Role of quorum sensing in the pathogenicity of Burkholderia pseudomallei

Distribution and functional analysis of two types of quorum sensing gene pairs, glaI1/glaR1 and glaI2/glaR2, in Burkholderia gladioli

Burkholderia gladioli produces a yellow-pigmented toxin called toxoflavin, and causes disease on a variety of plants. Previous studies have suggested that the pathogenicity of B. gladioli is regulated by an N-acyl-l-homoserine lactone (AHL)-mediated quorum sensing (QS) system. In this study, complete genome sequencing revealed that B. gladioli pv. gladioli MAFF 302385 possesses two types of AHL synthase and AHL receptor gene pairs: glaI1/glaR1 and glaI2/glaR2. Disruption of QS genes revealed that the glaI1/glaR1 QS system regulated swarming motility, biofilm formation, and colony formation via N-octanoyl-l-homoserine lactone. Although Escherichia coli harboring glaI2 produced N-(3-hydroxyoctanoyl)-l-homoserine lactone and N-(3-hydroxydecanoyl)-l-homoserine lactone, the expression of glaI2 was not confirmed in MAFF 302385 cells. We also found that toxoflavin production was regulated by the glaI1/glaR1 QS system in liquid medium, but not on agar medium. When pathogenicity tests were performed on gladiolus leaves, the wild-type and QS mutants showed a similar level of disease. Our results demonstrated that only the glaI1/glaR1-mediated QS system is active in MAFF 302385, but major virulence factors, especially toxoflavin, are not completely dependent on the QS system.

Role of bacterial multidrug efflux pumps during infection

Multidrug efflux pumps are protein complexes located in the cell envelope that enable bacteria to expel, not only antibiotics, but also a wide array of molecules relevant for infection. Hence, they are important players in microbial pathogenesis. On the one hand, efflux pumps can extrude exogenous compounds, including host-produced antimicrobial molecules. Through this extrusion, pathogens can resist antimicrobial agents and evade host defenses. On the other hand, efflux pumps also have a role in the extrusion of endogenous compounds, such as bacterial intercommunication signaling molecules, virulence factors or metabolites. Therefore, efflux pumps are involved in the modulation of bacterial behavior and virulence, as well as in the maintenance of the bacterial homeostasis under different stresses found within the host. This review delves into the multifaceted roles that efflux pumps have, shedding light on their impact on bacterial virulence and their contribution to bacterial infection. These observations suggest that strategies targeting bacterial efflux pumps could both reinvigorate the efficacy of existing antibiotics and modulate the bacterial pathogenicity to the host. Thus, a comprehensive understanding of bacterial efflux pumps can be pivotal for the development of new effective strategies for the management of infectious diseases.

Transitioning from Soil to Host: Comparative Transcriptome Analysis Reveals the Burkholderia pseudomallei Response to Different Niches

Burkholderia pseudomallei, a soil and water saprophyte, is responsible for the tropical human disease melioidosis. A hundred years since its discovery, there is still much to learn about B. pseudomallei proteins that are essential for the bacterium's survival in and interaction with the infected host, as well as their roles within the bacterium's natural soil habitat. To address this gap, bacteria grown under conditions mimicking the soil environment were subjected to transcriptome sequencing (RNA-seq) analysis. A dual RNA-seq approach was used on total RNA from spleens isolated from a B. pseudomallei mouse infection model at 5 days postinfection. Under these conditions, a total of 1,434 bacterial genes were induced, with 959 induced in the soil environment and 475 induced in bacteria residing within the host. Genes encoding metabolism and transporter proteins were induced when the bacteria were present in soil, while virulence factors, metabolism, and bacterial defense mechanisms were upregulated during active infection of mice. On the other hand, capsular polysaccharide and quorum-sensing pathways were inhibited during infection. In addition to virulence factors, reactive oxygen species, heat shock proteins, siderophores, and secondary metabolites were also induced to assist bacterial adaptation and survival in the host. Overall, this study provides crucial insights into the transcriptome-level adaptations which facilitate infection by soil-dwelling B. pseudomallei. Targeting novel therapeutics toward B. pseudomallei proteins required for adaptation provides an alternative treatment strategy given its intrinsic antimicrobial resistance and the absence of a vaccine. IMPORTANCE Burkholderia pseudomallei, a soil-dwelling bacterium, is the causative agent of melioidosis, a fatal infectious disease of humans and animals. The bacterium has a large genome consisting of two chromosomes carrying genes that encode proteins with important roles for survival in diverse environments as well as in the infected host. While a general mechanism of pathogenesis has been proposed, it is not clear which proteins have major roles when the bacteria are in the soil and whether the same proteins are key to successful infection and spread. To address this question, we grew the bacteria in soil medium and then in infected mice. At 5 days postinfection, bacteria were recovered from infected mouse organs and their gene expression was compared against that of bacteria grown in soil medium. The analysis revealed a list of genes expressed under soil growth conditions and a different set of genes encoding proteins which may be important for survival, replication, and dissemination in an infected host. These proteins are a potential resource for understanding the full adaptation mechanism of this pathogen. In the absence of a vaccine for melioidosis and with treatment being reliant on combinatorial antibiotic therapy, these proteins may be ideal targets for designing antimicrobials to treat melioidosis.

A virulence activator of a surface attachment protein in Burkholderia pseudomallei acts as a global regulator of other membrane-associated virulence factors

Burkholderia pseudomallei (Bp), causing a highly fatal disease called melioidosis, is a facultative intracellular pathogen that attaches and invades a variety of cell types. We previously identified BP1026B_I0091 as a surface attachment protein (Sap1) and an essential virulence factor, contributing to Bp pathogenesis in vitro and in vivo. The expression of sap1 is regulated at different stages of Bp intracellular lifecycle by unidentified regulator(s). Here, we identified SapR (BP1026B_II1046) as a transcriptional regulator that activates sap1, using a high-throughput transposon mutagenesis screen in combination with Tn-Seq. Consistent with phenotypes of the Δsap1 mutant, the ΔsapR activator mutant exhibited a significant reduction in Bp attachment to the host cell, leading to subsequent decreased intracellular replication. RNA-Seq analysis further revealed that SapR regulates sap1. The regulation of sap1 by SapR was confirmed quantitatively by qRT-PCR, which also validated the RNA-Seq data. SapR globally regulates genes associated with the bacterial membrane in response to diverse environments, and some of the genes regulated by SapR are virulence factors that are required for Bp intracellular infection (e.g., type III and type VI secretion systems). This study has identified the complex SapR regulatory network and its importance as an activator of an essential Sap1 attachment factor.

Metabolomics Analysis of Bacterial Pathogen Burkholderia thailandensis and Mammalian Host Cells in Co-culture

The Tier 1 HHS/USDA Select Agent Burkholderia pseudomallei is a bacterial pathogen that is highly virulent when introduced into the respiratory tract and intrinsically resistant to many antibiotics. Transcriptomic- and proteomic-based methodologies have been used to investigate mechanisms of virulence employed by B. pseudomallei and Burkholderia thailandensis, a convenient surrogate; however, analysis of the pathogen and host metabolomes during infection is lacking. Changes in the metabolites produced can be a result of altered gene expression and/or post-transcriptional processes. Thus, metabolomics complements transcriptomics and proteomics by providing a chemical readout of a biological phenotype, which serves as a snapshot of an organism's physiological state. However, the poor signal from bacterial metabolites in the context of infection poses a challenge in their detection and robust annotation. In this study, we coupled mammalian cell culture-based metabolomics with feature-based molecular networking of mono- and co-cultures to annotate the pathogen's secondary metabolome during infection of mammalian cells. These methods enabled us to identify several key secondary metabolites produced by B. thailandensis during infection of airway epithelial and macrophage cell lines. Additionally, the use of in silico approaches provided insights into shifts in host biochemical pathways relevant to defense against infection. Using chemical class enrichment analysis, for example, we identified changes in a number of host-derived compounds including immune lipids such as prostaglandins, which were detected exclusively upon pathogen challenge. Taken together, our findings indicate that co-culture of B. thailandensis with mammalian cells alters the metabolome of both pathogen and host and provides a new dimension of information for in-depth analysis of the host-pathogen interactions underlying Burkholderia infection.

Comprehensive approaches for the detection of Burkholderia pseudomallei and diagnosis of melioidosis in human and environmental samples

Melioidosis is endemic in Southeast Asia and northern Australia. The causative agent of melioidosis is a Gram-negative bacterium, Burkholderia pseudomallei. Its invasion can be fatal if melioidosis is not treated promptly. It is intrinsically resistant to a variety of antibiotics. In this paper, we present a comprehensive overview of the current trends on melioidosis cases, treatments, B. pseudomallei virulence factors, and molecular techniques to detect the bacterium from different samples. The clinical and microbial diagnosis methods of identification and detection of B. pseudomallei are commonly used for the rapid diagnosis and typing of strains, such as polymerase chain reaction or multi-locus sequence typing. The genotyping strategies and techniques have been constantly evolving to identify genomic loci linked to or associated with this human disease. More research strategies for detecting and controlling melioidosis should be encouraged and conducted to understand the current situation. In conclusion, we review existing diagnostic methodologies for melioidosis detection and provide insights on prospective diagnostic methods for the bacterium.

Dynamical behaviors of quorum sensing network mediated by combinatorial perturbation

The dynamical behaviors of the quorum sensing (QS) system are closely related to the release drugs and control the PH value in microorganisms and plants. However, the effect of the main molecules AiiA, LuxI, H$ _2 $O$ _2 $, and time delayed individual and combinatorial perturbation on the QS system dynamics and the above-mentioned biological phenomena is still unclear, which are seen as a key consideration in our paper. This paper formulates a QS computational model by incorporating these several substances. First, for the protein production time delay, a critical value is given by Hopf bifurcation theory. It is found that a larger time delay can lead to a larger amplitude and a longer period. This indicates that the length of time for protein synthesis has a regulatory effect on the release of drugs from the bacterial population. Second, hen the concentrations of AiiA, LuxI, and H$ _2 $O$ _2 $ is modulated individually, the QS system undergoes periodic oscillation and bistable state. Meanwhile, oscillatory and bistable regions can be significantly affected by simultaneously perturbing any two parameters related to AiiA, LuxI, and H$ _2 $O$ _2 $. This means that the individual or simultaneous changes of the three intrinsic molecular concentrations can effectively control the drugs release and the PH value in microorganisms and plants. Finally, the sensitivity relationship between the critical value of the delay and AiiA, LuxI, H$ _2 $O$ _2 $ parameters is analyzed.

Evaluation of four sampling devices for Burkholderia pseudomallei laboratory aerosol studies

Previous field and laboratory studies investigating airborne Burkholderia pseudomallei have used a variety of different aerosol samplers to detect and quantify concentrations of the bacteria in aerosols. However, the performance of aerosol samplers can vary in their ability to preserve the viability of collected microorganisms, depending on the resistance of the organisms to impaction, desiccation, or other stresses associated with the sampling process. Consequently, sampler selection is critical to maximizing the probability of detecting viable microorganisms in collected air samples in field studies and for accurate determination of aerosol concentrations in laboratory studies. To inform such decisions, the present study assessed the performance of four laboratory aerosol samplers, specifically the all-glass impinger (AGI), gelatin filter, midget impinger, and Mercer cascade impactor, for collecting aerosols containing B. pseudomallei generated from suspensions in two types of culture media. The results suggest that the relative performance of the sampling devices is dependent on the suspension medium utilized for aerosolization. Performance across the four samplers was similar for aerosols generated from suspensions supplemented with 4% glycerol. However, for aerosols generated from suspensions without glycerol, use of the filter sampler or an impactor resulted in significantly lower estimates of the viable aerosol concentration than those obtained with either the AGI or midget impinger. These results demonstrate that sampler selection has the potential to affect estimation of doses in inhalational animal models of melioidosis, as well as the likelihood of detection of viable B. pseudomallei in the environment, and will be useful to inform design of future laboratory and field studies.

ScmR, a Global Regulator of Gene Expression, Quorum Sensing, pH Homeostasis, and Virulence in Burkholderia thailandensis

The nonpathogenic soil saprophyte Burkholderia thailandensis is a member of the Burkholderia pseudomallei /B. thailandensis/B. mallei group, which also comprises the closely related human pathogens B. pseudomallei and Burkholderia mallei responsible for the melioidosis and glanders diseases, respectively. ScmR, a recently identified LysR-type transcriptional regulator in B. thailandensis, acts as a global transcriptional regulator throughout the stationary phase and modulates the production of a wide range of secondary metabolites, including N-acyl-l-homoserine lactones and 4-hydroxy-3-methyl-2-alkylquinolines and virulence in the Caenorhabditis elegans nematode worm host model, as well as several quorum sensing (QS)-dependent phenotypes. We have investigated the role of ScmR in B. thailandensis strain E264 during the exponential phase. We used RNA sequencing transcriptomic analyses to identify the ScmR regulon, which was compared to the QS-controlled regulon, showing a considerable overlap between the ScmR-regulated genes and those controlled by QS. We characterized several genes modulated by ScmR using quantitative reverse transcription-PCR or mini-CTX-lux transcriptional reporters, including the oxalate biosynthetic gene obc1 required for pH homeostasis, the orphan LuxR-type transcriptional regulator BtaR5-encoding gene, and the bsa (Burkholderia secretion apparatus) type III secretion system genes essential for both B. pseudomallei and B. mallei pathogenicity, as well as the scmR gene itself. We confirmed that the transcription of scmR is under QS control, presumably ensuring fine-tuned modulation of gene expression. Finally, we demonstrated that ScmR influences virulence using the fruit fly model host Drosophila melanogaster We conclude that ScmR represents a central component of the B. thailandensis QS regulatory network.IMPORTANCE Coordination of the expression of genes associated with bacterial virulence and environmental adaptation is often dependent on quorum sensing (QS). The QS circuitry of the nonpathogenic bacterium Burkholderia thailandensis, widely used as a model system for the study of the human pathogen Burkholderia pseudomallei, is complex. We found that the LysR-type transcriptional regulator, ScmR, which is highly conserved and involved in the control of virulence/survival factors in the Burkholderia genus, is a global regulator mediating gene expression through the multiple QS systems coexisting in B. thailandensis, as well as QS independently. We conclude that ScmR represents a key QS modulatory network element, ensuring tight regulation of the transcription of QS-controlled genes, particularly those required for acclimatization to the environment.

Host monitoring of quorum sensing during Pseudomonas aeruginosa infection

Pseudomonas aeruginosa rapidly adapts to altered conditions by quorum sensing (QS), a communication system that it uses to collectively modify its behavior through the production, release, and detection of signaling molecules. QS molecules can also be sensed by hosts, although the respective receptors and signaling pathways are poorly understood. We describe a pattern of regulation in the host by the aryl hydrocarbon receptor (AhR) that is critically dependent on qualitative and quantitative sensing of P. aeruginosa quorum. QS molecules bind to AhR and distinctly modulate its activity. This is mirrored upon infection with P. aeruginosa collected from diverse growth stages and with QS mutants. We propose that by spying on bacterial quorum, AhR acts as a major sensor of infection dynamics, capable of orchestrating host defense according to the status quo of infection.

The Chemistry and Biology of Bactobolin: A 10-Year Collaboration with Natural Product Chemist Extraordinaire Jon Clardy

Bactobolin is a hybrid natural product with potent cytotoxic activity. Its production from Burkholderia thailandensis was reported as part of a collaboration between the Greenberg and Clardy laboratories in 2010. The collaboration sparked a series of studies leading to the discovery of new analogues and associated structure-activity relationships, the identification of the bactobolin biosynthetic gene cluster and assembly of its unusual amino acid building block, the molecular target of and resistance to the antibiotic, and finally an X-ray crystal structure of the ribosome-bactobolin complex. Herein, we review the collaborations that led to our current understanding of the chemistry and biology of bactobolin.

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.

Betaproteobacteria are predominant in drinking water: are there reasons for concern?

Betaproteobacteria include some of the most abundant and ubiquitous bacterial genera that can be found in drinking water, including mineral water. The combination of physiology and ecology traits place some Betaproteobacteria in the list of potential, yet sometimes neglected, opportunistic pathogens that can be transmitted by water or aqueous solutions. Indeed, some drinking water Betaproteobacteria with intrinsic and sometimes acquired antibiotic resistance, harbouring virulence factors and often found in biofilm structures, can persist after water disinfection and reach the consumer. This literature review summarises and discusses the current knowledge about the occurrence and implications of Betaproteobacteria in drinking water. Although the sparse knowledge on the ecology and physiology of Betaproteobacteria thriving in tap or bottled natural mineral/spring drinking water (DW) is an evidence of this review, it is demonstrated that DW holds a high diversity of Betaproteobacteria, whose presence may not be innocuous. Frequently belonging to genera also found in humans, DW Betaproteobacteria are ubiquitous in different habitats, have the potential to resist antibiotics either due to intrinsic or acquired mechanisms, and hold different virulence factors. The combination of these factors places DW Betaproteobacteria in the list of candidates of emerging opportunistic pathogens. Improved bacterial identification of clinical isolates associated with opportunistic infections and additional genomic and physiological studies may contribute to elucidate the potential impact of these bacteria.

Do Global Regulators Hold the Key to Production of Bacterial Secondary Metabolites?

The emergence of multiple antibiotic resistant bacteria has pushed the available pool of antibiotics to the brink. Bacterial secondary metabolites have long been a valuable resource in the development of antibiotics, and the genus Burkholderia has recently emerged as a source of novel compounds with antibacterial, antifungal, and anti-cancer activities. Genome mining has contributed to the identification of biosynthetic gene clusters, which encode enzymes that are responsible for synthesis of such secondary metabolites. Unfortunately, these large gene clusters generally remain silent or cryptic under normal laboratory settings, which creates a hurdle in identification and isolation of these compounds. Various strategies, such as changes in growth conditions and antibiotic stress, have been applied to elicit the expression of these cryptic gene clusters. Although a number of compounds have been isolated from different Burkholderia species, the mechanisms by which the corresponding gene clusters are regulated remain poorly understood. This review summarizes the activity of well characterized secondary metabolites from Burkholderia species and the role of local regulators in their synthesis, and it highlights recent evidence for the role of global regulators in controlling production of secondary metabolites. We suggest that targeting global regulators holds great promise for the awakening of cryptic gene clusters and for developing better strategies for discovery of novel antibiotics.

Quorum sensing as a potential target for increased production of rhamnolipid biosurfactant in Burkholderia thailandensis E264

Burkholderia thailandensis E264 is a potential non-pathogenic substitute for producing rhamnolipid biosurfactant, replacing the pathogenic Pseudomonas aeruginosa. However, it has low rhamnolipid production and longer fermentation time. We have earlier suggested that media supplementation with exogenous quorum sensing (QS) molecules could lead to early onset of biosynthesis and increased rhamnolipid yield. Here, we assessed the effect of single, double or triple mutations in the various QS systems of B. thailandensis on rhamnolipid production, with the view to see which system(s) have the most impact on rhamnolipid yield and subsequently use the QS molecule to potentially increase yield in the wild-type B. thailandensis. The triple mutant strain had a rhamnolipid yield of 4.46 ± 0.345 g/l at 240 h of fermentation which was significantly higher than that of the wild type (0.94 ± 0.06 g/l), an unexpected outcome. To gain more insight as to how this might occur, we studied substrate metabolism and energy storage in the form of polyhydroxyalkanoate (PHA) by both the triple mutant and the wild type. We observed increased glycerol metabolism and reduced PHA production in the triple mutant compared with the wild type. Glycerol concentration at 240 h and maximum PHA productivity (g/gDCB) were 8.76 g/l or 16.19 g/l and 21.80% or 31.4% in either the triple mutant or the wild type respectively. Complementation of the triple-mutant cultures with exogenous QS molecules restored rhamnolipid production to similar levels as the wild type. QS therefore is a potential target for increased rhamnolipid production in B. thailandensis.

Two rsaM Homologues Encode Central Regulatory Elements Modulating Quorum Sensing in Burkholderia thailandensis

The bacterium Burkholderia thailandensis possesses three N-acyl-l-homoserine lactone (AHL) quorum sensing (QS) systems designated BtaI1/BtaR1 (QS-1), BtaI2/BtaR2 (QS-2), and BtaI3/BtaR3 (QS-3). These QS systems are associated with the biosynthesis of N-octanoyl-homoserine lactone (C₈-HSL), N-3-hydroxy-decanoyl-homoserine lactone (3OHC₁₀-HSL), and N-3-hydroxy-octanoyl-homoserine lactone (3OHC₈-HSL), which are produced by the LuxI-type synthases BtaI1, BtaI2, and BtaI3 and modulated by the LuxR-type transcriptional regulators BtaR1, BtaR2, and BtaR3. The btaR1-btaI1 and btaR2-btaI2 gene clusters each carry an additional gene encoding a homologue of the QS repressor RsaM originally identified in the phytopathogen Pseudomonas fuscovaginae and thus here named rsaM1 and rsaM2, respectively. We have characterized the functions of these two conserved rsaM homologues and demonstrated their involvement in the regulation of AHL biosynthesis in B. thailandensis strain E264. We quantified the production of C₈-HSL, 3OHC₁₀-HSL, and 3OHC₈-HSL by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in the wild-type strain and in the rsaM1 and rsaM2 mutants, and we monitored btaI1, btaI2, and btaI3 expression using chromosomal mini-CTX-lux transcriptional reporters. The transcription of btaR1, btaR2, and btaR3 was also measured by quantitative reverse transcription-PCR (qRT-PCR). We observed that RsaM1 mainly represses the QS-1 system, whereas RsaM2 principally represses the QS-2 system. We also found that both rsaM1 and rsaM2 are QS controlled and negatively autoregulated. We conclude that RsaM1 and RsaM2 are an integral part of the QS circuitry of B. thailandensis and play a major role in the hierarchical and homeostatic organization of the QS-1, QS-2, and QS-3 systems.IMPORTANCE Quorum sensing (QS) is commonly involved in the coordination of gene transcription associated with the establishment of host-pathogen interactions and acclimatization to the environment. We present the functional characterization of two rsaM homologues in the regulation of the multiple QS systems coexisting in the nonpathogenic bacterium Burkholderia thailandensis, which is widely used as a model system for the study of the human pathogen Burkholderia pseudomallei We found that inactivation of these rsaM homologues, which are clustered with the other QS genes, profoundly affects the QS circuitry of B. thailandensis We conclude that they constitute essential regulatory components of the QS modulatory network and provide additional layers of regulation to modulate the transcription of QS-controlled genes, particularly those linked to environmental adaptation.

Melioidosis

Burkholderia pseudomallei is a Gram-negative environmental bacterium and the aetiological agent of melioidosis, a life-threatening infection that is estimated to account for ∼89,000 deaths per year worldwide. Diabetes mellitus is a major risk factor for melioidosis, and the global diabetes pandemic could increase the number of fatalities caused by melioidosis. Melioidosis is endemic across tropical areas, especially in southeast Asia and northern Australia. Disease manifestations can range from acute septicaemia to chronic infection, as the facultative intracellular lifestyle and virulence factors of B. pseudomallei promote survival and persistence of the pathogen within a broad range of cells, and the bacteria can manipulate the host's immune responses and signalling pathways to escape surveillance. The majority of patients present with sepsis, but specific clinical presentations and their severity vary depending on the route of bacterial entry (skin penetration, inhalation or ingestion), host immune function and bacterial strain and load. Diagnosis is based on clinical and epidemiological features as well as bacterial culture. Treatment requires long-term intravenous and oral antibiotic courses. Delays in treatment due to difficulties in clinical recognition and laboratory diagnosis often lead to poor outcomes and mortality can exceed 40% in some regions. Research into B. pseudomallei is increasing, owing to the biothreat potential of this pathogen and increasing awareness of the disease and its burden; however, better diagnostic tests are needed to improve early confirmation of diagnosis, which would enable better therapeutic efficacy and survival.

The Complex Quorum Sensing Circuitry of Burkholderia thailandensis Is Both Hierarchically and Homeostatically Organized

The genome of the bacterium Burkholderia thailandensis encodes three complete LuxI/LuxR-type quorum sensing (QS) systems: BtaI1/BtaR1 (QS-1), BtaI2/BtaR2 (QS-2), and BtaI3/BtaR3 (QS-3). The LuxR-type transcriptional regulators BtaR1, BtaR2, and BtaR3 modulate the expression of target genes in association with various N-acyl-l-homoserine lactones (AHLs) as signaling molecules produced by the LuxI-type synthases BtaI1, BtaI2, and BtaI3. We have systematically dissected the complex QS circuitry of B. thailandensis strain E264. Direct quantification of N-octanoyl-homoserine lactone (C₈-HSL), N-3-hydroxy-decanoyl-homoserine lactone (3OHC₁₀-HSL), and N-3-hydroxy-octanoyl-homoserine lactone (3OHC₈-HSL), the primary AHLs produced by this bacterium, was performed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) in the wild-type strain and in QS deletion mutants. This was compared to the transcription of btaI1, btaI2, and btaI3 using chromosomal mini-CTX-lux transcriptional reporters. Furthermore, the levels of expression of btaR1, btaR2, and btaR3 were monitored by quantitative reverse transcription-PCR (qRT-PCR). We observed that C₈-HSL, 3OHC₁₀-HSL, and 3OHC₈-HSL are differentially produced over time during bacterial growth and correlate with the btaI1, btaI2, and btaI3 gene expression profiles, revealing a successive activation of the corresponding QS systems. Moreover, the transcription of the btaR1, btaR2, and btaR3 genes is modulated by cognate and noncognate AHLs, showing that their regulation depends on themselves and on other QS systems. We conclude that the three QS systems in B. thailandensis are interdependent, suggesting that they cooperate dynamically and function in a concerted manner in modulating the expression of QS target genes through a successive regulatory network.

Quorum sensing (QS) is a widespread bacterial communication system coordinating the expression of specific genes in a cell density-dependent manner and allowing bacteria to synchronize their activities and to function as multicellular communities. QS plays a crucial role in bacterial pathogenicity by regulating the expression of a wide spectrum of virulence/survival factors and is essential to environmental adaptation. The results presented here demonstrate that the multiple QS systems coexisting in the bacterium Burkholderia thailandensis, which is considered the avirulent version of the human pathogen Burkholderia pseudomallei and thus commonly used as an alternative study model, are hierarchically and homeostatically organized. We found these QS systems to be finely integrated into a complex regulatory network, including transcriptional and posttranscriptional interactions, and further incorporating growth stages and temporal expression. These results provide a unique, comprehensive illustration of a sophisticated QS network and will contribute to a better comprehension of the regulatory mechanisms that can be involved in the expression of QS-controlled genes, in particular those associated with the establishment of host-pathogen interactions and acclimatization to the environment.

Engineering Soluble Human Paraoxonase 2 for Quorum Quenching

Many pathogenic bacteria utilize quorum sensing (QS) systems to regulate the expression of their virulence genes and promote the formation of biofilm, which renders pathogens with extreme resistance to conventional antibiotic treatments. As a novel approach for attenuating antibiotic resistance and in turn fighting chronic infections, enzymatic inactivation of QS signaling molecules, such as N-acyl homoserine lactones (AHLs), holds great promises. Instead of using bacterial lactonases that can evoke immune response when administered, we focus on the human paraoxonase 2 (huPON2). However, insolubility when heterologously overexpressed hinders its application as anti-infection therapeutics. In this study, huPON2 was engineered for soluble expression with minimal introduction of foreign sequences. On the basis of structure modeling, degenerate linkers were exploited for the removal of hydrophobic helices of huPON2 without disrupting its folding structure and thus retaining its enzymatic function. High soluble expression levels were achieved with a yield of 76 mg of fully human PON2 variants per liter of culture media. Particularly, two clones, D2 and E3, showed significant quorum quenching (QQ) bioactivities and effectively impeded Pseudomonas aeruginosa swimming and swarming motilities, signs of an early stage of biofilm formation. In addition, by correlating QQ with luminescence signal readouts, quantitative analysis of QQ toward natural or non-natural AHL-regulator combinations suggested that D2 and E3 exhibited strong lactone hydrolysis activities toward five AHLs of different side chain lengths and modifications widely utilized by a variety of biomedically important pathogens.

Multidrug Efflux Pumps at the Crossroad between Antibiotic Resistance and Bacterial Virulence

Multidrug efflux pumps can be involved in bacterial resistance to antibiotics at different levels. Some efflux pumps are constitutively expressed at low levels and contribute to intrinsic resistance. In addition, their overexpression may allow higher levels of resistance. This overexpression can be transient, in the presence of an effector (phenotypic resistance), or constitutive when mutants in the regulatory elements of the expression of efflux pumps are selected (acquired resistance). Efflux pumps are present in all cells, from human to bacteria and are highly conserved, which indicates that they are ancient elements in the evolution of different organisms. Consequently, it has been suggested that, besides antibiotic resistance, bacterial multidrug efflux pumps would likely contribute to other relevant processes of the microbial physiology. In the current article, we discuss some specific examples of the role that efflux pumps may have in the bacterial virulence of animals’ and plants’ pathogens, including the processes of intercellular communication. Based in these evidences, we propose that efflux pumps are at the crossroad between resistance and virulence of bacterial pathogens. Consequently, the comprehensive study of multidrug efflux pumps requires addressing these functions, which are of relevance for the bacterial–host interactions during infection.

Coprinopsis cinerea intracellular lactonases hydrolyze quorum sensing molecules of Gram-negative bacteria

Biofilm formation on fungal hyphae and production of antifungal molecules are strategies of bacteria in their competition with fungi for nutrients. Since these strategies are often coordinated and under control of quorum sensing by the bacteria, interference with this bacterial communication system can be used as a counter-strategy by the fungi in this competition. Hydrolysis of N-acyl-homoserine lactones (HSL), a quorum sensing molecule used by Gram-negative bacteria, by fungal cultures has been demonstrated. However, the enzymes that are responsible for this activity, have not been identified. In this study, we identified and characterized two paralogous HSL hydrolyzing enzymes from the coprophilous fungus Coprinopsis cinerea. The C. cinerea HSL lactonases belong to the metallo-β-lactamase family and show sequence homology to and a similar biochemical activity as the well characterized lactonase AiiA from Bacillus thuringiensis. We show that the fungal lactonases, similar to the bacterial enzymes, are kept intracellularly and act as a sink for the bacterial quorum sensing signals both in C. cinerea and in Saccharomyces cerevisiae expressing C. cinerea lactonases, due to the ability of these signal molecules to diffuse over the fungal cell wall and plasma membrane. The two isogenes coding for the C. cinerea HSL lactonases are arranged in the genome as a tandem repeat and expressed preferentially in vegetative mycelium. The occurrence of orthologous genes in genomes of other basidiomycetes appears to correlate with a saprotrophic lifestyle.

Correlation between biofilm production, antibiotic susceptibility and exopolysaccharide composition in Burkholderia pseudomallei bpsI, ppk, and rpoS mutant strains

Burkholderia pseudomallei is the cause of melioidosis, a fatal tropical infectious disease, which has been reported to have a high rate of recurrence, even when an intensive dose of antibiotics is used. Biofilm formation is believed to be one of the possible causes of relapse because of its ability to increase drug resistance. EPS in biofilms have been reported to be related to the limitation of antibiotic penetration in B. pseudomallei. However, the mechanisms by which biofilms restrict the diffusion of antibiotics remain unclear. The present study presents a correlation between exopolysaccharide production in biofilm matrix and antibiotic resistance in B. pseudomallei using bpsI, ppk, and rpoS mutant strains. CLSM revealed a reduction in exopolysaccharide production and disabled micro-colony formation in B. pseudomallei mutants, which paralleled the antibiotic resistance. Different ratios of carbohydrate contents in the exopolysaccharides of the mutants were detected, although they have the same components, including glucose, galactose, mannose, and rhamnose, with the exception being that no detectable rhamnose peak was observed in the bpsI mutant. These results indicate that the correlation between these phenomena in the B. pseudomallei biofilm at least results from the exopolysaccharide, which may be under the regulation of bpsI, ppk, or rpoS genes.

Utilization of Whole-Cell MALDI-TOF Mass Spectrometry to Differentiate Burkholderia pseudomallei Wild-Type and Constructed Mutants

Whole-cell matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (whole-cell MALDI-TOF MS) has been widely adopted as a useful technology in the identification and typing of microorganisms. This study employed the whole-cell MALDI-TOF MS to identify and differentiate wild-type and mutants containing constructed single gene mutations of Burkholderia pseudomallei, a pathogenic bacterium causing melioidosis disease in both humans and animals. Candidate biomarkers for the B. pseudomallei mutants, including rpoS, ppk, and bpsI isolates, were determined. Taxon-specific and clinical isolate-specific biomarkers of B. pseudomallei were consistently found and conserved across all average mass spectra. Cluster analysis of MALDI spectra of all isolates exhibited separate distribution. A total of twelve potential mass peaks discriminating between wild-type and mutant isolates were identified using ClinProTools analysis. Two peaks (m/z 2721 and 2748 Da) were specific for the rpoS isolate, three (m/z 3150, 3378, and 7994 Da) for ppk, and seven (m/z 3420, 3520, 3587, 3688, 4623, 4708, and 5450 Da) for bpsI. Our findings demonstrated that the rapid, accurate, and reproducible mass profiling technology could have new implications in laboratory-based rapid differentiation of extensive libraries of genetically altered bacteria.

The Burkholderia pseudomallei Proteins BapA and BapC Are Secreted TTSS3 Effectors and BapB Levels Modulate Expression of BopE

Many Gram-negative pathogens use a type III secretion system (TTSS) for the injection of bacterial effector proteins into host cells. The injected effector proteins play direct roles in modulation of host cell pathways for bacterial benefit. Burkholderia pseudomallei, the causative agent of melioidosis, expresses three different TTSSs. One of these systems, the TTSS3, is essential for escape from host endosomes and therefore intracellular survival and replication. Here we have characterized three putative TTSS3 proteins; namely BapA, BapB and BapC. By employing a tetracysteine (TC)-FlAsH^™ labelling technique to monitor the secretion of TC-tagged fusion proteins, BapA and BapC were shown to be secreted during in vitro growth in a TTSS3-dependant manner, suggesting a role as TTSS3 effectors. Furthermore, we constructed B. pseudomallei bapA, bapB and bapC mutants and used the well-characterized TTSS3 effector BopE as a marker of secretion to show that BapA, BapB and BapC are not essential for the secretion process. However, BopE transcription and secretion were significantly increased in the bapB mutant, suggesting that BapB levels modulate BopE expression. In a BALB/c mouse model of acute melioidosis, the bapA, bapB and bapC mutants showed a minor reduction of in vivo fitness. Thus, this study defines BapA and BapC as novel TTSS3 effectors, BapB as a regulator of BopE production, and all three as necessary for full B. pseudomallei in vivo fitness.

Burkholderia pseudomallei kynB plays a role in AQ production, biofilm formation, bacterial swarming and persistence

Kynurenine formamidase (KynB) forms part of the kynurenine pathway which metabolises tryptophan to anthranilate. This metabolite can be used for downstream production of 2-alkyl-4-quinolone (AQ) signalling molecules that control virulence in Pseudomonas aeruginosa. Here we investigate the role of kynB in the production of AQs and virulence-associated phenotypes of Burkholderia pseudomallei K96243, the causative agent of melioidosis. Deletion of kynB resulted in reduced AQ production, increased biofilm formation, decreased swarming and increased tolerance to ciprofloxacin. Addition of exogenous anthranilic acid restored the biofilm phenotype, but not the persister phenotype. This study suggests the kynurenine pathway is a critical source of anthranilate and signalling molecules that may regulate B. pseudomallei virulence.

RovS and its associated signaling peptide form a cell-to-cell communication system required for Streptococcus agalactiae pathogenesis

Bacteria can communicate with each other to coordinate their biological functions at the population level. In a previous study, we described a cell-to-cell communication system in streptococci that involves a transcriptional regulator belonging to the Rgg family and short hydrophobic peptides (SHPs) that act as signaling molecules. Streptococcus agalactiae, an opportunistic pathogenic bacterium responsible for fatal infections in neonates and immunocompromised adults, has one copy of the shp/rgg locus. The SHP-associated Rgg is called RovS in S. agalactiae. In this study, we found that the SHP/RovS cell-to-cell communication system is active in the strain NEM316 of S. agalactiae, and we identified different partners that are involved in this system, such as the Eep peptidase, the PptAB, and the OppA1-F oligopeptide transporters. We also identified a new target gene controlled by this system and reexamined the regulation of a previously proposed target gene, fbsA, in the context of the SHP-associated RovS system. Furthermore, our results are the first to indicate the SHP/RovS system specificity to host liver and spleen using a murine model, which demonstrates its implication in streptococci virulence. Finally, we observed that SHP/RovS regulation influences S. agalactiae's ability to adhere to and invade HepG2 hepatic cells. Hence, the SHP/RovS cell-to-cell communication system appears to be an essential mechanism that regulates pathogenicity in S. agalactiae and represents an attractive target for the development of new therapeutic strategies.

IMPORTANCE

Rgg regulators and their cognate pheromones, called small hydrophobic peptides (SHPs), are present in nearly all streptococcal species. The general pathways of the cell-to-cell communication system in which Rgg and SHP take part are well understood. However, many other players remain unidentified, and the direct targets of the system, as well as its link to virulence, remain unclear. Here, we identified the different players involved in the SHP/Rgg system in S. agalactiae, which is the leading agent of severe infections in human newborns. We have identified a direct target of the Rgg regulator in S. agalactiae (called RovS) and examined a previously proposed target, all in the context of associated SHP. For the first time, we have also demonstrated the implication of the SHP/RovS mechanism in virulence, as well as its host organ specificity. Thus, this cell-to-cell communication system may represent a future target for S. agalactiae disease treatment.

Melioidosis: molecular aspects of pathogenesis

Burkholderia pseudomallei is a gram-negative bacterium that causes melioidosis, a multifaceted disease that is highly endemic in southeast Asia and northern Australia. This facultative intracellular pathogen possesses a large genome that encodes a wide array of virulence factors that promote survival in vivo by manipulating host cell processes and disarming elements of the host immune system. Antigens and systems that play key roles in B. pseudomallei virulence include capsular polysaccharide, lipopolysaccharide, adhesins, specialized secretion systems, actin-based motility and various secreted factors. This review provides an overview of the current and steadily expanding knowledge regarding the molecular mechanisms used by this organism to survive within a host and their contribution to the pathogenesis of melioidosis.

Cross-species comparison of the Burkholderia pseudomallei, Burkholderia thailandensis, and Burkholderia mallei quorum-sensing regulons

Burkholderia pseudomallei, Burkholderia thailandensis, and Burkholderia mallei (the Bptm group) are close relatives with very different lifestyles: B. pseudomallei is an opportunistic pathogen, B. thailandensis is a nonpathogenic saprophyte, and B. mallei is a host-restricted pathogen. The acyl-homoserine lactone quorum-sensing (QS) systems of these three species show a high level of conservation. We used transcriptome sequencing (RNA-seq) to define the quorum-sensing regulon in each species, and we performed a cross-species analysis of the QS-controlled orthologs. Our analysis revealed a core set of QS-regulated genes in all three species, as well as QS-controlled factors shared by only two species or unique to a given species. This global survey of the QS regulons of B. pseudomallei, B. thailandensis, and B. mallei serves as a platform for predicting which QS-controlled processes might be important in different bacterial niches and contribute to the pathogenesis of B. pseudomallei and B. mallei.

Unusual multiple production of N-acylhomoserine lactones a by Burkholderia sp. strain C10B isolated from dentine caries

Bacteria realize the ability to communicate by production of quorum sensing (QS) molecules called autoinducers, which regulate the physiological activities in their ecological niches. The oral cavity could be a potential area for the presence of QS bacteria. In this study, we report the isolation of a QS bacterial isolate C10B from dentine caries. Preliminary screening using Chromobacterium violaceum CV026 biosensor showed that isolate C10B was able to produce N-acylhomoserine lactones (AHLs). This bacterium was further identified as a member of Burkholderia, an opportunistic pathogen. The isolated Burkholderia sp. was confirmed to produce N-hexanoyl-L-homoserine lactone (C6-HSL), N-octanoyl-L-homoserine lactone (C8-HSL), N-decanoyl-L-homoserine lactone (C10-HSL) and N-dodecanoyl-L-homoserine lactone (C12-HSL).

Global analysis of the Burkholderia thailandensis quorum sensing-controlled regulon

Burkholderia thailandensis contains three acyl-homoserine lactone quorum sensing circuits and has two additional LuxR homologs. To identify B. thailandensis quorum sensing-controlled genes, we carried out transcriptome sequencing (RNA-seq) analyses of quorum sensing mutants and their parent. The analyses were grounded in the fact that we identified genes coding for factors shown previously to be regulated by quorum sensing among a larger set of quorum-controlled genes. We also found that genes coding for contact-dependent inhibition were induced by quorum sensing and confirmed that specific quorum sensing mutants had a contact-dependent inhibition defect. Additional quorum-controlled genes included those for the production of numerous secondary metabolites, an uncharacterized exopolysaccharide, and a predicted chitin-binding protein. This study provides insights into the roles of the three quorum sensing circuits in the saprophytic lifestyle of B. thailandensis, and it provides a foundation on which to build an understanding of the roles of quorum sensing in the biology of B. thailandensis and the closely related pathogenic Burkholderia pseudomallei and Burkholderia mallei.

Alteration of the phenotypic and pathogenic patterns of Burkholderia pseudomallei that persist in a soil environment

Melioidosis is caused by the soil-borne pathogen Burkholderia pseudomallei. To investigate whether the distinct phenotypic and virulent characteristics result from environmental adaptations in the soil or from the host body, two pairs of isogenic strains were generated by passages in soil or mice. After cultivation in soil, the levels of 3-hydroxytetradecanoic acid, biofilm formation, flagellar expression, and ultrastructure were altered in the bacteria. Uniformly fatal melioidosis developed as a result of infection with mouse-derived strains; however, the survival rates of mice infected with soil-derived strains prolonged. After primary infection or reinfection with soil-derived strains, the mice developed a low degree of bacterial hepatitis and bacterial colonization in the liver and bone marrow compared with mice that were infected with isogenic or heterogenic mouse-derived strains. We suggest that specific phenotypic and pathogenic patterns can be induced through infection with B. pseudomallei that has been cultured in different (soil versus mouse) environments.

Plant-associated symbiotic Burkholderia species lack hallmark strategies required in mammalian pathogenesis

Burkholderia is a diverse and dynamic genus, containing pathogenic species as well as species that form complex interactions with plants. Pathogenic strains, such as B. pseudomallei and B. mallei, can cause serious disease in mammals, while other Burkholderia strains are opportunistic pathogens, infecting humans or animals with a compromised immune system. Although some of the opportunistic Burkholderia pathogens are known to promote plant growth and even fix nitrogen, the risk of infection to infants, the elderly, and people who are immunocompromised has not only resulted in a restriction on their use, but has also limited the application of non-pathogenic, symbiotic species, several of which nodulate legume roots or have positive effects on plant growth. However, recent phylogenetic analyses have demonstrated that Burkholderia species separate into distinct lineages, suggesting the possibility for safe use of certain symbiotic species in agricultural contexts. A number of environmental strains that promote plant growth or degrade xenobiotics are also included in the symbiotic lineage. Many of these species have the potential to enhance agriculture in areas where fertilizers are not readily available and may serve in the future as inocula for crops growing in soils impacted by climate change. Here we address the pathogenic potential of several of the symbiotic Burkholderia strains using bioinformatics and functional tests. A series of infection experiments using Caenorhabditis elegans and HeLa cells, as well as genomic characterization of pathogenic loci, show that the risk of opportunistic infection by symbiotic strains such as B. tuberum is extremely low.

Novel Burkholderia mallei virulence factors linked to specific host-pathogen protein interactions

Burkholderia mallei is an infectious intracellular pathogen whose virulence and resistance to antibiotics makes it a potential bioterrorism agent. Given its genetic origin as a commensal soil organism, it is equipped with an extensive and varied set of adapted mechanisms to cope with and modulate host-cell environments. One essential virulence mechanism constitutes the specialized secretion systems that are designed to penetrate host-cell membranes and insert pathogen proteins directly into the host cell's cytosol. However, the secretion systems' proteins and, in particular, their host targets are largely uncharacterized. Here, we used a combined in silico, in vitro, and in vivo approach to identify B. mallei proteins required for pathogenicity. We used bioinformatics tools, including orthology detection and ab initio predictions of secretion system proteins, as well as published experimental Burkholderia data to initially select a small number of proteins as putative virulence factors. We then used yeast two-hybrid assays against normalized whole human and whole murine proteome libraries to detect and identify interactions among each of these bacterial proteins and host proteins. Analysis of such interactions provided both verification of known virulence factors and identification of three new putative virulence proteins. We successfully created insertion mutants for each of these three proteins using the virulent B. mallei ATCC 23344 strain. We exposed BALB/c mice to mutant strains and the wild-type strain in an aerosol challenge model using lethal B. mallei doses. In each set of experiments, mice exposed to mutant strains survived for the 21-day duration of the experiment, whereas mice exposed to the wild-type strain rapidly died. Given their in vivo role in pathogenicity, and based on the yeast two-hybrid interaction data, these results point to the importance of these pathogen proteins in modulating host ubiquitination pathways, phagosomal escape, and actin-cytoskeleton rearrangement processes.

The condition-dependent transcriptional landscape of Burkholderia pseudomallei

Burkholderia pseudomallei (Bp), the causative agent of the often-deadly infectious disease melioidosis, contains one of the largest prokaryotic genomes sequenced to date, at 7.2 Mb with two large circular chromosomes (1 and 2). To comprehensively delineate the Bp transcriptome, we integrated whole-genome tiling array expression data of Bp exposed to >80 diverse physical, chemical, and biological conditions. Our results provide direct experimental support for the strand-specific expression of 5,467 Sanger protein-coding genes, 1,041 operons, and 766 non-coding RNAs. A large proportion of these transcripts displayed condition-dependent expression, consistent with them playing functional roles. The two Bp chromosomes exhibited dramatically different transcriptional landscapes — Chr 1 genes were highly and constitutively expressed, while Chr 2 genes exhibited mosaic expression where distinct subsets were expressed in a strongly condition-dependent manner. We identified dozens of cis-regulatory motifs associated with specific condition-dependent expression programs, and used the condition compendium to elucidate key biological processes associated with two complex pathogen phenotypes — quorum sensing and in vivo infection. Our results demonstrate the utility of a Bp condition-compendium as a community resource for biological discovery. Moreover, the observation that significant portions of the Bp virulence machinery can be activated by specific in vitro cues provides insights into Bp's capacity as an “accidental pathogen”, where genetic pathways used by the bacterium to survive in environmental niches may have also facilitated its ability to colonize human hosts.

Bacterial transcriptomes are dynamic, context-specific and condition-dependent. Infection by the soil bacterium, Burkholderia pseudomallei, causes melioidosis, an often fatal infectious disease of humans and animals. Possessing a large multi-chromosomal genome, B. pseudomallei is able to persist and survive in a multitude of environments. To obtain a comprehensive overview of B. pseudomallei expressed transcripts, we initiated whole-genome transcriptome profiling covering a broad spectrum of conditions and exposures — a so-called “condition compendium”. Using the compendium, we confirmed many previously-annotated genes and operons, and also identified hundreds of novel transcripts including anti-sense transcripts and non-coding RNAs. By systematically examining genes exhibiting highly similar expression patterns, we ascribed putative functions to previously uncharacterized genes, and identified novel regulatory elements controlling these expression patterns. We also used the compendium to elucidate candidate virulence pathways associated with quorum-sensing and infection in mice. Our study showcases the power of a B. pseudomallei condition compendium as a valuable resource for understanding microbial physiology and the pathogenesis of melioidosis.

Quorum sensing negatively regulates multinucleate cell formation during intracellular growth of Burkholderia pseudomallei in macrophage-like cells

Burkholderia pseudomallei is a Gram-negative environmental bacterium and the causative agent of melioidosis, a potentially fatal, acute or chronic disease endemic in the tropics. Acyl homoserine lactone (AHL)-mediated quorum sensing and signalling have been associated with virulence and biofilm formation in numerous bacterial pathogens. In the canonical acyl-homoserine lactone signalling paradigm, AHLs are detected by a response regulator. B. pseudomallei encodes three AHL synthases, encoded by bpsI1, bpsI2 and bpsI3, and five regulator genes. In this study, we mutated the B. pseudomallei AHL synthases individually and in double and triple combination. Five AHLs were detected and quantified by tandem liquid chromatography-mass spectroscopy. The major AHLs produced were N-octanoylhomoserine lactone and N-(3-hydroxy-decanoyl)homoserine lactone, the expression of which depended on bpsI1 and bpsI2, respectively. B. pseudomallei infection of macrophage cells causes cell fusion, leading to multinucleated cells (3 or more nuclei per cell). A triple mutant defective in production of all three AHL synthases was associated with a striking phenotype of massively enhanced host cellular fusion in macrophages. However, neither abrogation of host cell fusion, achieved by mutation of bimA or hcp1, nor enhancement of fusion altered intracellular replication of B. pseudomallei. Furthermore, when tested in murine models of acute melioidosis the AHL synthase mutants were not attenuated for virulence. Collectively, this study identifies important new aspects of the genetic basis of AHL synthesis in B. pseudomallei and the roles of these AHLs in systemic infection and in cell fusion in macrophages for this important human pathogen.

Virulence of Burkholderia mallei quorum-sensing mutants

Many Proteobacteria use acyl-homoserine lactone-mediated quorum-sensing (QS) to activate specific sets of genes as a function of cell density. QS often controls the virulence of pathogenic species, and in fact a previous study indicated that QS was important for Burkholderia mallei mouse lung infections. To gain in-depth information on the role of QS in B. mallei virulence, we constructed and characterized a mutant of B. mallei strain GB8 that was unable to make acyl-homoserine lactones. The QS mutant showed virulence equal to that of its wild-type parent in an aerosol mouse infection model, and growth in macrophages was indistinguishable from that of the parent strain. Furthermore, we assessed the role of QS in B. mallei ATCC 23344 by constructing and characterizing a mutant strain producing AiiA, a lactonase enzyme that degrades acyl-homoserine lactones. Although acyl-homoserine lactone levels in cultures of this strain are very low, it showed full virulence. Contrary to the previous report, we conclude that QS is not required for acute B. mallei infections of mice. QS may be involved in some stage of chronic infections in the natural host of horses, or the QS genes may be remnants of the QS network in B. pseudomallei from which this host-adapted pathogen evolved.

Efficacy of post exposure administration of doxycycline in a murine model of inhalational melioidosis

Burkholderia pseudomallei is the causative agent of melioidosis. Treatment of melioidosis is suboptimal and developing improved melioidosis therapies requires animal models. In this report, we exposed male BALB/c mice to various amounts of aerosolized B. pseudomallei 1026b to determine lethality. After establishing a median lethal dose (LD(50)) of 2,772 colony forming units (cfu)/animal, we tested the ability of doxycycline administered 6 hours after exposure to a uniformly lethal dose of ~20 LD(50) to prevent death and eliminate bacteria from the lung and spleens. Tissue bacterial burdens were examined by PCR analysis. We found that 100% of mice treated with doxycycline survived and B. pseudomallei DNA was not amplified from the lungs or spleens of most surviving mice. We conclude the BALB/c mouse is a useful model of melioidosis. Furthermore, the data generated in this mouse model indicate that doxycycline is likely to be effective in post-exposure prophylaxis of melioidosis.

Transition States, analogues, and drug development

Enzymes achieve their transition states by dynamic conformational searches on the femtosecond to picosecond time scale. Mimics of reactants at enzymatic transition states bind tightly to enzymes by stabilizing the conformation optimized through evolution for transition state formation. Instead of forming the transient transition state geometry, transition state analogues convert the short-lived transition state to a stable thermodynamic state. Enzymatic transition states are understood by combining kinetic isotope effects and computational chemistry. Analogues of the transition state can bind millions of times more tightly than substrates and show promise for drug development for several targets.

Bacterial quorum sensing, cooperativity, and anticipation of stationary-phase stress

Acyl-homoserine lactone-mediated quorum sensing (QS) regulates diverse activities in many species of Proteobacteria. QS-controlled genes commonly code for production of secreted or excreted public goods. The acyl-homoserine lactones are synthesized by members of the LuxI signal synthase family and are detected by cognate members of the LuxR family of transcriptional regulators. QS affords a means of population density-dependent gene regulation. Control of public goods via QS provides a fitness benefit. Another potential role for QS is to anticipate overcrowding. As population density increases and stationary phase approaches, QS might induce functions important for existence in stationary phase. Here we provide evidence that in three related species of the genus Burkholderia QS allows individuals to anticipate and survive stationary-phase stress. Survival requires QS-dependent activation of cellular enzymes required for production of excreted oxalate, which serves to counteract ammonia-mediated alkaline toxicity during stationary phase. Our findings provide an example of QS serving as a means to anticipate stationary phase or life at the carrying capacity of a population by activating the expression of cytoplasmic enzymes, altering cellular metabolism, and producing a shared resource or public good, oxalate.

Malleilactone, a polyketide synthase-derived virulence factor encoded by the cryptic secondary metabolome of Burkholderia pseudomallei group pathogens

Sequenced bacterial genomes are routinely found to contain gene clusters that are predicted to encode metabolites not seen in fermentation-based studies. Pseudomallei group Burkholderia are emerging pathogens whose genomes are particularly rich in cryptic natural product biosynthetic gene clusters. We systematically probed the influence of the cryptic secondary metabolome on the virulence of these bacteria and found that disruption of the MAL gene cluster, which is natively silent in laboratory fermentation experiments and conserved across this group of pathogens, attenuates virulence in animal models. Using a promoter exchange strategy to activate the MAL cluster, we identified malleilactone, a polyketide synthase-derived cytotoxic siderophore encoded by this gene cluster. Small molecules targeting malleilactone biosynthesis either alone or in conjunction with antibiotics could prove useful as therapeutics to combat melioidosis and glanders.

Common features of environmental and potentially beneficial plant-associated Burkholderia

The genus Burkholderia comprises more than 60 species isolated from a wide range of niches. Although they have been shown to be diverse and ubiquitously distributed, most studies have thus far focused on the pathogenic species due to their clinical importance. However, the increasing number of recently described Burkholderia species associated with plants or with the environment has highlighted the division of the genus into two main clusters, as suggested by phylogenetical analyses. The first cluster includes human, animal, and plant pathogens, such as Burkholderia glumae, Burkholderia pseudomallei, and Burkholderia mallei, as well as the 17 defined species of the Burkholderia cepacia complex, while the other, more recently established cluster comprises more than 30 non-pathogenic species, which in most cases have been found to be associated with plants, and thus might be considered to be potentially beneficial. Several species from the latter group share characteristics that are of use when associating with plants, such as a quorum sensing system, the presence of nitrogen fixation and/or nodulation genes, and the ability to degrade aromatic compounds. This review examines the commonalities in this growing subgroup of Burkholderia species and discusses their prospective biotechnological applications.

GacA directly regulates expression of several virulence genes in Pseudomonas syringae pv. tabaci 11528

A two-component system comprising GacS and GacA affects a large number of traits in many Gram-negative bacteria. However, the signals to which GacS responds, the regulation mechanism for GacA expression, and the genes GacA controls are not yet clear. In this study, several phenotypic tests and tobacco-leaf pathogenicity assays were conducted using a gacA deletion mutant strain (BL473) of Pseudomonas syringae pv. tabaci 11528. To determine the regulation mechanism for gacA gene expression and to identify GacA-regulated genes, we conducted quantitative RT-PCR and electrophoretic mobility shift assay (EMSA) experiments. The results indicated that virulence traits related to the pathogenesis of P. syringae pv. tabaci 11528 are regulated coordinately by GacA and iron availability. They also revealed that several systems coordinately regulate gacA gene expression in response to iron concentration and bacterial cell density and that GacA and iron together control the expression of several virulence genes. EMSA results provided genetic and molecular evidence for direct control of virulence genes by GacA.

N-Octanoylhomoserine lactone signalling mediated by the BpsI–BpsR quorum sensing system plays a major role in biofilm formation of Burkholderia pseudomallei

The genome of Burkholderia pseudomallei encodes three acylhomoserine lactone (AHL) quorum sensing systems, each comprising an AHL synthase and a signal receptor/regulator. The BpsI–BpsR system produces N-octanoylhomoserine lactone (C8HL) and is positively auto-regulated by its AHL product. The products of the remaining two systems have not been identified. In this study, tandem MS was used to identify and quantify the AHL species produced by three clinical B. pseudomallei isolates – KHW, K96243 and H11 – three isogenic KHW mutants that each contain a null mutation in an AHL synthase gene, and recombinant Escherichia coli heterologously expressing each of the three B. pseudomallei AHL synthase genes. BpsI synthesized predominantly C8HL, which accounted for more than 95 % of the extracellular AHLs produced in stationary-phase KHW cultures. The major products of BpsI2 and BpsI3 were N-(3-hydroxy-octanoyl)homoserine lactone (OHC8HL) and N-(3-hydroxy-decanoyl)homoserine lactone, respectively, and their corresponding transcriptional regulators, BpsR2 and BpsR3, were capable of driving reporter gene expression in the presence of these cognate lactones. Formation of biofilm by B. pseudomallei KHW was severely impaired in mutants lacking either BpsI or BpsR but could be restored to near wild-type levels by exogenous C8HL. BpsI2 was not required, and BpsI3 was partially required for biofilm formation. Unlike the bpsI mutant, biofilm formation in the bpsI3 mutant could not be restored to wild-type levels in the presence of OHC8HL, the product of BpsI3. C8HL and OHC8HL had opposite effects on biofilm formation; exogenous C8HL enhanced biofilm formation in both the bpsI3 mutant and wild-type KHW while exogenous OHC8HL suppressed the formation of biofilm in the same strains. We propose that exogenous OHC8HL antagonizes biofilm formation in B. pseudomallei, possibly by competing with endogenous C8HL for binding to BpsR.

A product analog bound form of 3-oxoadipate-enol-lactonase (PcaD) reveals a multifunctional role for the divergent cap domain

Lactones are a class of structurally diverse molecules that serve essential roles in biological processes ranging from quorum sensing to the aerobic catabolism of aromatic compounds. Not surprisingly, enzymes involved in the bioprocessing of lactones are often targeted for protein engineering studies with the potential, for example, of optimized bioremediation of aromatic pollutants. The enol-lactone hydrolase (ELH) represents one such class of targeted enzymes and catalyzes the conversion of 3-oxoadipate-enol-lactone into the linear β-ketoadipate. To define the structural details that govern ELH catalysis and assess the impact of divergent features predicted by sequence analysis, we report the first structural characterization of an ELH (PcaD) from Burkholderia xenovorans LB400 in complex with the product analog levulinic acid. The overall dimeric structure of PcaD reveals an α-helical cap domain positioned atop a core α/β-hydrolase domain. Despite the localization of the conserved catalytic triad to the core domain, levulinic acid is bound largely within the region of the active site defined by the cap domain, suggesting a key role for this divergent substructure in mediating product release. Furthermore, the architecture of the cap domain results in an unusually deep active-site pocket with topological features to restrict binding to small or kinked substrates. The evolutionary basis for this substrate selectivity is discussed with respect to the homologous dienelactone hydrolase. Overall, the PcaD costructure provides a detailed insight into the intimate role of the cap domain in influencing all aspects of substrate binding, turnover, and product release.