Identification of a quorum-sensing signal molecule in the facultative intracellular pathogen Brucella melitensis

Mycoplasma synoviae lipid-associated membrane proteins identification and expression changes when exposed to chicken cells

Mycoplasma synoviae is a significant cause of respiratory disease and synovitis among chickens, and has an adverse economic impact on broiler breeding efforts. The present study was designed to develop a systematic understanding of the role that M. synoviae lipid-associated membrane proteins (LAMPs) may play in the virulence of this pathogen. Bioinformatics tools were used to identify 146 predicted membrane proteins and lipoproteins in the M. synoviae proteome. Then, Triton X-114 was used to extract LAMPs that were subsequently identified via LC-MS/MS. This approach enabled the detection of potential LAMPs, and the top 200 most abundant proteins detected using this strategy were subject to further analysis. M. synoviae cells (100 MOI) were exposed to chicken fibroblasts (DF-1) and macrophages (HD-11) in a 1:1 mixed culture. Analysis of LAMP transcripts identified 72 up-regulated LAMP genes which were analyzed in depth by bioinformatics. GO analysis revealed these genes to be enriched in the nucleotide binding, sulfur amino acid transmembrane transporter activity, tRNA binding, rRNA modification, and transition metal ion transport pathways. Moreover, KEGG enrichment analysis suggested that these genes were enriched in the biosynthesis of secondary metabolites, carbon metabolism, glycolysis/gluconeogenesis, and nitrogen metabolism pathways.

Contribution of quorum sensing to virulence and antibiotic resistance in zoonotic bacteria

Quorum sensing (QS), which is a key part of cell/cell communication, is widely distributed in microorganisms, especially in bacteria. Bacteria can produce and detect the presence of QS signal molecule, perceive the composition and density of microorganisms in their complex habitat, and then dynamically regulate their own gene expression to adapt to their environment. Among the many traits controlled by QS in pathogenic bacteria is the expression of virulence factors and antibiotic resistance. Many pathogenic bacteria rely on QS to govern the production of virulence factors and express drug-resistance, especially in zoonotic bacteria. The threat of antibiotic resistant zoonotic bacteria has called for alternative antimicrobial strategies that would mitigate the increase of classical resistance mechanism. Targeting QS has proven to be a promising alternative to conventional antibiotic for controlling infections. Here we review the QS systems in common zoonotic pathogenic bacteria and outline how QS may control the virulence and antibiotic resistance of zoonotic bacteria.

Transcriptome Landscape of Intracellular Brucella ovis Surviving in RAW264.7 Macrophage Immune System

Brucella ovis infection results in genital damage and epididymitis in rams, placental inflammation and rare abortion in ewes, and neonatal mortality in lambs. However, the mechanism underlying B. ovis infection remains unclear. In the present study, we used prokaryotic transcriptome sequencing to identify the differentially expressed genes (DEGs) between wild-type B. ovis and intracellular B. ovis in RAW264.7 macrophages. Gene ontology (GO) term enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed, and quantitative reverse transcriptase PCR (qRT-PCR) was used to validate the top 10 upregulated and downregulated DEGs. The results showed that 212 genes were differentially expressed, including 68 upregulated and 144 downregulated genes, which were mainly enriched in 30 GO terms linked to biological process, cellular component, and molecular function. KEGG analysis showed that the DEGs were enriched in the hypoxia-inducible factor 1 (HIF-1) signaling pathway, mitogen-activated protein kinase (MAPK) signaling pathway, beta-alanine metabolism, and quorum sensing pathway. BME_RS01160, BME_RS04270, BME_RS08185, BME_RS12880, BME_RS25875, predicted_RNA865, and predicted_RNA953 were confirmed with the transcriptome sequencing data. Hence, our findings not only reveal the intracellular parasitism of B. ovis in the macrophage immune system, but also help to understand the mechanism of chronic B. ovis infection.

Regulatory Networks of the T4SS Control: From Host Cell Sensing to the Biogenesis and the Activity during the Infection

Delivery of effectors, DNA or proteins, that hijack host cell processes to the benefit of bacteria is a mechanism widely used by bacterial pathogens. It is achieved by complex effector injection devices, the secretion systems, among which Type 4 Secretion Systems (T4SSs) play a key role in bacterial virulence of numerous animal and plant pathogens. Considerable progress has recently been made in the structure-function analyses of T4SSs. Nevertheless, the signals and processes that trigger machine assembly and activity during infection, as well as those involved in substrate recognition and transfer, are complex and still poorly understood. In this review, we aim at summarizing the last updates of the knowledge on signaling pathways that regulate the biogenesis and the activity of T4SSs in important bacterial pathogens.

Uncovering the Hidden Credentials of Brucella Virulence

Bacteria in the genus Brucella are important human and veterinary pathogens. The abortion and infertility they cause in food animals produce economic hardships in areas where the disease has not been controlled, and human brucellosis is one of the world's most common zoonoses. Brucella strains have also been isolated from wildlife, but we know much less about the pathobiology and epidemiology of these infections than we do about brucellosis in domestic animals. The brucellae maintain predominantly an intracellular lifestyle in their mammalian hosts, and their ability to subvert the host immune response and survive and replicate in macrophages and placental trophoblasts underlies their success as pathogens. We are just beginning to understand how these bacteria evolved from a progenitor alphaproteobacterium with an environmental niche and diverged to become highly host-adapted and host-specific pathogens. Two important virulence determinants played critical roles in this evolution: (i) a type IV secretion system that secretes effector molecules into the host cell cytoplasm that direct the intracellular trafficking of the brucellae and modulate host immune responses and (ii) a lipopolysaccharide moiety which poorly stimulates host inflammatory responses. This review highlights what we presently know about how these and other virulence determinants contribute to Brucella pathogenesis. Gaining a better understanding of how the brucellae produce disease will provide us with information that can be used to design better strategies for preventing brucellosis in animals and for preventing and treating this disease in humans.

The Role of the Flagellar Protein FlgJ in the Virulence of Brucella abortus

Brucella abortus is a facultative intracellular pathogen that causes a zoonosis called brucellosis. This disease leads to abortion and infertility in cattle, and diverse complications in humans. B. abortus is a successful intracellular bacterium that has developed the ability to evade the host's immune system and it replicates in professional and non-professional phagocytic cells, persisting in the different tissues, and organs of its hosts. It has been described that Brucella expresses a polar flagellum under certain conditions, but its function is still unknown. In this study we evaluated the role of the FlgJ, a protein, presumably a peptidoglycan hydrolase involved in flagellum formation and in the virulence of B. abortus strain 2308. B. abortus 2308 ΔflgJ mutant and complemented strains were constructed to study the function of the FlgJ protein in the context of the virulence of this pathogen in in vitro and in vivo assays. The results showed that the elimination of the flgJ gene delays the growth rate of B. abortus in culture, reduces its intracellular survival capacity in professional and non-professional phagocytic cells, rendering it unable to escape from the endocytic route and not reaching the endoplasmic reticulum. It also negatively affects their persistence in BALB/c mice. Functionally, the B. abortus 2308 flgJ gene restored motility to an E. coli flgJ mutant gene. Furthermore, it was discovered that the production of FlgJ protein is associated with the bacterial adherence by B. abortus. Therefore, although the specific function of the polar flagellum for Brucella is unknown, the data indicates that the flagellar flgJ gene and its product are required for full virulence of B. abortus 2308, since its deletion significantly reduces the fitness of this pathogen in vitro and in vivo.

Development and evaluation of in murine model, of an improved live-vaccine candidate against brucellosis from to Brucella melitensis vjbR deletion mutant

Brucellosis is an infectious disease that brings enormous economic burdens for developing countries. The Brucella melitensis (B. melitensis) M5-90 vaccine strain (M5-90) has been used on a large scale in China, but may cause abortions if given to pregnant goats or sheep subcutaneously during the late stages of gestation. Moreover, the vaccine M5-90 cannot differentiate natural from vaccinated infection. Therefore, a safer and more potent M5-90 vaccine is required. In this study, a vjbR mutant of M5-90 (M5-90ΔvjbR) was constructed and overcame these drawbacks. M5-90ΔvjbR strain showed reduced survival capability in murine macrophages (RAW 264.7) and BALB/c mice and induced high protective immunity in mice. In addition, M5-90ΔvjbR induced an anti-Brucella-specific immunoglobulin G (IgG) response and stimulated the expression of gamma interferon (INF-γ) and interleukin-4 (IL-4) in vaccinated mice. Furthermore, M5-90ΔvjbR induced IgG response and stimulated the secretion of IFN-γ and IL-4 in immunized sheep. Moreover, the VjbR antigen allowed serological differentiation between infected and vaccinated animals. These results suggest that M5-90ΔvjbR is an ideal live attenuated and efficacious live vaccine candidate against B. melitensis 16 M infection.

Deciphering Physiological Functions of AHL Quorum Quenching Acylases

N-Acylhomoserine lactone (AHL)-acylase (also known as amidase or amidohydrolase) is a class of enzyme that belongs to the Ntn-hydrolase superfamily. As the name implies, AHL-acylases are capable of hydrolysing AHLs, the most studied signaling molecules for quorum sensing in Gram-negative bacteria. Enzymatic degradation of AHLs can be beneficial in attenuating bacterial virulence, which can be exploited as a novel approach to fight infection of human pathogens, phytopathogens or aquaculture-related contaminations. Numerous acylases from both prokaryotic and eukaryotic sources have been characterized and tested for the interference of quorum sensing-regulated functions. The existence of AHL-acylases in a multitude of organisms from various ecological niches, raises the question of what the physiological roles of AHL-acylases actually are. In this review, we attempt to bring together recent studies to extend our understanding of the biological functions of these enzymes in nature.

A functional and phylogenetic comparison of quorum sensing related genes in Brucella melitensis 16M

A quorum-sensing (QS) system is involved in Brucella melitensis survival inside the host cell. Two transcriptional regulators identified in B. melitensis, BlxR and VjbR, regulate the expression of virB, an operon required for bacterial intracellular persistence. In this work, 628 genes affected by VjbR and 124 by BlxR were analyzed to gain insights into their functional and taxonomical distributions among the Bacteria and Archaea cellular domains. In this regard, the Cluster of Orthologous Groups (COG) genes and orthologous genes in 789 nonredundant bacterial and archaeal genomes were obtained and compared against a group of randomly selected genes. From these analyses, we found 71 coaffected genes between VjbR and BlxR. In the COG comparison, VjbR activated genes associated with intracellular trafficking, secretion and vesicular transport and defense mechanisms, while BlxR affected genes related to energy production and conversion (with an equal effect) and translation, ribosomal structure and biogenesis, posttranslational modifications and carbohydrate and amino acid metabolism (with a negative effect). When the taxonomical distribution of orthologous genes was evaluated, the VjbR- and BlxR-related genes presented more orthologous genes in Crenarchaeota (Archaea), Firmicutes, and Tenericutes and fewer genes in Proteobacteria than expected by chance. These findings suggest that QS system exert a fine-tuning modulation of gene expression, by which VjbR activates genes related to infection persistence and defense, while BlxR represses general bacterial metabolism for intracellular adaptations. Finally, these affected genes present a degree of presence among Bacteria and Archaea genomes that is different from that expected by chance.

Quorum sensing and self-quorum quenching in the intracellular pathogen Brucellamelitensis

Brucella quorum sensing has been described as an important regulatory system controlling crucial virulence determinants such as the VirB type IV secretion system and the flagellar genes. However, the basis of quorum sensing, namely the production of autoinducers in Brucella has been questioned. Here, we report data obtained from the use of a genetic tool allowing the in situ detection of long-chain N-acyl-homoserine lactones (AHL) activity at single bacterium level in Brucella melitensis. These data are consistent with an intrinsic production of AHL by B. melitensis in low concentration both during in vitro growth and macrophage infection. Moreover, we identified a protein, named AibP, which is homologous to the AHL-acylases of various bacterial species. In vitro and during infection, expression of aibP coincided with a decrease in endogenous AHL activity within B. melitensis, suggesting that AibP could efficiently impair AHL accumulation. Furthermore, we showed that deletion of aibP in B. melitensis resulted in enhanced virB genes expression and VirB8 production as well as in a reduced flagellar genes expression and production of FlgE (hook protein) and FliC (flagellin) in vitro. Altogether, these results suggest that AHL-dependent quorum sensing and AHL-quorum quenching coexist in Brucella, at least to regulate its virulence.

Regulation of virulence in Brucella: an eclectic repertoire of transcription factors defines the complex architecture of the virB promoter

Many intracellular bacterial pathogens use type IV secretion systems to deliver effector molecules and subvert the eukaryotic host cell defenses. The genus Brucella comprises facultative intracellular bacteria that cause brucellosis, a disease affecting a wide range of mammals including humans. The virB operon codes for a type IV secretion system that plays a central role in intracellular survival and replication of Brucella within the host. Expression of the virB genes is under the control of various transcription factors that allow this system to respond to different types of environmental signals, and display binding site structures and arrangements that define the intrinsic complexity of the virB promoter. This review focuses on summarizing the current state of research concerning regulation of the Brucella virB operon, with special emphasis on describing the nature and function of the implicated regulatory elements and examining the involved protein–DNA interactions.

Impact of Hfq on global gene expression and intracellular survival in Brucella melitensis

Brucella melitensis is a facultative intracellular bacterium that replicates within macrophages. The ability of brucellae to survive and multiply in the hostile environment of host macrophages is essential to its virulence. The RNA-binding protein Hfq is a global regulator that is involved in stress resistance and pathogenicity. Here we demonstrate that Hfq is essential for stress adaptation and intracellular survival in B. melitensis. A B. melitensis hfq deletion mutant exhibits reduced survival under environmental stresses and is attenuated in cultured macrophages and mice. Microarray-based transcriptome analyses revealed that 359 genes involved in numerous cellular processes were dysregulated in the hfq mutant. From these same samples the proteins were also prepared for proteomic analysis to directly identify Hfq-regulated proteins. Fifty-five proteins with significantly affected expression were identified in the hfq mutant. Our results demonstrate that Hfq regulates many genes and/or proteins involved in metabolism, virulence, and stress responses, including those potentially involved in the adaptation of Brucella to the oxidative, acid, heat stress, and antibacterial peptides encountered within the host. The dysregulation of such genes and/or proteins could contribute to the attenuated hfq mutant phenotype. These findings highlight the involvement of Hfq as a key regulator of Brucella gene expression and facilitate our understanding of the role of Hfq in environmental stress adaptation and intracellular survival of B. melitensis.

The Aggregation of Brucella abortus Occurs Under Microaerobic Conditions and Promotes Desiccation Tolerance and Biofilm Formation

Brucella abortus causes brucellosis mainly in cattle. The infection is transmitted to humans by ingestion of animal products or direct contact with infected material. While the intracellular lifestyle of Brucella is well characterized, its extracellular survival is poorly understood. In nature, bacterial persistence is associated with biofilms, where aggregated cells are protected from adversity. The inability of Brucella abortus to aggregate under aerobiosis and that fact that the replicative niche of Brucella is characterized by microaerobic conditions prompted us to investigate the capacity of this pathogen to aggregate and grow in biofilms under microaerobiotic conditions. The results show that B. abortus aggregates and produces biofilms. The aggregates tolerate desiccation better than planktonic cells do, adhere and displace even in the absence of the lipopolysaccharide-O antigen, flagella, the transcriptional regulator VjbR, or the enzymes that synthesize, transport, and modify cyclic β (1,2) glucan.

Brucella T4SS: the VIP pass inside host cells

For many Gram-negative bacteria, like Brucella, the type IV secretion system (T4SS) has a critical role in bacterial virulence. In Brucella, the VirB T4SS permits the injection of bacterial effectors inside host cells, leading to subversion of signaling pathways and favoring bacterial growth and pathogenesis. The virB operon promoter is tightly regulated by a combination of transcriptional activators and repressors that are expressed according to the environmental conditions encountered by Brucella. Recent advances have shed light on the Brucella T4SS regulatory mechanisms and also its substrates. Characterization of the targets and functions of these translocated effectors is underway and will help understand the role of the T4SS in the establishment of a replication niche inside host cells.

Expression of VjbR under nutrient limitation conditions is regulated at the post-transcriptional level by specific acidic pH values and urocanic acid

VjbR is a LuxR homolog that regulates transcription of many genes including important virulence determinants of the facultative intracellular pathogen Brucella abortus. This transcription factor belongs to a family of regulators that participate in a cell-cell communication process called quorum sensing, which enables bacteria to respond to changes in cell population density by monitoring concentration of self produced autoinducer molecules. Unlike almost all other LuxR-type proteins, VjbR binds to DNA and activates transcription in the absence of any autoinducer signal. To investigate the mechanisms by which Brucella induces VjbR-mediated transcriptional activation, and to determine how inappropriate spatio-temporal expression of the VjbR target genes is prevented, we focused on the study of expression of vjbR itself. By assaying different parameters related to the intracellular lifestyle of Brucella, we identified a restricted set of conditions that triggers VjbR protein expression. Such conditions required the convergence of two signals of different nature: a specific pH value of 5.5 and the presence of urocanic acid, a metabolite involved in the connection between virulence and metabolism of Brucella. In addition, we also observed an urocanic acid, pH-dependent expression of RibH2 and VirB7, two additional intracellular survival-related proteins of Brucella. Analysis of promoter activities and determination of mRNA levels demonstrated that the urocanic acid-dependent mechanisms that induced expression of VjbR, RibH2, and VirB7 act at the post-transcriptional level. Taken together, our findings support a model whereby Brucella induces VjbR-mediated transcription by modulating expression of VjbR in response to specific signals related to the changing environment encountered within the host.

Brucellosis and type IV secretion

Brucellosis is a global disease of domestic and wild mammals that is caused by intracellular bacteria of the genus Brucella. Although humans are not a natural reservoir for Brucella, infection in the human population is common in many countries, and brucellosis is one of the most common zoonotic infections. Brucella species have evolved to avoid the host's immune system and infection is usually characterized by long-term persistence of the bacteria. One important Brucella virulence factor for intracellular survival and persistence in the host is the type IV secretion system. This review will discuss the Brucella type IV secretion system in detail, including current knowledge of architecture and regulation, as well as the newly identified effector substrates that this system transports into host cells.

Internal affairs: investigating the Brucella intracellular lifestyle

Bacteria of the genus Brucella are Gram-negative pathogens of several animal species that cause a zoonotic disease in humans known as brucellosis or Malta fever. Within their hosts, brucellae reside within different cell types where they establish a replicative niche and remain protected from the immune response. The aim of this article is to discuss recent advances in the field in the specific context of the Brucella intracellular 'lifestyle'. We initially discuss the different host cell targets and their relevance during infection. As it represents the key to intracellular replication, the focus is then set on the maturation of the Brucella phagosome, with particular emphasis on the Brucella factors that are directly implicated in intracellular trafficking and modulation of host cell signalling pathways. Recent data on the role of the type IV secretion system are discussed, novel effector molecules identified and how some of them impact on trafficking events. Current knowledge on Brucella gene regulation and control of host cell death are summarized, as they directly affect intracellular persistence. Understanding how Brucella molecules interplay with their host cell targets to modulate cellular functions and establish the intracellular niche will help unravel how this pathogen causes disease.

The RNA chaperone Hfq independently coordinates expression of the VirB type IV secretion system and the LuxR-type regulator BabR in Brucella abortus 2308

The type IV secretion system encoded by the virB operon is required for full virulence of Brucella sp., and the present study links the RNA chaperone Hfq to wild-type expression of virB in Brucella abortus 2308. Studies employing virB-lacZ fusions, quantitative reverse transcription-PCR, and immunoblot analysis showed that both transcription and translation of virB are decreased in an isogenic hfq mutant compared to those in the parental strain. These results led to the hypothesis that Hfq regulation of virB is mediated through an intermediate transcriptional regulator. Subsequent experiments determined that expression of the gene encoding the putative Brucella quorum-sensing regulator BabR (also known as BlxR), a known virB regulator, is also controlled by Hfq at the posttranscriptional level, and a cis-acting element in the 5' untranslated region of the babR transcript responsible for this regulation was identified. Consistent with its role as a virB regulator, recombinant Brucella BabR binds to the virB promoter region in electrophoretic mobility shift assays. However, experiments employing a babR mutant strain determined that BabR is a repressor, not an activator, of virB transcription. These findings suggest that Hfq regulates virB expression through both BabR-dependent and BabR-independent pathways.

Brucella adaptation and survival at the crossroad of metabolism and virulence

"In vivo" bacterial nutrition, i.e. the nature of the metabolic network and substrate(s) used by bacteria within their host, is a fundamental aspect of pathogenic or symbiotic lifestyles. A typical example are the Brucella spp., facultative intracellular pathogens responsible for chronic infections of animals and humans. Their virulence relies on their ability to modulate immune response and the physiology of host cells, but the fine-tuning of their metabolism in the host during infection appears increasingly crucial. Here we review new insights on the links between Brucella virulence and metabolism, pointing out the need to investigate both aspects to decipher Brucella infectious strategies.

Evolution of an endofungal lifestyle: Deductions from the Burkholderia rhizoxinica genome

BACKGROUND

Burkholderia rhizoxinica is an intracellular symbiont of the phytopathogenic zygomycete Rhizopus microsporus, the causative agent of rice seedling blight. The endosymbiont produces the antimitotic macrolide rhizoxin for its host. It is vertically transmitted within vegetative spores and is essential for spore formation of the fungus. To shed light on the evolution and genetic potential of this model organism, we analysed the whole genome of B. rhizoxinica HKI 0454 - a type strain of endofungal Burkholderia species.

RESULTS

The genome consists of a structurally conserved chromosome and two plasmids. Compared to free-living Burkholderia species, the genome is smaller in size and harbors less transcriptional regulator genes. Instead, we observed accumulation of transposons over the genome. Prediction of primary metabolic pathways and transporters suggests that endosymbionts consume host metabolites like citrate, but might deliver some amino acids and cofactors to the host. The rhizoxin biosynthesis gene cluster shows evolutionary traces of horizontal gene transfer. Furthermore, we analysed gene clusters coding for nonribosomal peptide synthetases (NRPS). Notably, B. rhizoxinica lacks common genes which are dedicated to quorum sensing systems, but is equipped with a large number of virulence-related factors and putative type III effectors.

CONCLUSIONS

B. rhizoxinica is the first endofungal bacterium, whose genome has been sequenced. Here, we present models of evolution, metabolism and tools for host-symbiont interaction of the endofungal bacterium deduced from whole genome analyses. Genome size and structure suggest that B. rhizoxinica is in an early phase of adaptation to the intracellular lifestyle (genome in transition). By analysis of tranporters and metabolic pathways we predict how metabolites might be exchanged between the symbiont and its host. Gene clusters for biosynthesis of secondary metabolites represent novel targets for genomic mining of cryptic natural products. In silico analyses of virulence-associated genes, secreted proteins and effectors might inspire future studies on molecular mechanisms underlying bacterial-fungal interaction.

The 16MΔvjbR as an ideal live attenuated vaccine candidate for differentiation between Brucella vaccination and infection

Brucellosis brings great economic burdens for developing countries. Live attenuated vaccines are the most efficient means for prevention and control of animal Brucellosis. However, the difficulties of differentiating of infection from vaccine immunization, which is essential for eradication programs, limit their applications. Therefore, the development of a vaccine that could differentiate infection from immunization will overcome the limitations and get extensive application. VjbR is a quorum sensing regulator involving in Brucella's intracellular survival. The vjbR∷Tn5 mutants have been proven effective against wild type strain challenge, implying its possibility of use in vaccine candidate development. To further evaluate this candidate gene, in the present study, the antigenicity of purified recombinant VjbR protein was analyzed. Antibodies to Brucella melitensis VjbR could be detected in sera from patients and animals with brucellosis but not in control ones, implying the potential use of this protein as a diagnostic antigen. Then a vjbR mutant of B. melitensis 16M was constructed by replacing the vjbR with kanamycin gene. The mutant showed reduced survival in macrophage and mice. Vaccination of BALB/c mice with 16MΔvjbR conferred significant protective immunity against B. melitensis strain 16M challenges, being equivalent to which induced by the license vaccine Rev.1. The vjbR deletion mutant elicited an anti-Brucella-specific immunoglobulin G response and induced the secretion of gamma interferon and interleukin-10. The most importance is that, the use of vjbR mutants as vaccines in association with diagnostic tests based on the VjbR antigen would allow the serological differentiation between infected and vaccinated animals. These results suggest that 16MΔvjbR is an ideal live attenuated vaccine candidate against B. melitensis and deserves further evaluation for vaccine development.

Global analysis of quorum sensing targets in the intracellular pathogen Brucella melitensis 16 M

Many pathogenic bacteria use a regulatory process termed quorum sensing (QS) to produce and detect small diffusible molecules to synchronize gene expression within a population. In Gram-negative bacteria, the detection of, and response to, these molecules depends on transcriptional regulators belonging to the LuxR family. Such a system has been discovered in the intracellular pathogen Brucella melitensis, a Gram-negative bacterium responsible for brucellosis, a worldwide zoonosis that remains a serious public health concern in countries were the disease is endemic. Genes encoding two LuxR-type regulators, VjbR and BabR, have been identified in the genome of B. melitensis 16 M. A ΔvjbR mutant is highly attenuated in all experimental models of infection tested, suggesting a crucial role for QS in the virulence of Brucella. At present, no function has been attributed to BabR. The experiments described in this report indicate that 5% of the genes in the B. melitensis 16 M genome are regulated by VjbR and/or BabR, suggesting that QS is a global regulatory system in this bacterium. The overlap between BabR and VjbR targets suggest a cross-talk between these two regulators. Our results also demonstrate that VjbR and BabR regulate many genes and/or proteins involved in stress response, metabolism, and virulence, including those potentially involved in the adaptation of Brucella to the oxidative, pH, and nutritional stresses encountered within the host. These findings highlight the involvement of QS as a major regulatory system in Brucella and lead us to suggest that this regulatory system could participate in the spatial and sequential adaptation of Brucella strains to the host environment.

Some pathogens use the regulatory process termed Quorum Sensing (QS) to synchronize gene expression within bacterial population. We report here the first genome scale study of the Quorum Sensing system of the intracellular pathogen Brucella melitensis. Our combined proteomic and transcriptomic data suggest that Quorum Sensing is involved in the spatial and sequential adaptation of B. melitensis to the host environment.

Biological properties and structure of the lipopolysaccharide of a vaccine strain of Francisella tularensis generated by inactivation of a quorum sensing system gene qseC

A knockout mutant with a deletion in a quorum sensing system gene qseC was generated from the vaccine strain Francisella tularensis 15 by site-directed mutagenesis. The variant with the inactivated gene qseC differed from the parental strain in growth rate on solid nutrient medium but had the same growth dynamics in liquid nutrient medium. The mutation abolished almost completely the resistance of the vaccine strain to normal rabbit serum and its ability to survive in macrophages; in addition, the strain lost the residual virulence. A significant phenotypic alteration was observed in the lipopolysaccharide of F. tularensis. Particularly, the mutant strain synthesized no noticeable amount of the lipopolysaccharide with the high-molecular-mass O-polysaccharide, presumably as a result of impairing biosynthesis of the repeating unit, namely, a loss of the ability to incorporate a formyl group, an N-acyl substituent of 4-amino-4,6-dideoxy-D-glucose.

Brucella melitensis VjbR and C12-HSL regulons: contributions of the N-dodecanoyl homoserine lactone signaling molecule and LuxR homologue VjbR to gene expression

Background

Quorum sensing is a communication system that regulates gene expression in response to population density and often regulates virulence determinants. Deletion of the luxR homologue vjbR highly attenuates intracellular survival of Brucella melitensis and has been interpreted to be an indication of a role for QS in Brucella infection. Confirmation for such a role was suggested, but not confirmed, by the demonstrated in vitro synthesis of an auto-inducer (AI) by Brucella cultures. In an effort to further delineate the role of VjbR to virulence and survival, gene expression under the control of VjbR and AI was characterized using microarray analysis.

Results

Analyses of wildtype B. melitensis and isogenic ΔvjbR transciptomes, grown in the presence and absence of exogenous N-dodecanoyl homoserine lactone (C₁₂-HSL), revealed a temporal pattern of gene regulation with variances detected at exponential and stationary growth phases. Comparison of VjbR and C₁₂-HSL transcriptomes indicated the shared regulation of 127 genes with all but 3 genes inversely regulated, suggesting that C₁₂-HSL functions via VjbR in this case to reverse gene expression at these loci. Additional analysis using a ΔvjbR mutant revealed that AHL also altered gene expression in the absence of VjbR, up-regulating expression of 48 genes and a luxR homologue blxR 93-fold at stationary growth phase. Gene expression alterations include previously un-described adhesins, proteases, antibiotic and toxin resistance genes, stress survival aids, transporters, membrane biogenesis genes, amino acid metabolism and transport, transcriptional regulators, energy production genes, and the previously reported fliF and virB operons.

Conclusions

VjbR and C₁₂-HSL regulate expression of a large and diverse number of genes. Many genes identified as virulence factors in other bacterial pathogens were found to be differently expressed, suggesting an important contribution to intracellular survival of Brucella. From these data, we conclude that VjbR and C₁₂-HSL contribute to virulence and survival by regulating expression of virulence mechanisms and thus controlling the ability of the bacteria to survive within the host cell. A likely scenario occurs via QS, however, operation of such a mechanism remains to be demonstrated.

Brucella melitensis 16M produces a mannan and other extracellular matrix components typical of a biofilm

Mutations in the Brucella melitensis quorum-sensing (QS) system are involved in the formation of clumps containing an exopolysaccharide. Here, we show that the overexpression of a gene called aiiD in B. melitensis gives rise to a similar clumping phenotype. The AiiD enzyme degrades AHL molecules and leads therefore to a QS-deficient strain. We demonstrated the presence of exopolysaccharide and DNA, two classical components of extracellular matrices, in clumps produced by this strain. We also observed that the production of outer membrane vesicles is strongly increased in the aiiD-overexpressing strain. Moreover, this strain allowed us to purify the exopolysaccharide and to obtain its composition and the first structural information on the complex exopolysaccharide produced by B. melitensis 16M, which was found to have a molecular weight of about 16 kDa and to be composed of glucosamine, glucose and mostly mannose. In addition, we found the presence of 2- and/or 6-substituted mannosyl residues, which provide the first insights into the linkages involved in this polymer. We used a classical biofilm attachment assay and an HeLa cell infection model to demonstrate that the clumping strain is more adherent to polystyrene plates and to HeLa cell surfaces than the wild-type one. Taken together, these data reinforce the evidence that B. melitensis could form biofilms in its lifecycle.

Identification of the quorum-sensing target DNA sequence and N-Acyl homoserine lactone responsiveness of the Brucella abortus virB promoter

VjbR is a LuxR-type quorum-sensing (QS) regulator that plays an essential role in the virulence of the intracellular facultative pathogen Brucella, the causative agent of brucellosis. It was previously described that VjbR regulates a diverse group of genes, including the virB operon. The latter codes for a type IV secretion system (T4SS) that is central for the pathogenesis of Brucella. Although the regulatory role of VjbR on the virB promoter (P(virB)) was extensively studied by different groups, the VjbR-binding site had not been identified so far. Here, we identified the target DNA sequence of VjbR in P(virB) by DNase I footprinting analyses. Surprisingly, we observed that VjbR specifically recognizes a sequence that is identical to a half-binding site of the QS-related regulator MrtR of Mesorhizobium tianshanense. As shown by DNase I footprinting and electrophoretic mobility shift assays, generation of a palindromic MrtR-like-binding site in P(virB) increased both the affinity and the stability of the VjbR-DNA complex, which confirmed that the QS regulator of Brucella is highly related to that of M. tianshanense. The addition of N-dodecanoyl homoserine lactone dissociated VjbR from the promoter, which confirmed previous reports that indicated a negative effect of this signal on the VjbR-mediated activation of P(virB). Our results provide new molecular evidence for the structure of the virB promoter and reveal unusual features of the QS target DNA sequence of the main regulator of virulence in Brucella.

Quorum sensing and bacterial biofilms

This review describes the chemistry of the bacterial biofilms including the chemistry of their constituents and signalling compounds that mediate or inhibit the formation of biofilms. Systems are described with special emphasis, in which quorum sensing molecules (autoinducers) trigger the formation of biofilms. In the first instance, N-acyl-L-homoserine lactones (AHLs) are the focus of this review, whereas the inter-species signal known as furanosyl borate diester and peptide autoinducers used by Gram-positive bacteria are not discussed in detail. Since the first discovery of an AHL autoinducer from Vibrio fischeri a large and further increasing number of different AHL structures from Gram-negative bacteria have been identified. This review gives a summary of all known AHL autoinducers and producing bacterial species. A few systems are discussed, where biofilm formation is suppressed by enzymatic degradation of AHL molecules or interference of secondary metabolites from other species with the quorum sensing systems of communicating bacteria. Finally, the multi-channel quorum sensing system, the intracellular downstream processing of the signal, and the resulting response of whole populations including biofilm formation are discussed for the Vibrio genus that has been extensively investigated.

Cytotoxicity of Brucella smooth strains for macrophages is mediated by increased secretion of the type IV secretion system

Some Brucella rough mutants cause cytotoxicity that resembles oncosis and necrosis in macrophages. This cytotoxicity requires the type IV secretion system (T4SS). In rough mutants, the cell-surface O antigen is shortened and the T4SS structure is thus exposed on the surface. Cytotoxicity effector proteins can therefore be more easily secreted. This enhanced secretion of effector proteins might cause the increased levels of cytotoxicity observed. However, whether this cytotoxicity is unique to the rough mutant and is mediated by overexpression of the T4SS has not been definitively determined. To test this, in the present study, a virB inactivation mutant (BMΔvirB) and an overexpression strain (BM-VIR) of a smooth Brucella melitensis strain (BM) were constructed and their cytotoxicity for macrophages and intracellular survival capability were analysed and compared. Cytotoxicity was detected in macrophages infected with higher concentrations of strains BM or BM-VIR, but not in those infected with BMΔvirB. The quorum sensing signal molecule N-dodecanoyl-dl-homoserine lactone (C12-HSL), a molecule that can inhibit expression of virB, inhibited the cytotoxicity of BM and BM-VIR, but not of BMΔvirB. These results indicated that overexpression of virB is responsible for Brucella cytotoxicity in macrophages. Transcription analysis showed that virB is regulated in a cell-density-dependent manner both in in vitro culture and during macrophage infection. When compared with BM, BM-VIR showed a reduced survival capacity in macrophages and mice, but both strains demonstrated similar resistance to in vitro stress conditions designed to simulate intracellular environments. Taken together, the cytotoxicity of Brucella for macrophages is probably mediated by increased secretion of effector proteins that results from overexpression of virB or an increase in the number of bacterial cells. The observation that both inactivation and overexpression of virB are detrimental for Brucella intracellular survival also indicated that the expression of virB is tightly regulated in a cell-density-dependent manner.

Survival of the fittest: how Brucella strains adapt to their intracellular niche in the host

Brucella strains produce abortion and infertility in their natural hosts and a zoonotic disease in humans known as undulant fever. These bacteria do not produce classical virulence factors, and their capacity to successfully survive and replicate within a variety of host cells underlies their pathogenicity. Extensive replication of the brucellae in placental trophoblasts is associated with reproductive tract pathology in natural hosts, and prolonged persistence in macrophages leads to the chronic infections that are a hallmark of brucellosis in both natural hosts and humans. This review describes how Brucella strains have efficiently adapted to their intracellular lifestyle in the host.

Brucella regulators: self-control in a hostile environment

Brucella is an important zoonotic pathogen for which no human vaccine exists. In an infected host, Brucella resides in macrophages but must coordinate expression of multiple virulence factors for successful cell entry and trafficking to acquire this replicative niche. Brucella responds to environmental signals to regulate virulence strategies that circumvent or blunt the host immune response. The Brucella quorum sensing system is a nexus of control for several Brucella virulence factors including flagellar genes and the type IV secretion system. Other sensory transduction systems, such as BvrRS and the newly described LOV-HK, sense environmental factors to control virulence. Here, we examine the contributions of various regulatory systems to Brucella virulence.

Intracellular adaptation of Brucella abortus

Macrophages were infected with virulent Brucella abortus strain 2308 or attenuated strain 19. Intracellular bacteria were recovered at different times after infection and their proteomes compared. The virulent strain initially reduced most biosynthesis and altered its respiration; adaptations reversed later in infection. The attenuated strain was unable to match the magnitude of the virulent strain's adjustments. The results provide insight into mechanisms utilized by Brucella to establish intracellular infections.

Comparative proteomics analyses reveal the virB of B. melitensis affects expression of intracellular survival related proteins

BACKGROUND

Brucella melitensis is a facultative, intracellular, pathogenic bacterium that replicates within macrophages. The type IV secretion system encoded by the virB operon (virB) is involved in Brucella intracellular survival. However, the underlying molecular mechanisms, especially the target proteins affected by the virB, remain largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

In order to define the proteins affected by virB, the proteomes of wild-type and the virB mutant were compared under in vitro conditions where virB was highly activated. The differentially expressed proteins were identified by MALDI-TOF-MS. Forty-four down-regulated and eighteen up-regulated proteins which exhibited a 2-fold or greater change were identified. These proteins included those involved in amino acid transport and metabolism, lipid metabolism, energy production, cell membrane biogenesis, translation, post-translational modifications and protein turnover, as well as unknown proteins. Interestingly, several important virulence related proteins involved in intracellular survival, including VjbR, DnaK, HtrA, Omp25, and GntR, were down-regulated in the virB mutant. Transcription analysis of virB and vjbR at different growth phase showed that virB positively affect transcription of vjbR in a growth phase dependent manner. Quantitative RT-PCR showed that transcription of these genes was also affected by virB during macrophage cell infection, consistent with the observed decreased survival of the virB mutant in macrophage.

CONCLUSIONS/SIGNIFICANCE

These data indicated that the virB operon may control the intracellular survival of Brucella by affecting the expression of relevant proteins.

LuxR-family 'solos': bachelor sensors/regulators of signalling molecules

N-Acylhomoserine lactone (AHL) quorum-sensing (QS) signalling is the best-understood chemical language in proteobacteria. In the last 15 years a large amount of research in several bacterial species has revealed in detail the genetic, molecular and biochemical mechanisms underlying AHL signalling. These studies have revealed the role played by protein pairs of the AHL synthase belonging to the LuxI family and cognate LuxR-family AHL sensor-regulator. Proteobacteria however commonly possess a QS LuxR-family protein for which there is no obvious cognate LuxI synthase; these proteins are found in bacteria which possess a complete AHL QS system(s) as well as in bacteria that do not. Scientists are beginning to address the roles played by these proteins and it is emerging that they could allow bacteria to respond to endogenous and exogenous signals produced by their neighbours. AHL QS research thus far has mainly focused on a cell-density response involving laboratory monoculture studies. Recent findings on the role played by the unpaired LuxR-family proteins highlight the need to address bacterial behaviour and response to signals in mixed communities. Here we review recent progress with respect to these LuxR proteins, which we propose to call LuxR 'solos' since they act on their own without the need for a cognate signal generator. Initial investigations have revealed that LuxR solos have diverse roles in bacterial interspecies and interkingdom communication.

Orphan LuxR regulators of quorum sensing

Bacteria can modulate their behavior by releasing and responding to the accumulation of signal molecules. This population co-ordination, referred to as quorum sensing, is prevalent in Gram-negative and Gram-positive bacteria. The essential constituents of quorum-sensing systems include a signal producer, or synthase, and a cognate transcriptional regulator that responds to the accumulated signal molecules. With the availability of bacterial genome sequences and an increased elucidation of quorum-sensing circuits, genes that code for additional transcriptional regulators, usually in excess of the synthase, have been identified. These additional regulators are referred to as 'orphan' regulators, because they are not directly associated with a synthase. Here, we review orphan regulators characterized in various Gram-negative bacteria and their role in expanding the bacterial regulatory network.

Interplay between two RND systems mediating antimicrobial resistance in Brucella suis

The RND-type efflux pumps are responsible for the multidrug resistance phenotype observed in many clinically relevant species. Also, RND pumps have been implicated in physiological processes, with roles in the virulence mechanisms of several pathogenic bacteria. We have previously shown that the BepC outer membrane factor of Brucella suis is involved in the efflux of diverse drugs, probably as part of a tripartite complex with an inner membrane translocase. In the present work, we characterize two membrane fusion protein-RND translocases of B. suis encoded by the bepDE and bepFG loci. MIC assays showed that the B. suis DeltabepE mutant was more sensitive to deoxycholate (DOC), ethidium bromide, and crystal violet. Furthermore, multicopy bepDE increased resistance to DOC and crystal violet and also to other drugs, including ampicillin, norfloxacin, ciprofloxacin, tetracycline, and doxycycline. In contrast to the DeltabepE mutant, the resistance profile of B. suis remained unaltered when the other RND gene (bepG) was deleted. However, the DeltabepE DeltabepG double mutant showed a more severe phenotype than the DeltabepE mutant, indicating that BepFG also contributes to drug resistance. An open reading frame (bepR) coding for a putative regulatory protein of the TetR family was found upstream of the bepDE locus. BepR strongly repressed the activity of the bepDE promoter, but DOC released the repression mediated by BepR. A clear induction of the bepFG promoter activity was observed only in the BepDE-defective mutant, indicating a regulatory interplay between the two RND efflux pumps. Although only the BepFG-defective mutant showed a moderate attenuation in model cells, the activities of both bepDE and bepFG promoters were induced in the intracellular environment of HeLa cells. Our results show that B. suis harbors two functional RND efflux pumps that may contribute to virulence.

Identification of VceA and VceC, two members of the VjbR regulon that are translocated into macrophages by the Brucella type IV secretion system

Survival and replication inside host cells by Brucella spp. requires a type IV secretion system (T4SS), encoded by the virB locus. However, the identity of the molecules secreted by the T4SS has remained elusive. We hypothesized that proteins translocated by the T4SS would be co-regulated with the virB operon. The LuxR family regulator VjbR, known to regulate virB, bound a fragment of the virB promoter containing an 18 bp palindromic motif (virB promoter box), showing that VjbR regulated the virB operon directly. To identify virB co-regulated genes, we searched the Brucella suis 1330 and B. abortus 2308 genomes for genes with an upstream virB promoter box. One hundred and forty-four promoters in the two genomes contained the virB promoter box, including those of fliC encoding flagellin and cgs encoding cyclic beta-glucan synthetase. Thirteen of these proteins were tested for VirB-dependent translocation into macrophages using a beta-lactamase reporter assay. This analysis resulted in the identification of the proteins encoded by BAB1_1652 (VceA) and BR1038/BAB1_1058 (VceC) as novel protein substrates of the Brucella T4SS. VceC could also be translocated by the Legionella pneumophila Dot/Icm T4SS into host cells. Our results suggest that VjbR co-ordinates expression of the T4SS and at least two of its secreted substrates.

Putative quorum-sensing regulator BlxR of Brucella melitensis regulates virulence factors including the type IV secretion system and flagella

Brucella melitensis is an intracellular pathogen that establishes a replicative niche within macrophages. While the intracellular lifestyle of Brucella is poorly understood and few virulence factors have been identified, components of a quorum-sensing pathway in Brucella have recently been identified. The LuxR-type regulatory protein, VjbR, and an N-acylhomoserine lactone signaling molecule are both involved in regulating expression of the virB-encoded type IV secretion system. We have identified a second LuxR-type regulatory protein (BlxR) in Brucella. Microarray analysis of a blxR mutant suggests that BlxR regulates the expression of a number of genes, including those encoding the type IV secretion system and flagella. Confirming these results, deletion of blxR in B. melitensis reduced the transcriptional activities of promoters for the virB operon, flagellar genes, and another putative virulence factor gene, bopA. Furthermore, our data suggested that both BlxR and VjbR are positively autoregulated and cross-regulate the expression of each other. The blxR deletion strain exhibited reduced growth in macrophages, similar to that observed for a vjbR deletion strain. However, unlike the vjbR deletion, the blxR deletion did not fully attenuate virulence in mice. More strikingly, bioluminescent imaging revealed that dissemination of the blxR mutant was similar to that of wild-type B. melitensis, while the vjbR mutant was defective for systemic spread in IRF-1(-/-) mice, suggesting that these regulators are not functionally redundant but that they converge in a common pathway regulating bacterial processes.

Regulation of type IV secretion apparatus genes during Ehrlichia chaffeensis intracellular development by a previously unidentified protein

The type IV secretion (T4S) system is critical for the virulence of several pathogens. In the rickettsial pathogen Ehrlichia chaffeensis, the virBD genes are split into two operons, the virB3-virB6 (preceded by sodB) and virB8-virD4 operons. Between these two operons, there are duplications of virB4, virB8, and virB9. In this study we found that transcription of all five loci was downregulated prior to the release of E. chaffeensis from host THP-1 cells and was upregulated at the initiation of exponential growth. Electrophoretic mobility shift assays revealed an E. chaffeensis-encoded protein that specifically bound to the promoter regions upstream of the virBD loci. The protein was purified from the bacterial lysate by affinity chromatography using a biotinylated promoter region upstream of sodB. Mass spectrometry identified the protein as an E. chaffeensis 12.3-kDa hypothetical protein, which was designated EcxR. Recombinant EcxR bound to the promoter regions upstream of five individual virBD loci. EcxR also activated transcription of all five virBD loci in lacZ reporter constructs. The expression of ecxR was positively autoregulated by EcxR. These results suggest that the five virBD loci are coordinately regulated by EcxR to allow developmental stage-specific expression of the T4S system in E. chaffeensis.

Cytotoxicity in macrophages infected with rough Brucella mutants is type IV secretion system dependent

Smooth Brucella spp. inhibit macrophage apoptosis, whereas rough Brucella mutants induce macrophage oncotic and necrotic cell death. However, the mechanisms and genes responsible for Brucella cytotoxicity have not been identified. In the current study, a random mutagenesis approach was used to create a mutant bank consisting of 11,354 mutants by mariner transposon mutagenesis using Brucella melitensis rough mutant 16M delta manBA as the parental strain. Subsequent screening identified 56 mutants (0.49% of the mutant bank) that failed to cause macrophage cell death (release of 10% or less of the lactate dehydrogenase). The absence of cytotoxicity during infection with these mutants was independent of demonstrable defects in in vitro bacterial growth or uptake and survival in macrophages. Interrupted genes in 51 mutants were identified by DNA sequence analysis, and the mutations included interruptions in virB encoding the type IV secretion system (T4SS) (n = 36) and in vjbR encoding a LuxR-like regulatory element previously shown to be required for virB expression (n = 3), as well as additional mutations (n = 12), one of which also has predicted roles in virB expression. These results suggest that the T4SS is associated with Brucella cytotoxicity in macrophages. To verify this, deletion mutants were constructed in B. melitensis 16M by removing genes encoding phosphomannomutase/phosphomannoisomerase (delta manBA) and the T4SS (delta virB). As predicted, deletion of virB from 16M delta manBA and 16M resulted in a complete loss of cytotoxicity in rough strains, as well as the low level cytotoxicity observed with smooth strains at extreme multiplicities of infection (>1,000). Taken together, these results demonstrate that Brucella cytotoxicity in macrophages is T4SS dependent.

Mutations of the quorum sensing-dependent regulator VjbR lead to drastic surface modifications in Brucella melitensis

Successful establishment of infection by bacterial pathogens requires fine-tuning of virulence-related genes. Quorum sensing (QS) is a global regulation process based on the synthesis of, detection of, and response to small diffusible molecules, called N-acyl-homoserine lactones (AHL), in gram-negative bacteria. In numerous species, QS has been shown to regulate genes involved in the establishment of pathogenic interactions with the host. Brucella melitensis produces N-dodecanoyl homoserine lactones (C(12)-HSL), which down regulate the expression of flagellar genes and of the virB operon (encoding a type IV secretion system), both of which encode surface virulence factors. A QS-related regulator, called VjbR, was identified as a transcriptional activator of these genes. We hypothesized that VjbR mediates the C(12)-HSL effects described above. vjbR alleles mutated in the region coding for the AHL binding domain were constructed to test this hypothesis. These alleles expressed in trans in a DeltavjbR background behave as constitutive regulators both in vitro and in a cellular model of infection. Interestingly, the resulting B. melitensis strains, unable to respond to AHLs, aggregate spontaneously in liquid culture. Preliminary characterization of these strains showed altered expression of some outer membrane proteins and overproduction of a matrix-forming exopolysaccharide, suggesting for the first time that B. melitensis could form biofilms. Together, these results indicate that QS through VjbR is a major regulatory system of important cell surface structures of Brucella and as such plays a key role in host-pathogen interactions.

Identification of bacterial N-acylhomoserine lactones (AHLs) with a combination of ultra-performance liquid chromatography (UPLC), ultra-high-resolution mass spectrometry, and in-situ biosensors

N-Acylated homoserine lactones (AHLs) are produced by Gram-negative bacteria as communication signals and are frequently studied as mediators of the "quorum sensing" response of bacterial communities. Several reports have recently been published on the identification of AHLs from different species and attempts have been made to study their role in natural habitats, for example the surface of plant roots in the rhizosphere. In this article, different analytical methods, including bacterial biosensors and chromatographic techniques, are reviewed. A concept for assignment of the structures of AHLs is also presented. The retention behaviour of derivatives of AHLs containing beta-keto or hydroxyl groups and/or double bonds has been evaluated in relation to the separation behaviour of AHLs with saturated and unsubstituted alkanoyl chains. Samples have also been analysed by high resolution mass spectrometry (Fourier-transform ion-cyclotron-resonance mass spectrometry, FTICR-MS), nano liquid chromatography-electrospray ionization ion trap mass spectrometry (nano-LC-MS) and by the aid of a biosensor. The results obtained from ultra performance liquid chromatography (UPLC), FTICR-MS, nano-LC-MS, and bioassays have been compared to attempt structural characterisation of AHL without chemical synthesis of analytical standards. The method was used to identify the major AHL compound produced by the rhizosphere bacterium Acidovorax sp. N35 as N-(3-hydroxydecanoyl)homoserine lactone.

Messing with bacterial quorum sensing

Quorum sensing is widely recognized as an efficient mechanism to regulate expression of specific genes responsible for communal behavior in bacteria. Several bacterial phenotypes essential for the successful establishment of symbiotic, pathogenic, or commensal relationships with eukaryotic hosts, including motility, exopolysaccharide production, biofilm formation, and toxin production, are often regulated by quorum sensing. Interestingly, eukaryotes produce quorum-sensing-interfering (QSI) compounds that have a positive or negative influence on the bacterial signaling network. This eukaryotic interference could result in further fine-tuning of bacterial quorum sensing. Furthermore, recent work involving the synthesis of structural homologs to the various quorum-sensing signal molecules has resulted in the development of additional QSI compounds that could be used to control pathogenic bacteria. The creation of transgenic plants that express bacterial quorum-sensing genes is yet another strategy to interfere with bacterial behavior. Further investigation on the manipulation of quorum-sensing systems could provide us with powerful tools against harmful bacteria.

The TolC homologue of Brucella suis is involved in resistance to antimicrobial compounds and virulence

Brucella spp., like other pathogens, must cope with the environment of diverse host niches during the infection process. In doing this, pathogens evolved different type of transport systems to help them survive and disseminate within the host. Members of the TolC family have been shown to be involved in the export of chemically diverse molecules ranging from large protein toxins to small toxic compounds. The role of proteins from the TolC family in Brucella and other alpha-2-proteobacteria has been explored little. The gene encoding the unique member of the TolC family from Brucella suis (BepC) was cloned and expressed in an Escherichia coli mutant disrupted in the gene encoding TolC, which has the peculiarity of being involved in diverse transport functions. BepC fully complemented the resistance to drugs such as chloramphenicol and acriflavine but was incapable of restoring hemolysin secretion in the tolC mutant of E. coli. An insertional mutation in the bepC gene strongly affected the resistance phenotype of B. suis to bile salts and toxic chemicals such as ethidium bromide and rhodamine and significantly decreased the resistance to antibiotics such as erythromycin, ampicillin, tetracycline, and norfloxacin. Moreover, the B. suis bepC mutant was attenuated in the mouse model of infection. Taken together, these results suggest that BepC-dependent efflux processes of toxic compounds contribute to B. suis survival inside the host.

The stringent response mediator Rsh is required for Brucella melitensis and Brucella suis virulence, and for expression of the type IV secretion system virB

Physiological adaptation of intracellular bacteria is critical for timely interaction with eukaryotic host cells. One mechanism of adaptation, the stringent response, is induced by nutrient stress via its effector molecule (p)ppGpp, synthesized by the action of RelA/SpoT homologues. The intracellular pathogen Brucella spp., causative agent of brucellosis, possesses a gene homologous to relA/spoT, named rsh, encoding a (p)ppGpp synthetase as confirmed by heterologous complementation of a relA mutant of Sinorhizobium meliloti. The Rsh deletion mutants in Brucella suis and Brucella melitensis were characterized by altered morphology, and by reduced survival under starvation conditions and in cellular and murine models of infection. Most interestingly, we evidenced that expression of virB, encoding the type IV secretion system, a major virulence factor of Brucella, was Rsh-dependent. All mutant phenotypes, including lack of VirB proteins, were complemented with the rsh gene of Brucella. In addition, RelA of S. meliloti functionally replaced Brucella Rsh, describing the capacity of a gene from a plant symbiont to restore virulence in a mammalian pathogen. We therefore concluded that in the intramacrophagic environment encountered by Brucella, Rsh might participate in the adaptation of the pathogen to low-nutrient environments, and indirectly in the VirB-mediated formation of the final replicative niche.

Proteomic analysis of quorum sensing in Rhizobium leguminosarum biovar viciae UPM791

Production of quorum-sensing signal molecules of the acyl-homoserine lactone (AHL) type by Rhizobium leguminosarum bv. viciae UPM791 is dependent on its plasmid content. Curing of two of its four native plasmids, pUPM791d and pSym, resulted in loss of production of the largest (C(14)) and the three smaller (C(6)-C(8)) AHLs, respectively. Introduction of a lactonase-containing plasmid resulted in AHL signal degradation and quorum quenching. The quorum-dependent proteome was studied in these strains by DIGE. Quorum quenching affected a small (1.7%) fraction of the detected spots in the wild-type and a smaller (0.6%) fraction in the pSym-cured strain. Unexpectedly, quorum quenching affected up to 3.3% of the detected spots in the pUPM791d-cured strain, suggesting that C(14)-AHL normally interferes with the quorum response mediated by other AHLs. This, together with the observation that ca. 50% of the quorum-regulated proteins in strain UPM791 showed AHL-mediated repression, suggests that an important part of their functionality can be exerted through repression, although AHLs are usually considered as gene expression inducers. The three main quorum-induced polypeptides were identified by MALDI-MS as charge isoforms of the rhizospheric RhiA protein. Another major quorum-induced polypeptide was only present in the pUPM791d-cured strain and could not be identified.

Immune modulation by Pseudomonas aeruginosa quorum-sensing signal molecules

There is burgeoning recent interest in the potential of bacterial quorum-sensing signal molecules (QSSMs) such as the long chain N-acylhomoserine lactones (AHLs) and 4-quinolones produced by Pseudomonas aeruginosa for modulating immune function. While it is clear that QSSMs have well defined immune modulatory potential in vitro, and are detectable in body fluids (such as sputum from cystic fibrosis patients infected with P. aeruginosa) at levels which might be expected to modify immune competence, the true impact of bacterial QSSMs on host physiology in vivo has yet to be fully determined.