results

Xoo genome harbours an rpf cluster comprising rpfB, rpfF, rpfC and rpfG. The proteins encoded by these genes are highly homologous to their counterparts in X. campestris pv. campestris (Xcc), suggesting that Xcc and Xoo might use similar mechanisms for DSF biosynthesis and autoregulation. Consistent with in silico analysis, the rpfF mutant was DSF-deficient and the rpfC mutant produced about 25 times higher DSF-like activity than the wild type Xoo strain KACC10331. From the supernatants of rpfC mutant, we purified three compounds showing strong DSF-like activity. Mass spectrometry and NMR analysis revealed that two of them were the previously characterized DSF and BDSF; the third one was a novel unsaturated fatty acid with 2 double bonds and was designated as CDSF in this study. Further analysis showed that all the three DSF-family signals were synthesized via the enzyme RpfF encoded by Xoo2868. DSF and BDSF at a final concentration of 3 μM to the rpfF mutant could fully restore its extracellular xylanase activity and EPS production to the wild type level, but CDSF was less active than DSF and BDSF in induction of EPS and xylanase. DSF and CDSF shared a similar cell density-dependent production time course with the maximum production being detected at 42 h after inoculation, whereas the maximum production of BDSF was observed at 36 h after inoculation. When grown in a rich medium such as YEB, LB, PSA, and NYG, Xoo produced all the three signals with the majority being DSF. Whereas in nutritionally poor XOLN medium Xoo only produced BDSF and DSF but the majority was BDSF.

Conclusions

This study demonstrates that Xoo and Xcc share the conserved mechanisms for DSF biosynthesis and autoregulation. Xoo produces DSF, BDSF and CDSF signals in rich media and CDSF is a novel signal in DSF-family with two double bonds. All the three DSF-family signals promote EPS production and xylanase activity in Xoo, but CDSF is less active than its analogues DSF and BDSF. The composition and ratio of the three DSF-family signals produced by Xoo are influenced by the composition of culture media.

Structural and functional characterization of diffusible signal factor family quorum-sensing signals produced by members of the Burkholderia cepacia complex

Previous work has shown that Burkholderia cenocepacia produces the diffusible signal factor (DSF) family signal cis-2-dodecenoic acid (C(12):Delta(2), also known as BDSF), which is involved in the regulation of virulence. In this study, we determined whether C(12):Delta(2) production is conserved in other members of the Burkholderia cepacia complex (Bcc) by using a combination of high-performance liquid chromatography, mass spectrometry, and bioassays. Our results show that five Bcc species are capable of producing C(12):Delta(2) as a sole DSF family signal, while four species produce not only C(12):Delta(2) but also a new DSF family signal, which was identified as cis,cis-11-methyldodeca-2,5-dienoic acid (11-Me-C(12):Delta(2,5)). In addition, we demonstrate that the quorum-sensing signal cis-11-methyl-2-dodecenoic acid (11-Me-C(12):Delta(2)), which was originally identified in Xanthomonas campestris supernatants, is produced by Burkholderia multivorans. It is shown that, similar to 11-Me-C(12):Delta(2) and C(12):Delta(2), the newly identified molecule 11-Me-C(12):Delta(2,5) is a potent signal in the regulation of biofilm formation, the production of virulence factors, and the morphological transition of Candida albicans. These data provide evidence that DSF family molecules are highly conserved bacterial cell-cell communication signals that play key roles in the ecology of the organisms that produce them.

Combining Biofilm-Controlling Compounds and Antibiotics as a Promising New Way to Control Biofilm Infections

Many bacteria grow on surfaces forming biofilms. In this structure, they are well protected and often high dosages of antibiotics cannot clear infectious biofilms. The formation and stabilization of biofilms are mediated by diffusible autoinducers (e.g. N-acyl homoserine lactones, small peptides, furanosyl borate diester). Metabolites interfering with this process have been identified in plants, animals and microbes, and synthetic analogues are known. Additionally, this seems to be not the only way to control biofilms. Enzymes capable of cleaving essential components of the biofilm matrix, e.g. polysaccharides or extracellular DNA, and thus weakening the biofilm architecture have been identified. Bacteria also have mechanisms to dissolve their biofilms and return to planktonic lifestyle. Only a few compounds responsible for the signalling of these processes are known, but they may open a completely novel line of biofilm control. All these approaches lead to the destruction of the biofilm but not the killing of the pathogens. Therefore, a combination of biofilm-destroying compounds and antibiotics to handle biofilm infections is proposed. In this article, different approaches to combine biofilm-controlling compounds and antibiotics to fight biofilm infections are discussed, as well as the balance between biofilm formation and virulence.

Surface translocation by Legionella pneumophila: a form of sliding motility that is dependent upon type II protein secretion

Legionella pneumophila exhibits surface translocation when it is grown on a buffered charcoal yeast extract (BCYE) containing 0.5 to 1.0% agar. After 7 to 22 days of incubation, spreading legionellae appear in an amorphous, lobed pattern that is most manifest at 25 to 30 degrees C. All nine L. pneumophila strains examined displayed the phenotype. Surface translocation was also exhibited by some, but not all, other Legionella species examined. L. pneumophila mutants that were lacking flagella and/or type IV pili behaved as the wild type did when plated on low-percentage agar, indicating that the surface translocation is not swarming or twitching motility. A translucent film was visible atop the BCYE agar, advancing ahead of the spreading legionellae. Based on its abilities to disperse water droplets and to promote the spreading of heterologous bacteria, the film appeared to manipulate surface tension and, as such, acted like a surfactant. Indeed, a sample obtained from the film rapidly dispersed when it was spotted onto a plastic surface. L. pneumophila type II secretion (Lsp) mutants, but not their complemented derivatives, were defective for both surface translocation and film production. In contrast, mutants defective for type IV secretion exhibited normal surface translocation. When lsp mutants were spotted onto film produced by the wild type, they were able to spread, suggesting that type II secretion promotes the elaboration of the Legionella surfactant. Together, these data indicate that L. pneumophila exhibits a form of surface translocation that is most akin to "sliding motility" and uniquely dependent upon type II secretion.

Diffusible signals and interspecies communication in bacteria

Many bacteria use cell-cell communication mediated by diffusible signal molecules to monitor their population density or confinement to niches and to modulate their behaviour in response to these aspects of their environment. Work on signalling systems within individual species has formed a platform for studies of interspecies interactions that can occur within polymicrobial communities in nature. In addition to signalling between organisms that synthesize the same or related signal molecules, it is becoming evident that bacteria can sense signal molecules that they do not synthesize, thereby eavesdropping on signalling by other organisms in their immediate environment. Furthermore, molecules such as antibiotics that are considered not to be signals for the producing species can have effects on gene expression in other bacteria that indicate a signalling function. Interspecies signalling can lead to alteration in factors contributing to the virulence or persistence of bacterial pathogens as well as influencing the development of beneficial microbial communities. Here we review our current understanding of interspecies signalling in bacteria and the signals involved, what is known of the underlying signal transduction mechanisms and their influences on bacterial behaviour.

Quorum sensing and virulence regulation in Xanthomonas campestris

It is now clear that cell-cell communication, often referred to as quorum sensing (QS), is the norm in the prokaryotic kingdom and this community-wide genetic regulatory mechanism has been adopted for regulation of many important biological functions. Since the 1980s, several types of QS signals have been identified, which are associated commonly with different types of QS mechanisms. Among them, the diffusible signal factor (DSF)-dependent QS system, originally discovered from bacterial pathogen Xanthomonas campestris pv. campestris, is a relatively new regulatory mechanism. The rapid research progress over the last few years has identified the chemical structure of the QS signal DSF, established the DSF regulon, and unveiled the general signaling pathways and mechanisms. Particular noteworthy are that DSF biosynthesis is modulated by a novel posttranslational autoinduction mechanism involving protein-protein interaction between the DSF synthase RpfF and the sensor RpfC, and that QS signal sensing is coupled to intracellular regulatory networks through a second messenger cyclic-di-GMP and a global regulator Clp. Genomic and genetic analyses show that the DSF QS-signaling pathway regulates diverse biological functions including virulence, biofilm dispersal, and ecological competence. Moreover, evidence is emerging that the DSF QS system is conserved in a range of plant and human bacterial pathogens.