The Burkholderia pseudomallei RpoE (AlgU) operon is involved in environmental stress tolerance and biofilm formation

Comparison of the colonization ability of Burkholderia strain B23 in the citrus rhizoplane and rhizosphere and assessment of the underlying mechanisms using full-length 16S rDNA amplicon and metatranscriptomic analyses

The characterization of bacterial strains with efficient root colonization ability and the mechanisms responsible for their efficient colonization is critical for the identification and application of beneficial bacteria. In this study, we found that Burkholderia strain B23 exhibited a strong niche differentiation between the rhizosphere and rhizoplane (a niche with more abundant easy-to-use nutrients but stronger selective pressures compared with the tightly adjacent rhizosphere) when inoculated into the field-grown citrus trees. Full-length 16S rDNA amplicon analysis demonstrated that the relative abundance of B23 in the rhizoplane microbiome at 3, 5, and 9 days post-inoculation (dpi) was always higher than that at 1 dpi, whereas its relative abundance in the rhizosphere microbiome was decreased continuously, as demonstrated by a 3.18-fold decrease at 9 dpi compared to 1 dpi. Time-series comparative expression profiling of B23 between the rhizoplane and rhizosphere was performed at representative time points (1, 3, and 9 dpi) through metatranscriptomic analysis, and the results demonstrated that multiple genes involved in the uptake and utilization of easy-to-use carbohydrates and amino acids and those involved in metabolism, energy production, replication, and translation were upregulated in the rhizoplane compared with the rhizosphere at 1 dpi and 3 dpi. Several genes involved in resistance to plant- and microbial competitor-derived stresses exhibited higher expression activities in the rhizoplane compared with the rhizosphere. Furthermore, gene loci responsible for the biosynthesis of the key antifungal and antibacterial metabolites occidiofungin and ornibactin were induced, and their expression levels remained relatively stable from 3 dpi to 9 dpi in the rhizoplane but not in the rhizosphere. Collectively, our findings provide novel lights into the mechanisms underlying the root colonization of the inoculated bacterial strains and serve as a basis for the identification of strains with efficient colonization ability, thus contributing to the development of beneficial bacteria applications.

Pathogenicity and virulence of Burkholderia pseudomallei

The soil saprophyte, Burkholderia pseudomallei, is the causative agent of melioidosis, a disease endemic in South East Asia and northern Australia. Exposure to B. pseudomallei by either inhalation or inoculation can lead to severe disease. B. pseudomallei rapidly shifts from an environmental organism to an aggressive intracellular pathogen capable of rapidly spreading around the body. The expression of multiple virulence factors at every stage of intracellular infection allows for rapid progression of infection. Following invasion or phagocytosis, B. pseudomallei resists host-cell killing mechanisms in the phagosome, followed by escape using the type III secretion system. Several secreted virulence factors manipulate the host cell, while bacterial cells undergo a shift in energy metabolism allowing for overwhelming intracellular replication. Polymerisation of host cell actin into “actin tails” propels B. pseudomallei to the membranes of host cells where the type VI secretion system fuses host cells into multinucleated giant cells (MNGCs) to facilitate cell-to-cell dissemination. This review describes the various mechanisms used by B. pseudomallei to survive within cells.

Cpx-signalling facilitates Hms-dependent biofilm formation by Yersinia pseudotuberculosis

Bacteria often reside in sessile communities called biofilms, where they adhere to a variety of surfaces and exist as aggregates in a viscous polymeric matrix. Biofilms are resistant to antimicrobial treatments, and are a major contributor to the persistence and chronicity of many bacterial infections. Herein, we determined that the CpxA-CpxR two-component system influenced the ability of enteropathogenic Yersinia pseudotuberculosis to develop biofilms. Mutant bacteria that accumulated the active CpxR~P isoform failed to form biofilms on plastic or on the surface of the Caenorhabditis elegans nematode. A failure to form biofilms on the worm surface prompted their survival when grown on the lawns of Y. pseudotuberculosis. Exopolysaccharide production by the hms loci is the major driver of biofilms formed by Yersinia. We used a number of molecular genetic approaches to demonstrate that active CpxR~P binds directly to the promoter regulatory elements of the hms loci to activate the repressors of hms expression and to repress the activators of hms expression. Consequently, active Cpx-signalling culminated in a loss of exopolysaccharide production. Hence, the development of Y. pseudotuberculosis biofilms on multiple surfaces is controlled by the Cpx-signalling, and at least in part this occurs through repressive effects on the Hms-dependent exopolysaccharide production.

AlgU, a Conserved Sigma Factor Regulating Abiotic Stress Tolerance and Promoting Virulence in Pseudomonas syringae

Pseudomonas syringae can rapidly deploy specialized functions to deal with abiotic and biotic stresses. Host niches pose specific sets of environmental challenges driven, in part, by immune defenses. Bacteria use a "just-in-time" strategy of gene regulation, meaning that they only produce the functions necessary for survival as needed. Extracytoplasmic function (ECF) sigma factors transduce a specific set of environmental signals and change gene expression patterns by altering RNA polymerase promoter specificity, to adjust bacterial physiology, structure, or behavior, singly or in combination, to improve chances of survival. The broadly conserved ECF sigma factor AlgU affects virulence in both animal and plant pathogens. Pseudomonas syringae AlgU controls expression of more than 800 genes, some of which contribute to suppression of plant immunity and bacterial fitness in plants. This review discusses AlgU activation mechanisms, functions controlled by AlgU, and how these functions contribute to P. syringae survival in plants.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law. 2021.

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.

New insights into the responses of soil microorganisms to polycyclic aromatic hydrocarbon stress by combining enzyme activity and sequencing analysis with metabolomics

Polycyclic aromatic hydrocarbons (PAHs), some of the most widespread organic contaminants, are highly toxic to soil microorganisms. Whether long-term polluted soils can still respond to the fresh input of pollutants is unknown. In this study, the soil enzyme activity, soil microbial community structure and function and microbial metabolism pathways were examined to systematically investigate the responses of soil microorganisms to fresh PAH stress. Microbial activity as determined by soil dehydrogenase and urease activity was inhibited upon microbe exposure to PAH stress. In addition, the soil microbial community and function were obviously shifted under PAH stress. Both microbial diversity and richness were decreased by PAH stress. Rhizobacter, Sphingobium, Mycobacterium, Massilia, Bacillus and Pseudarthrobacter were significantly affected by PAH stress and can be considered important indicators of PAH contamination in agricultural soils. Moreover, the majority of microbial metabolic function predicted to respond to PAH stress were affected adversely. Finally, soil metabolomics further revealed specific inhibition of soil metabolism pathways associated with fatty acids, carbohydrates and amino acids. Therefore, the soil metabolic composition distinctively changed, reflecting a change in the soil metabolism. In summary, fresh contaminant introduction into long-term polluted soils inhibited microbial activity and metabolism, which might profoundly affect the whole soil quality.

Burkholderia pseudomallei Adaptation for Survival in Stressful Conditions

Burkholderia pseudomallei is a Gram-negative bacterium that causes melioidosis, which can be fatal in humans. Melioidosis is prevalent in the tropical regions of Southeast Asia and Northern Australia. Ecological data have shown that this bacterium can survive as a free-living organism in environmental niches, such as soil and water, as well as a parasite living in host organisms, such as ameba, plants, fungi, and animals. This review provides an overview of the survival and adaptation of B. pseudomallei to stressful conditions induced by hostile environmental factors, such as salinity, oxidation, and iron levels. The adaptation of B. pseudomallei in host cells is also reviewed. The adaptive survival mechanisms of this pathogen mainly involve modulation of gene and protein expression, which could cause alterations in the bacteria's cell membrane, metabolism, and virulence. Understanding the adaptations of this organism to environmental factors provides important insights into the survival and pathogenesis of B. pseudomallei, which may lead to the development of novel strategies for the control, prevention, and treatment of melioidosis in the future.

Effects of sodium chloride on heat resistance, oxidative susceptibility, motility, biofilm and plaque formation of Burkholderia pseudomallei

Burkholderia pseudomallei is an environmental saprophyte and the causative agent of melioidosis, a severe infectious disease prevalent in tropical areas, including southeast Asia and northern Australia. In Thailand, the highest incidence of melioidosis is in the northeast region, where saline soil and water are abundant. We hypothesized that B. pseudomallei develops an ability to thrive in saline conditions and gains a selective ecological advantage over other soil-dwelling microorganisms. However, little is known about how an elevated NaCl concentration affects survival and adaptive changes in this pathogen. In this study, we examined the adaptive changes in six isolates of B. pseudomallei after growth in Luria-Bertani medium containing different concentrations of NaCl at 37°C for 6 hr. The bacteria were then investigated for resistance to heat at 50°C and killing by hydrogen peroxide (H₂ O₂ ). In addition, flagellar production, biofilm formation, and the plaque formation efficiency of B. pseudomallei after culture in saline conditions were observed. In response to exposure to 150 and 300 mmol L^-1 NaCl, all B. pseudomallei isolates showed significantly increased thermal tolerance, oxidative resistance, and plaque-forming efficiency. However, NaCl exposure notably decreased the number of B. pseudomallei flagella. Taken together, these results provide insight into the adaptations of B. pseudomallei that might be crucial for survival and persistence in the host and/or endemic environments with high salinity.

The Secreted Protease PrtA Controls Cell Growth, Biofilm Formation and Pathogenicity in Xylella fastidiosa

Pierce's disease (PD) is a deadly disease of grapevines caused by the Gram-negative bacterium Xylella fastidiosa. Though disease symptoms were formerly attributed to bacteria blocking the plant xylem, this hypothesis is at best overly simplistic. Recently, we used a proteomic approach to characterize the secretome of X. fastidiosa, both in vitro and in planta, and identified LesA as one of the pathogenicity factors of X. fastidiosa in grapevines that leads to leaf scorching and chlorosis. Herein, we characterize another such factor encoded by PD0956, designated as an antivirulence secreted protease "PrtA" that displays a central role in controlling in vitro cell proliferation, length, motility, biofilm formation, and in planta virulence. The mutant in X. fastidiosa exhibited reduced cell length, hypermotility (and subsequent lack of biofilm formation) and hypervirulence in grapevines. These findings are supported by transcriptomic and proteomic analyses with corresponding plant infection data. Of particular interest, is the hypervirulent response in grapevines observed when X. fastidiosa is disrupted for production of PrtA, and that PD-model tobacco plants transformed to express PrtA exhibited decreased symptoms after infection by X. fastidiosa.

Transcriptional profiles of Burkholderia pseudomallei reveal the direct and indirect roles of Sigma E under oxidative stress conditions

Background

Burkholderia pseudomallei, the causative agent of melioidosis, is a Gram-negative bacterium widely distributed in soil and water in endemic areas. This soil saprophyte can survive harsh environmental conditions, even in soils where herbicides (containing superoxide generators) are abundant. Sigma factor E (σ^E) is a key regulator of extra-cytoplasmic stress response in Gram-negative bacteria. In this study, we identified the B. pseudomallei σ^E regulon and characterized the indirect role that σ^E plays in the regulation of spermidine, contributing to the successful survival of B. pseudomallei in stressful environments.

Results

Changes in the global transcriptional profiles of B. pseudomallei wild type and σ^E mutant under physiological and oxidative stress (hydrogen peroxide) conditions were determined. We identified 307 up-regulated genes under oxidative stress condition. Comparison of the transcriptional profiles of B. pseudomallei wild type and σ^E mutant under control or oxidative stress conditions identified 85 oxidative-responsive genes regulated by σ^E, including genes involved in cell membrane repair, maintenance of protein folding and oxidative stress response and potential virulence factors such as a type VI secretion system (T6SS). Importantly, we identified that the speG gene, encoding spermidine-acetyltransferase, is a novel member of the B. pseudomallei σ^E regulon. The expression of speG was regulated by σ^E, implying that σ^E plays an indirect role in the regulation of physiological level of spermidine to protect the bacteria during oxidative stress.

Conclusion

This study identified B. pseudomallei genes directly regulated by σ^E in response to oxidative stress and revealed the indirect role of σ^E in the regulation of the polyamine spermidine (via regulation of speG) for bacterial cell protection during oxidative stress. This study provides new insights into the regulatory mechanisms by which σ^E contributes to the survival of B. pseudomallei under stressful conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-787) contains supplementary material, which is available to authorized users.

Antimicrobial agents and Burkholderia pseudomallei: perspectives from Thailand

Background: Burkholderia pseudomallei are the causative agent of melioidosis, a potentially life-threatening disease in humans and animals. It is a common public health threat in parts of Thailand.

Objective: To summarizes the current knowledge regarding antimicrobial agents and B. pseudomallei.

Methods: A literature search using MEDLINE (PubMed), SCOPUS, and OVID/LWWW databases.

Results: B. pseudomallei are intrinsically resistant to a wide range of antimicrobial agents including β-lactam antibiotics, aminoglycosides, and macrolides. Antimicrobial therapy for melioidosis is divided into an acute phase and an eradication phase. The current recommendations for the acute phase are parenteral antimicrobial agents for ≥10 days using ceftazidime or a carbapenem. The eradication phase involves oral antimicrobial agents for ≥180 days using trimethoprim-sulfamethoxazole. Amoxicillin-clavulanic acid may be used as an alternative. Ceftazidime revealed rare primary resistance and a high relapse rate.

Conclusion: Patients with acute melioidosis usually need intensive care and appropriate antibiotics for the acute and eradication phases. Ceftazidime is remains an effective agent in Thailand. A trend for decreasing susceptibility to antibiotics requires monitoring.

Identification of a predicted trimeric autotransporter adhesin required for biofilm formation of Burkholderia pseudomallei

The autotransporters are a large and diverse family of bacterial secreted and outer membrane proteins, which are present in many Gram-negative bacterial pathogens and play a role in numerous environmental and virulence-associated interactions. As part of a larger systematic study on the autotransporters of Burkholderia pseudomallei, the causative agent of the severe tropical disease melioidosis, we have constructed an insertion mutant in the bpss1439 gene encoding an unstudied predicted trimeric autotransporter adhesin. The bpss1439 mutant demonstrated a significant reduction in biofilm formation at 48 hours in comparison to its parent 10276 wild-type strain. This phenotype was complemented to wild-type levels by the introduction of a full-length copy of the bpss1439 gene in trans. Examination of the wild-type and bpss1439 mutant strains under biofilm-inducing conditions by microscopy after 48 hours confirmed that the bpss1439 mutant produced less biofilm compared to wild-type. Additionally, it was observed that this phenotype was due to low levels of bacterial adhesion to the abiotic surface as well as reduced microcolony formation. In a murine melioidosis model, the bpss1439 mutant strain demonstrated a moderate attenuation for virulence compared to the wild-type strain. This attenuation was abrogated by in trans complementation, suggesting that bpss1439 plays a subtle role in the pathogenesis of B. pseudomallei. Taken together, these studies indicate that BPSS1439 is a novel predicted autotransporter involved in biofilm formation of B. pseudomallei; hence, this factor was named BbfA, Burkholderia biofilm factor A.

Role of Bordetella pertussis RseA in the cell envelope stress response and adenylate cyclase toxin release

Bordetella pertussis is the bacterial agent of the human disease such as whooping cough. In many bacteria, the extracellular function sigma factor σ^E is central to the response to envelope stress, and its activity is negatively controlled by the RseA anti-sigma factor. In this study, the role of RseA in B. pertussis envelope stress responses was investigated. Compared with the wild-type strain, an rseA mutant showed elevated resistance to envelope stress and enhanced growth at 25 °C. rpoH and other predicted σ^E target genes demonstrated increased transcription in the rseA mutant compared with the wild-type parent. Transcription of those genes was also increased in wild-type B. pertussis and Escherichia coli under envelope stress, whereas no stress-induced increase in transcription was observed in the rseA mutant. rseA inactivation was also associated with altered levels of certain proteins in culture supernatant fluids, which showed increased adenylate cyclase toxin (CyaA) levels. The increased CyaA in the mutant was correlated with an apparent increased stability of the extracellular toxin and increased production of CyaA-containing outer membrane vesicles. Consistent with this, compared with the wild-type strain, rseA mutant cells produced increased numbers of large surface-associated vesicles.

sigE facilitates the adaptation of Bordetella bronchiseptica to stress conditions and lethal infection in immunocompromised mice

Background

The cell envelope of a bacterial pathogen can be damaged by harsh conditions in the environment outside a host and by immune factors during infection. Cell envelope stress responses preserve the integrity of this essential compartment and are often required for virulence. Bordetella species are important respiratory pathogens that possess a large number of putative transcription factors. However, no cell envelope stress responses have been described in these species. Among the putative Bordetella transcription factors are a number of genes belonging to the extracytoplasmic function (ECF) group of alternative sigma factors, some of which are known to mediate cell envelope stress responses in other bacteria. Here we investigate the role of one such gene, sigE, in stress survival and pathogenesis of Bordetella bronchiseptica.

Results

We demonstrate that sigE encodes a functional sigma factor that mediates a cell envelope stress response. Mutants of B. bronchiseptica strain RB50 lacking sigE are more sensitive to high temperature, ethanol, and perturbation of the envelope by SDS-EDTA and certain β-lactam antibiotics. Using a series of immunocompromised mice deficient in different components of the innate and adaptive immune responses, we show that SigE plays an important role in evading the innate immune response during lethal infections of mice lacking B cells and T cells. SigE is not required, however, for colonization of the respiratory tract of immunocompetent mice. The sigE mutant is more efficiently phagocytosed and killed by peripheral blood polymorphonuclear leukocytes (PMNs) than RB50, and exhibits decreased cytotoxicity toward macrophages. These altered interactions with phagocytes could contribute to the defects observed during lethal infection.

Conclusions

Much of the work on transcriptional regulation during infection in B. bronchiseptica has focused on the BvgAS two-component system. This study reveals that the SigE regulon also mediates a discrete subset of functions associated with virulence. SigE is the first cell envelope stress-sensing system to be described in the bordetellae. In addition to its role during lethal infection of mice deficient in adaptive immunity, our results indicate that SigE is likely to be important for survival in the face of stresses encountered in the environment between hosts.

Biofilm formation in bacterial pathogens of veterinary importance

Bacterial biofilms are structured communities of bacterial cells enclosed in a self-produced polymer matrix that is attached to a surface. Biofilms protect and allow bacteria to survive and thrive in hostile environments. Bacteria within biofilms can withstand host immune responses, and are much less susceptible to antibiotics and disinfectants when compared with their planktonic counterparts. The ability to form biofilms is now considered a universal attribute of micro-organisms. Diseases associated with biofilms require novel methods for their prevention, diagnosis and treatment; this is largely due to the properties of biofilms. Surprisingly, biofilm formation by bacterial pathogens of veterinary importance has received relatively little attention. Here, we review the current knowledge of bacterial biofilms as well as studies performed on animal pathogens.

Global transcriptional profiling of Burkholderia pseudomallei under salt stress reveals differential effects on the Bsa type III secretion system

Background

Burkholderia pseudomallei is the causative agent of melioidosis where the highest reported incidence world wide is in the Northeast of Thailand, where saline soil and water are prevalent. Moreover, recent reports indicate a potential pathogenic role for B. pseudomallei in cystic fibrosis lung disease, where an increased sodium chloride (NaCl) concentration in airway surface liquid has been proposed. These observations raise the possibility that high salinity may represent a favorable niche for B. pseudomallei. We therefore investigated the global transcriptional response of B. pseudomallei to increased salinity using microarray analysis.

Results

Transcriptome analysis of B. pseudomallei under salt stress revealed several genes significantly up-regulated in the presence of 320 mM NaCl including genes associated with the bsa-derived Type III secretion system (T3SS). Microarray data were verified by reverse transcriptase-polymerase chain reactions (RT-PCR). Western blot analysis confirmed the increased expression and secretion of the invasion-associated type III secreted proteins BipD and BopE in B. pseudomallei cultures at 170 and 320 mM NaCl relative to salt-free medium. Furthermore, salt-treated B. pseudomallei exhibited greater invasion efficiency into the lung epithelial cell line A549 in a manner partly dependent on a functional Bsa system.

Conclusions

B. pseudomallei responds to salt stress by modulating the transcription of a relatively small set of genes, among which is the bsa locus associated with invasion and virulence. Expression and secretion of Bsa-secreted proteins was elevated in the presence of exogenous salt and the invasion efficiency was enhanced. Our data indicate that salinity has the potential to influence the virulence of B. pseudomallei.

Regulation of pga operon expression and biofilm formation in Actinobacillus pleuropneumoniae by sigmaE and H-NS

Clinical isolates of the porcine pathogen Actinobacillus pleuropneumoniae often form adherent colonies on agar plates due to expression of an operon, pgaABCD, encoding a poly-beta-1,6-N-acetyl-D-glucosamine (PGA) extracellular matrix. The adherent colony phenotype, which correlates with the ability to form biofilms on the surfaces of polystyrene plates, is lost following serial passage in broth culture, and repeated passage of the nonadherent variants on solid media does not result in reversion to the adherent colony phenotype. In order to investigate the regulation of PGA expression and biofilm formation in A. pleuropneumoniae, we screened a bank of transposon mutants of the nonadherent serovar 1 strain S4074(T) and identified mutations in two genes, rseA and hns, which resulted in the formation of the adherent colony phenotype. In other bacteria, including the Enterobacteriaceae, H-NS acts as a global gene regulator, and RseA is a negative regulator of the extracytoplasmic stress response sigma factor sigma(E). Transcription profiling of A. pleuropneumoniae rseA and hns mutants revealed that both sigma(E) and H-NS independently regulate expression of the pga operon. Transcription of the pga operon is initiated from a sigma(E) promoter site in the absence of H-NS, and upregulation of sigma(E) is sufficient to displace H-NS, allowing transcription to proceed. In A. pleuropneumoniae, H-NS does not act as a global gene regulator but rather specifically regulates biofilm formation via repression of the pga operon. Positive regulation of the pga operon by sigma(E) indicates that biofilm formation is part of the extracytoplasmic stress response in A. pleuropneumoniae.

The molecular and cellular basis of pathogenesis in melioidosis: how does Burkholderia pseudomallei cause disease?

Melioidosis, a febrile illness with disease states ranging from acute pneumonia or septicaemia to chronic abscesses, was first documented by Whitmore & Krishnaswami (1912). The causative agent, Burkholderia pseudomallei, was subsequently identified as a motile, gram-negative bacillus, which is principally an environmental saprophyte. Melioidosis has become an increasingly important disease in endemic areas such as northern Thailand and Australia (Currie et al., 2000). This health burden, plus the classification of B. pseudomallei as a category B biological agent (Rotz et al., 2002), has resulted in an escalation of research interest. This review focuses on the molecular and cellular basis of pathogenesis in melioidosis, with a comprehensive overview of the current knowledge on how B. pseudomallei can cause disease. The process of B. pseudomallei movement from the environmental reservoir to attachment and invasion of epithelial and macrophage cells and the subsequent intracellular survival and spread is outlined. Furthermore, the diverse assortment of virulence factors that allow B. pseudomallei to become an effective opportunistic pathogen, as well as to avoid or subvert the host immune response, is discussed. With the recent increase in genomic and molecular studies, the current understanding of the infection process of melioidosis has increased substantially, yet, much still remains to be elucidated.

Comparative analysis of extracellular enzymes and virulence exhibited by Burkholderia pseudomallei from different sources

To evaluate the potential role of extracellular proteins in the pathogenicity and virulence of Burkholderia pseudomallei, the activities of several enzymes in the culture filtrates of nine clinical and six environmental isolates were investigated in vitro and in vivo in ICR strain of mice. The production of protease, phosphatase, phospholipase C, superoxide dismutase, catalase and peroxidase were detected in the culture filtrates of all the 15 isolates at different time points of growth 4-24h. Over time, activity of each enzyme at each time point varied. Profile of secretion was similar among the 15 isolates irrespective of source, that is clinical or environmental. Catalase, phosphatase and phospholipase C were found to be increased in 60-100% of the isolates post-passage in mice. In vivo inoculation studies in ICR mice demonstrated a wide difference in their ability to cause bacteraemia, splenic or external abscesses and mortality rate ranged from few days to several weeks.

Burkholderia cenocepacia requires RpoE for growth under stress conditions and delay of phagolysosomal fusion in macrophages

Burkholderia cenocepacia is an opportunistic pathogen causing serious infections in patients with cystic fibrosis. The widespread distribution of this bacterium in the environment suggests that it must adapt to stress to be able to survive. We identified in B. cenocepacia K56-2 a gene predicted to encode RpoE, the extra-cytoplasmic stress response regulator. The rpoE gene is the first gene of a predicted operon encoding proteins homologous to RseA, RseB, MucD and a protein of unknown function. The genomic organization and the co-transcription of these genes were confirmed by PCR and RT-PCR. The mucD and rpoE genes were mutated, giving rise to B. cenocepacia RSF24 and RSF25, respectively. While mutant RSF24 did not demonstrate any growth defects under the conditions tested, RSF25 was compromised for growth under temperature (44 degrees C) and osmotic stress (426 mM NaCl). Expression of RpoE in trans could complement the osmotic growth defect but exacerbated temperature sensitivity in both RSF25 and wild-type K56-2. Inactivation of rpoE altered the bacterial cell surface, as indicated by increased binding of the fluorescent dye calcofluor white and by an altered outer-membrane protein profile. These cell surface changes were restored by complementation with a plasmid encoding rpoE. Macrophage infections in which bacterial colocalization with fluorescent dextran was examined demonstrated that the rpoE mutant could not delay the fusion of B. cenocepacia-containing vacuoles with lysosomes, in contrast to the parental strain K56-2. These data show that B. cenocepacia rpoE is required for bacterial growth under certain stress conditions and for the ability of intracellular bacteria to delay phagolysosomal fusion in macrophages.

Characterization of two distinct phospholipase C enzymes from Burkholderia pseudomallei

Burkholderia pseudomallei is a serious bacterial pathogen that can cause a lethal infection in humans known as melioidosis. In this study two of its phospholipase C (PLC) enzymes (Plc-1 and Plc-2) were characterized. Starting with a virulent strain, two single mutants were constructed, each with one plc gene inactivated, and one double mutant with both plc genes inactivated. The single plc mutants exhibited decreased extracellular PLC activity in comparison to the wild-type strain, thereby demonstrating that the two genes encoded functional extracellular PLCs. Growth comparisons between the wild-type and PLC mutants in egg-yolk-supplemented medium indicated that both PLCs contributed to egg-yolk phospholipid utilization. Both PLCs hydrolysed phosphatidylcholine and sphingomyelin but neither was haemolytic for human erythrocytes. Experimental infections of eukaryotic cells demonstrated that Plc-1 itself had no effect on plaque-forming efficiency but it had an additive effect on increasing the efficiency of Plc-2 to form plaques. Only Plc-2 had a significant role in host cell cytotoxicity. In contrast, neither Plc-1 nor Plc-2 appeared to play any role in multinucleated giant cell (MNGC) formation or induction of apoptotic death in the cells studied. These data suggested that PLCs contribute, at least in part, to B. pseudomallei virulence and support the view that Plc-1 and Plc-2 are not redundant virulence factors.

Altered Proteome in Burkholderia pseudomallei rpoE Operon Knockout Mutant: Insights into Mechanisms of rpoE Operon in Stress Tolerance, Survival, and Virulence

We have previously shown that the alternative sigma factor sigmaE (RpoE), encoded by rpoE, is involved in stress tolerance and survival of Burkholderia pseudomallei. However, its molecular and pathogenic mechanisms remain unclear. In the present study, we applied gel-based, differential proteomics to compare the cellular proteome of an rpoE operon knockout mutant (RpoE Mut) to that of wild-type (K96243 WT) B. pseudomallei. Quantitative intensity analysis (n = 5 gels from 5 individual culture flasks in each group) revealed significantly differential expression of 52 proteins, which were subsequently identified by Q-TOF MS/MS. These included oxidative, osmotic, and other stress response proteins; chaperones; transcriptional/translational regulators; metabolic enzymes; proteins involved in cell wall synthesis, fatty synthesis, glycogen synthesis, and storage; exported proteins; secreted proteins; adhesion molecule; protease/peptidase; protease inhibitor; signaling proteins; and other miscellaneous proteins. The down-regulation of several stress response proteins, chaperones, transcriptional/translational regulators, and proteins involved in cell wall synthesis in RpoE Mut provided some new insights into the mechanisms of the rpoE operon for the stress tolerance and survival of B. pseudomallei. In addition, the proteomic data and in vivo study indicated that the rpoE operon is also involved in the virulence of B. pseudomallei. Our findings underscore the usefulness of proteomics for unraveling pathogenic mechanisms of diseases at the molecular level.

Melioidosis

PURPOSE OF REVIEW

Melioidosis is increasingly recognized around the world. Despite several decades of clinical research, the mortality rate for melioidosis remains high. This review focuses on studies that relate to patient management, including risk factors, diagnosis, treatment and prediction of the outcome. A brief summary of studies relating to genomics and genotyping, immunology and pathogenesis is provided.

RECENT FINDINGS

Involvement in the tsunami of December 2004 is a risk factor for melioidosis, and risk may extend to individuals who were uninjured bystanders. Several standard microbiological techniques used to culture and identify Burkholderia pseudomallei have been evaluated. Polymerase chain reaction has been developed for bacterial identification, although limited evaluation has been performed in the clinical setting. Two trials of antimicrobial therapy provide evidence with which to refine existing treatment protocols. Inexpensive clinical and laboratory predictors for poor outcome have been described. Several putative vaccine candidates have been proposed and studied in animals, but no vaccine is on the immediate horizon.

SUMMARY

None of the studies reviewed here report strategies that reduce mortality. A key area for future research is the identification of affordable interventions that lower the death rate, and are applicable to low-resource settings.

Temperature-regulated microcolony formation by Burkholderia pseudomallei requires pilA and enhances association with cultured human cells

Burkholderia pseudomallei is the causative agent of melioidosis, a potentially fatal disease that is endemic to Northern Australia and Southeast Asia and is acquired from soil or water. Adherence of B. pseudomallei 08 to cultured cells increases dramatically following prior growth at 30 degrees C or less compared to that following prior growth at 37 degrees C. Here, we show that this occurs almost entirely as the result of microcolony formation (bacterium-bacterium interactions) following growth at 27 degrees C but not at 37 degrees C, which considerably enhances bacterial association with eukaryotic cells. Further, we demonstrate that the type IVA pilin-encoding gene, pilA, is essential for microcolony development by B. pseudomallei 08, and thus optimum association with eukaryotic cells, but is not required for direct adherence (bacterium-cell interactions). In contrast, although the B. pseudomallei genome sequence strain, K96243, also contains transcriptionally active pilA, microcolony formation rarely occurs following growth at either 27 degrees C or 37 degrees C and cell association occurs significantly less than with strain 08. Analysis of pilA transcription in 08 identified that pilA is dramatically upregulated under microcolony-forming conditions, viz., growth at low temperature, and association with eukaryotic cells; the pattern of transcription of pilA in K96243 differed from that in 08. Our study also suggests that biofilm formation by B. pseudomallei 08 and K96243 on polyvinylchloride is not mediated by pilA. Adherence and microcolony formation, and pilA transcription, vary between strains, consistent with known genomic variation in B. pseudomallei, and these phenotypes may be relevant to colonization from the environment.

Global map of growth-regulated gene expression in Burkholderia pseudomallei, the causative agent of melioidosis

Many microbial pathogens express specific virulence traits at distinct growth phases. To understand the molecular pathways linking bacterial growth to pathogenicity, we have characterized the growth transcriptome of Burkholderia pseudomallei, the causative agent of melioidosis. Using a fine-scale sampling approach, we found approximately 17% of all B. pseudomallei genes displaying regulated expression during growth in rich medium, occurring as broad waves of functionally coherent gene expression tightly associated with distinct growth phases and transition points. We observed regulation of virulence genes across all growth phases and identified serC as a potentially new virulence factor by virtue of its coexpression with other early-phase virulence genes. serC-disrupted B. pseudomallei strains were serine auxotrophs and in mouse infection assays exhibited a dramatic attenuation of virulence compared to wild-type B. pseudomallei. Immunization of mice with serC-disrupted B. pseudomallei also conferred protection against subsequent challenges with different wild-type B. pseudomallei strains. At a genomic level, early-phase genes were preferentially localized on chromosome 1, while stationary-phase genes were significantly biased towards chromosome 2. We detected a significant level of chromosomally clustered gene expression, allowing us to predict approximately 100 potential operons in the B. pseudomallei genome. We computationally and experimentally validated these operons by showing that genes in these regions are preferentially transcribed in the same 5'-->3' direction, possess significantly shorter intergenic lengths than the overall genome, and are expressed as a common mRNA transcript. The availability of this transcriptome map provides an important resource for understanding the transcriptional architecture of B. pseudomallei.