Elongation factor P modulates Acinetobacter baumannii physiology and virulence as a cyclic dimeric guanosine monophosphate effector.

Cyclic diguanosine monophosphate (c-di-GMP) is widely used by bacteria to control biological functions in response to diverse signals or cues. A previous study showed that potential c-di-GMP metabolic enzymes play a role in the regulation of biofilm formation and motility in Acinetobacter baumannii. However, it was unclear whether and how A. baumannii cells use c-di-GMP signaling to modulate biological functions. Here, we report that c-di-GMP is an important intracellular signal in the modulation of biofilm formation, motility, and virulence in A. baumannii. The intracellular level of c-di-GMP is principally controlled by the diguanylate cyclases (DGCs) A1S_1695, A1S_2506, and A1S_3296 and the phosphodiesterase (PDE) A1S_1254. Intriguingly, we revealed that A1S_2419 (an elongation factor P [EF-P]), is a novel c-di-GMP effector in A. baumannii. Response to a c-di-GMP signal boosted A1S_2419 activity to rescue ribosomes from stalling during synthesis of proteins containing consecutive prolines and thus regulate A. baumannii physiology and pathogenesis. Our study presents a unique and widely conserved effector that controls bacterial physiology and virulence by sensing the second messenger c-di-GMP.

An anthranilic acid-responsive transcriptional regulator controls the physiology and pathogenicity of Ralstonia solanacearum.

Quorum sensing (QS) is widely employed by bacterial cells to control gene expression in a cell density-dependent manner. A previous study revealed that anthranilic acid from Ralstonia solanacearum plays a vital role in regulating the physiology and pathogenicity of R. solanacearum. We reported here that anthranilic acid controls the important biological functions and virulence of R. solanacearum through the receptor protein RaaR, which contains helix-turn-helix (HTH) and LysR substrate binding (LysR_substrate) domains. RaaR regulates the same processes as anthranilic acid, and both are present in various bacterial species. In addition, anthranilic acid-deficient mutant phenotypes were rescued by in trans expression of RaaR. Intriguingly, we found that anthranilic acid binds to the LysR_substrate domain of RaaR with high affinity, induces allosteric conformational changes, and then enhances the binding of RaaR to the promoter DNA regions of target genes. These findings indicate that the components of the anthranilic acid signaling system are distinguished from those of the typical QS systems. Together, our work presents a unique and widely conserved signaling system that might be an important new type of cell-to-cell communication system in bacteria.

Membrane-enclosed Pseudomonas quinolone signal attenuates bacterial virulence by interfering with quorum sensing.

Outer membrane vesicle (OMV)-delivered Pseudomonas quinolone signal (PQS) plays a critical role in cell-cell communication in Pseudomonas aeruginosa. However, the functions and mechanisms of membrane-enclosed PQS in interspecies communication in microbial communities are not clear. Here, we demonstrate that PQS delivered by both OMVs from P. aeruginosa and liposome reduces the competitiveness of Burkholderia cenocepacia, which usually shares the same niche in the lungs of cystic fibrosis patients, by interfering with quorum sensing (QS) in B. cenocepacia through the LysR-type regulator ShvR. Intriguingly, we found that ShvR regulates the production of the QS signals cis-2-dodecenoic acid (BDSF) and N-acyl homoserine lactone (AHL) by directly binding to the promoters of signal synthase-encoding genes. Perception of PQS influences the regulatory activity of ShvR and thus ultimately reduces QS signal production and virulence in B. cenocepacia. Our findings provide insights into the interspecies communication mediated by the membrane-enclosed QS signal among bacterial species residing in the same microbial community.IMPORTANCEQuorum sensing (QS) is a ubiquitous cell-to-cell communication mechanism. Previous studies showed that Burkholderia cenocepacia mainly employs cis-2-dodecenoic acid (BDSF) and N-acyl homoserine lactone (AHL) QS systems to regulate biological functions and virulence. Here, we demonstrate that Pseudomonas quinolone signal (PQS) delivered by outer membrane vesicles from Pseudomonas aeruginosa or liposome attenuates B. cenocepacia virulence by targeting the LysR-type regulator ShvR, which regulates the production of the QS signals BDSF and AHL in B. cenocepacia. Our results not only suggest the important roles of membrane-enclosed PQS in interspecies and interkingdom communications but also provide a new perspective on the use of functional nanocarriers loaded with QS inhibitors for treating pathogen infections.

Anti-virulence and bactericidal activities of Stattic against Shigella sonnei.

IMPORTANCE: Shigella sonnei is a major human enteric pathogen that causes bacillary dysentery. The increasing spread of drug-resistant S. sonnei strains has caused an emergent need for the development of new antimicrobial agents against this pathogenic bacterium. In this study, we demonstrate that Stattic employs two antibacterial mechanisms against S. sonnei. It exerted both anti-virulence activity and bactericidal activity against S. sonnei, suggesting that it shows advantages over traditional antibiotics. Moreover, Stattic showed excellent synergistic effects with kanamycin, ampicillin, chloramphenicol, and gentamicin against S. sonnei. Our findings suggest that Stattic has promising potential for development as a new antibiotic or as an adjuvant to antibiotics for infections caused by S. sonnei.

A ProQ/FinO family protein involved in plasmid copy number control favours fitness of bacteria carrying mcr-1-bearing IncI2 plasmids.

The plasmid-encoded colistin resistance gene mcr-1 challenges the use of polymyxins and poses a threat to public health. Although IncI2-type plasmids are the most common vector for spreading the mcr-1 gene, the mechanisms by which these plasmids adapt to host bacteria and maintain resistance genes remain unclear. Herein, we investigated the regulatory mechanism for controlling the fitness cost of an IncI2 plasmid carrying mcr-1. A putative ProQ/FinO family protein encoded by the IncI2 plasmid, designated as PcnR (plasmid copy number repressor), balances the mcr-1 expression and bacteria fitness by repressing the plasmid copy number. It binds to the first stem-loop structure of the repR mRNA to repress RepA expression, which differs from any other previously reported plasmid replication control mechanism. Plasmid invasion experiments revealed that pcnR is essential for the persistence of the mcr-1-bearing IncI2 plasmid in the bacterial populations. Additionally, single-copy mcr-1 gene still exerted a fitness cost to host bacteria, and negatively affected the persistence of the IncI2 plasmid in competitive co-cultures. These findings demonstrate that maintaining mcr-1 plasmid at a single copy is essential for its persistence, and explain the significantly reduced prevalence of mcr-1 following the ban of colistin as a growth promoter in China.

The cis-2-Dodecenoic Acid (BDSF) Quorum Sensing System in Burkholderia cenocepacia.

It has been demonstrated that quorum sensing (QS) is widely employed by bacterial cells to coordinately regulate various group behaviors. Diffusible signal factor (DSF)-type signals have emerged as a growing family of conserved cell-cell communication signals. In addition to the DSF signal initially identified in Xanthomonas campestris pv. campestris, Burkholderiadiffusible signal factor (BDSF) (cis-2-dodecenoic acid) has been recognized as a conserved DSF-type signal with specific characteristics in both signal perception and transduction from DSF signals. Here, we review the history and current progress of the research on this type of signal, especially focusing on its biosynthesis, signaling pathways, and biological functions. We also discuss and explore the huge potential of targeting this kind of QS system as a new therapeutic strategy to control bacterial infections and diseases.

Orchestrated actin nucleation by the Candida albicans polarisome complex enables filamentous growth.

Candida albicans is a dimorphic fungus that converts from a yeast form to a hyphae form during infection. This switch requires the formation of actin cable to coordinate polarized cell growth. It's known that nucleation of this cable requires a multiprotein complex localized at the tip called the polarisome, but the mechanisms underpinning this process were unclear. Here, we found that C. albicans Aip5, a homolog of polarisome component ScAip5 in Saccharomyces cerevisiae that nucleates actin polymerization and synergizes with the formin ScBni1, regulates actin assembly and hyphae growth synergistically with other polarisome proteins Bni1, Bud6, and Spa2. The C terminus of Aip5 binds directly to G-actin, Bni1, and the C-terminal of Bud6, which form the core of the nucleation complex to polymerize F-actin. Based on insights from structural biology and molecular dynamic simulations, we propose a possible complex conformation of the actin nucleation core, which provides cooperative positioning and supports the synergistic actin nucleation activity of a tri-protein complex Bni1-Bud6-Aip5. Together with known interactions of Bni1 with Bud6 and Aip5 in S. cerevisiae, our findings unravel molecular mechanisms of C. albicans by which the tri-protein complex coordinates the actin nucleation in actin cable assembly and hyphal growth, which is likely a conserved mechanism in different filamentous fungi and yeast.

A LysR Family Transcriptional Regulator Modulates Burkholderia cenocepacia Biofilm Formation and Protease Production.

Quorum-sensing (QS) signals are widely employed by bacteria to regulate biological functions in response to cell densities. Previous studies showed that Burkholderia cenocepacia mostly utilizes two types of QS systems, including the N-acylhomoserine lactone (AHL) and cis-2-dodecenoic acid (BDSF) systems, to regulate biological functions. We demonstrated here that a LysR family transcriptional regulator, Bcal3178, controls the QS-regulated phenotypes, including biofilm formation and protease production, in B. cenocepacia H111. Expression of Bcal3178 at the transcriptional level was obviously downregulated in both the AHL-deficient and BDSF-deficient mutant strains compared to the wild-type H111 strain. It was further identified that Bcal3178 regulated target gene expression by directly binding to the promoter DNA regions. We also revealed that Bcal3178 was directly controlled by the AHL system regulator CepR. These results show that Bcal3178 is a new downstream component of the QS signaling network that modulates a subset of genes and functions coregulated by the AHL and BDSF QS systems in B. cenocepacia. IMPORTANCE Burkholderia cenocepacia is an important opportunistic pathogen in humans that utilizes the BDSF and AHL quorum-sensing (QS) systems to regulate biological functions and virulence. We demonstrated here that a new downstream regulator, Bcal3178 of the QS signaling network, controls biofilm formation and protease production. Bcal3178 is a LysR family transcriptional regulator modulated by both the BDSF and AHL QS systems. Furthermore, Bcal3178 controls many target genes, which are regulated by the QS systems in B. cenocepacia. Collectively, our findings depict a novel molecular mechanism with which QS systems regulate some target gene expression and biological functions by modulating the expression level of a LysR family transcriptional regulator in B. cenocepacia.

Burkholderia cenocepacia integrates cis-2-dodecenoic acid and cyclic dimeric guanosine monophosphate signals to control virulence.

Quorum sensing (QS) signals are used by bacteria to regulate biological functions in response to cell population densities. Cyclic diguanosine monophosphate (c-di-GMP) regulates cell functions in response to diverse environmental chemical and physical signals that bacteria perceive. In Burkholderia cenocepacia, the QS signal receptor RpfR degrades intracellular c-di-GMP when it senses the QS signal cis-2-dodecenoic acid, also called Burkholderia diffusible signal factor (BDSF), as a proxy for high cell density. However, it was unclear how this resulted in control of BDSF-regulated phenotypes. Here, we found that RpfR forms a complex with a regulator named GtrR (BCAL1536) to enhance its binding to target gene promoters under circumstances where the BDSF signal binds to RpfR to stimulate its c-di-GMP phosphodiesterase activity. In the absence of BDSF, c-di-GMP binds to the RpfR-GtrR complex and inhibits its ability to control gene expression. Mutations in rpfR and gtrR had overlapping effects on both the B. cenocepacia transcriptome and BDSF-regulated phenotypes, including motility, biofilm formation, and virulence. These results show that RpfR is a QS signal receptor that also functions as a c-di-GMP sensor. This protein thus allows B. cenocepacia to integrate information about its physical and chemical surroundings as well as its population density to control diverse biological functions including virulence. This type of QS system appears to be widely distributed in beta and gamma proteobacteria.

Xanthomonas campestris Promotes Diffusible Signal Factor Biosynthesis and Pathogenicity by Utilizing Glucose and Sucrose from Host Plants.

The plant pathogen Xanthomonas campestris pv. campestris produces diffusible signal factor (DSF) quorum sensing (QS) signals to regulate its biological functions and virulence. Our previous study showed that X. campestris pv. campestris utilizes host plant metabolites to enhance the biosynthesis of DSF family signals. However, it is unclear how X. campestris pv. campestris benefits from the metabolic products of the host plant. In this study, we observed that the host plant metabolites not only boosted the production of the DSF family signals but also modulated the expression levels of DSF-regulated genes in X. campestris pv. campestris. Infection with X. campestris pv. campestris induced changes in the expression of many sugar transporter genes in Arabidopsis thaliana. Exogenous addition of sucrose or glucose, which are the major products of photosynthesis in plants, enhanced DSF signal production and X. campestris pv. campestris pathogenicity in the Arabidopsis model. In addition, several sucrose hydrolase-encoding genes in X. campestris pv. campestris and sucrose invertase-encoding genes in the host plant were notably upregulated during the infection process. These enzymes hydrolyzed sucrose to glucose and fructose, and in trans expression of one of these enzymes, CINV1 of A. thaliana or XC_0805 of X. campestris pv. campestris, enhanced DSF signal biosynthesis in X. campestris pv. campestris in the presence of sucrose. Taken together, our findings demonstrate that X. campestris pv. campestris applies multiple strategies to utilize host plant sugars to enhance QS and pathogenicity.

A novel two-component system modulates quorum sensing and pathogenicity in Burkholderia cenocepacia.

Quorum sensing (QS) is widely utilized by bacterial pathogens to regulate biological functions and pathogenicity. Recent evidence has shown that QS is subject to regulatory cascades, especially two-component systems that often respond to environmental stimulation. At least two different types of QS systems regulate pathogenesis in Burkholderia cenocepacia. However, it remains unclear how this bacterial pathogen controls these QS systems. Here, we demonstrate a novel two-component system RqpSR (Regulating Quorum sensing and Pathogenicity), which plays an important role in modulating QS and pathogenesis in B. cenocepacia. We demonstrate strong protein-protein binding affinity between RqpS and RqpR. Mutations in rqpS and rqpR exerted overlapping effects on B. cenocepacia transcriptomes and phenotypes, including motility, biofilm formation and virulence. In trans expression of rqpR rescued the defective phenotypes in the rqpS mutant. RqpR controls target gene expression by direct binding to DNA promoters, including the cis-2-dodecenoic acid (BDSF) and N-acylhomoserine lactone (AHL) signal synthase gene promoters. These findings suggest that the RqpSR system strongly modulates physiology by forming a complicated hierarchy with QS systems. This type of two-component system appears to be widely distributed and coexists with the BDSF QS system in various bacterial species.

Inhibition of Yeast-to-Hypha Transition and Virulence of Candida albicans by 2-Alkylaminoquinoline Derivatives.

A rapid increase in Candida albicans infection and drug resistance has caused an emergent need for new clinical strategies against this fungal pathogen. In this study, we evaluated the inhibitory activity of a series of 2-alkylaminoquinoline derivatives against C. albicans isolates. A total of 28 compounds were assessed for their efficacy in inhibiting the yeast-to-hypha transition, which is considered one of the key virulence factors in C. albicans Several compounds showed strong activity to decrease the morphological transition and virulence of C. albicans cells. The two leading compounds, compound 1 (2-[piperidin-1-yl]quinolone) and compound 12 (6-methyl-2-[piperidin-1-yl]quinoline), remarkably attenuated C. albicans hyphal formation and cytotoxicity in a dose-dependent manner, but they showed no toxicity to either C. albicans cells or human cells. Intriguingly, compound 12 showed an excellent ability to inhibit C. albicans infection in the mouse oral mucosal infection model. This leading compound also interfered with the expression levels of hypha-specific genes in the cyclic AMP-protein kinase A and mitogen-activated protein kinase signaling pathways. Our findings suggest that 2-alkylaminoquinoline derivatives could potentially be developed as novel therapeutic agents against C. albicans infection due to their interference with the yeast-to-hypha transition.

Disruption of Quorum Sensing and Virulence in Burkholderia cenocepacia by a Structural Analogue of the cis-2-Dodecenoic Acid Signal.

Quorum sensing (QS) signals are widely used by bacterial pathogens to control biological functions and virulence in response to changes in cell population densities. Burkholderia cenocepacia employs a molecular mechanism in which the cis-2-dodecenoic acid (named Burkholderiadiffusible signal factor [BDSF]) QS system regulates N-acyl homoserine lactone (AHL) signal production and virulence by modulating intracellular levels of cyclic diguanosine monophosphate (c-di-GMP). Thus, inhibition of BDSF signaling may offer a non-antibiotic-based therapeutic strategy against BDSF-regulated bacterial infections. In this study, we report the synthesis of small-molecule mimics of the BDSF signal and evaluate their ability to inhibit BDSF QS signaling in B. cenocepacia A novel structural analogue of BDSF, 14-Me-C16:Δ2 (cis-14-methylpentadec-2-enoic acid), was observed to inhibit BDSF production and impair BDSF-regulated phenotypes in B. cenocepacia, including motility, biofilm formation, and virulence, while it did not inhibit the growth rate of this pathogen. 14-Me-C16:Δ2 also reduced AHL signal production. Genetic and biochemical analyses showed that 14-Me-C16:Δ2 inhibited the production of the BDSF and AHL signals by decreasing the expression of their synthase-encoding genes. Notably, 14-Me-C16:Δ2 attenuated BDSF-regulated phenotypes in various Burkholderia species. These findings suggest that 14-Me-C16:Δ2 could potentially be developed as a new therapeutic agent against pathogenic Burkholderia species by interfering with their QS signaling.IMPORTANCEBurkholderia cenocepacia is an important opportunistic pathogen which can cause life-threatening infections in susceptible individuals, particularly in cystic fibrosis and immunocompromised patients. It usually employs two types of quorum sensing (QS) systems, including the cis-2-dodecenoic acid (BDSF) system and N-acyl homoserine lactone (AHL) system, to regulate virulence. In this study, we have designed and identified an unsaturated fatty acid compound (cis-14-methylpentadec-2-enoic acid [14-Me-C16:Δ2]) that is capable of interfering with B. cenocepacia QS signaling and virulence. We demonstrate that 14-Me-C16:Δ2 reduced BDSF and AHL signal production in B. cenocepacia It also impaired QS-regulated phenotypes in various Burkholderia species. These results suggest that 14-Me-C16:Δ2 could interfere with QS signaling in many Burkholderia species and might be developed as a new antibacterial agent.

Efficacy of Compounds Isolated from Streptomyces olivaceus against the Morphogenesis and Virulence of Candida albicans.

Candida albicans is a type of commensal fungi which causes serious infections in immunocompromised patients and contributes to high mortality. In the present study, we identified that the extract from Streptomyces olivaceus SCSIO T05 inhibited hypha and biofilm formation of C. albicans. Seven compounds were isolated and evaluated for their effects on the biological functions and virulence of C. albicans. Two leading compounds, compound 1 (sorbicillin) and compound 2 (3-methyl-N-(2'-phenethyl)-butyrylamide) were identified as exhibiting strong activity against C. albicans morphological transition, adhesion activity, cytotoxicity, and adhesion to human cells, in a dose-dependent manner. Notably, compound 2 inhibited C. albicans infection in mouse oral mucosal models. Transcriptomic analysis and real-time PCR results revealed that compound 2 most likely inhibited the biological functions of C. albicans cells by regulating the expression levels of HWP1, TEC1, ALS1, IFD6, and CSH1, which are associated with filament formation and cell adhesion. Our results suggest that the candidate compounds present excellent efficacy against C. albicans pathogenicity and that they can be developed as potential options for the clinical treatment of candidiasis.

Identification of Cyclic Dipeptides from Escherichia coli as New Antimicrobial Agents against Ralstonia Solanacearum.

Ralstonia solanacearum is a causative agent of bacterial wilt in many important crops throughout the world. How to control bacterial wilt caused by R. solanacearum is a major problem in agriculture. In this study, we aim to isolate the biocontrol agents that have high efficacy in the control of bacterial wilt. Three new bacterial strains with high antimicrobial activity against R. solanacearum GMI1000 were isolated and identified. Our results demonstrated that these bacteria could remarkably inhibit the disease index of host plant infected by R. solanacearum. It was indicated that strain GZ-34 (CCTCC No. M 2016353) showed an excellent protective effect to tomato under greenhouse conditions. Strain GZ-34 was characterized as Escherichia coli based on morphology, biochemistry, and 16S rRNA analysis. We identified that the main antimicrobial compounds produced by E. coli GZ-34 were cyclo(l-Pro-d-Ile) and cyclo(l-Pro-l-Phe) using electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) analysis. The two active compounds also interfered with the expression levels of some pathogenicity-contributors of R. solanacearum. Furthermore, cyclo(l-Pro-l-Phe) effectively inhibited spore formation of Magnaporthe grisea, which is a vital pathogenesis process of the fungal pathogen, suggesting cyclic dipeptides from E. coli are promising potential antimicrobial agents with broad-spectrum activity to kill pathogens or interfere with their pathogenesis.

A Cyclic di-GMP-binding Adaptor Protein Interacts with Histidine Kinase to Regulate Two-component Signaling.

The bacterial messenger cyclic di-GMP (c-di-GMP) binds to a diverse range of effectors to exert its biological effect. Despite the fact that free-standing PilZ proteins are by far the most prevalent c-di-GMP effectors known to date, their physiological function and mechanism of action remain largely unknown. Here we report that the free-standing PilZ protein PA2799 from the opportunistic pathogen Pseudomonas aeruginosa interacts directly with the hybrid histidine kinase SagS. We show that PA2799 (named as HapZ: histidine kinase associated PilZ) binds directly to the phosphoreceiver (REC) domain of SagS, and that the SagS-HapZ interaction is further enhanced at elevated c-di-GMP concentration. We demonstrate that binding of HapZ to SagS inhibits the phosphotransfer between SagS and the downstream protein HptB in a c-di-GMP-dependent manner. In accordance with the role of SagS as a motile-sessile switch and biofilm growth factor, we show that HapZ impacts surface attachment and biofilm formation most likely by regulating the expression of a large number of genes. The observations suggest a previously unknown mechanism whereby c-di-GMP mediates two-component signaling through a PilZ adaptor protein.

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.

Cytoglobosins H and I, New Antiproliferative Cytochalasans from Deep-Sea-Derived Fungus Chaetomium globosum.

Cytoglobosins H (1) and I (2), together with seven known cytochalasan alkaloids (3-9), were isolated from the deep-sea-derived fungus Chaetomium globosum. The structures of new compounds 1 and 2 were elucidated by extensive 1D and 2D NMR and mass spectroscopic data. All the compounds were evaluated for their antiproliferative activities against MDA-MB-231 human breast cancer cells, LNCaP human prostate cancer cells, and B16F10 mouse melanoma cells. Compound 6 showed significant antiproliferative activity against LNCaP and B16F10 cell lines with IC50 values of 0.62 and 2.78 µM, respectively. Further testing confirmed that compound 6 inhibited the growth of LNCaP cells by inducing apoptosis.

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.

A cell-cell communication signal integrates quorum sensing and stress response.

Pseudomonas aeruginosa uses a hierarchical quorum sensing (QS) network consisting of las, pqs and rhl regulatory elements to coordinate the expression of bacterial virulence genes. However, clinical isolates frequently contain loss-of-function mutations in the central las system. This motivated us to search for a mechanism that may functionally substitute las. Here we report identification of a new QS signal, IQS. Disruption of IQS biosynthesis paralyzes the pqs and rhl QS systems and attenuates bacterial virulence. Production of IQS is tightly controlled by las under normal culture conditions but is also activated by phosphate limitation, a common stressor that bacteria encounter during infections. Thus, these results have established an integrated QS system that connects the central las system and phosphate-stress response mechanism to the downstream pqs and rhl regulatory systems. Our discovery highlights the complexity of QS signaling systems and extends the gamut of QS and stress-response mechanisms.