Interaction of insulin with Pseudomonas pseudomallei

Marmosets as models of infectious diseases

Animal models of infectious disease often serve a crucial purpose in obtaining licensure of therapeutics and medical countermeasures, particularly in situations where human trials are not feasible, i.e., for those diseases that occur infrequently in the human population. The common marmoset (Callithrix jacchus), a Neotropical new-world (platyrrhines) non-human primate, has gained increasing attention as an animal model for a number of diseases given its small size, availability and evolutionary proximity to humans. This review aims to (i) discuss the pros and cons of the common marmoset as an animal model by providing a brief snapshot of how marmosets are currently utilized in biomedical research, (ii) summarize and evaluate relevant aspects of the marmoset immune system to the study of infectious diseases, (iii) provide a historical backdrop, outlining the significance of infectious diseases and the importance of developing reliable animal models to test novel therapeutics, and (iv) provide a summary of infectious diseases for which a marmoset model exists, followed by an in-depth discussion of the marmoset models of two studied bacterial infectious diseases (tularemia and melioidosis) and one viral infectious disease (viral hepatitis C).

Neuromodulators as Interdomain Signaling Molecules Capable of Occupying Effector Binding Sites in Bacterial Transcription Factors

Hormones and neurotransmitters are important components of inter-kingdom signaling systems that ensure the coexistence of eukaryotes with their microbial community. Their ability to affect bacterial physiology, metabolism, and gene expression was evidenced by various experimental approaches, but direct penetration into bacteria has only recently been reported. This opened the possibility of considering neuromodulators as potential effectors of bacterial ligand-dependent regulatory proteins. Here, we assessed the validity of this assumption for the neurotransmitters epinephrine, dopamine, and norepinephrine and two hormones (melatonin and serotonin). Using flexible molecular docking for transcription factors with ligand-dependent activity, we assessed the ability of neuromodulators to occupy their effector binding sites. For many transcription factors, including the global regulator of carbohydrate metabolism, CRP, and the key regulator of lactose assimilation, LacI, this ability was predicted based on the analysis of several 3D models. By occupying the ligand binding site, neuromodulators can sterically hinder the interaction of the target proteins with the natural effectors or even replace them. The data obtained suggest that the direct modulation of the activity of at least some bacterial transcriptional factors by neuromodulators is possible. Therefore, the natural hormonal background may be a factor that preadapts bacteria to the habitat through direct perception of host signaling molecules.

Isolated splenic abscess due to melioidosis in type 1 diabetes mellitus: laboratory diagnosis of Burkholderia pseudomallei in resource-restricted setting

Diabetes mellitus, type 1 in particular, is a well-recognised risk factor for melioidosis, a disease caused by Burkholderia pseudomallei Melioidosis is endemic in Southeast Asia and in northern Australia and has a variety of clinical presentation, isolated splenic abscess being one of them. B. pseudomallei, however, is an uncommon aetiology of splenic abscess. The diagnosis of melioidosis is often overlooked unless the clinician and the microbiologist are suspicious of the condition. Multiple splenic abscesses and perisplenic collection were noted in CT scan of the abdomen in a patient of type 1 diabetes, presenting with fever for preceding 4 weeks. B. pseudomallei was isolated from the splenic aspirate and the diagnosis was made based on gram stain and routine biochemical tests. He was successfully treated with antibiotics. We postulate that the likely route of infection was inoculation through skin, the integrity of which was compromised by multiple subcutaneous insulin injections.

Characterization of the Burkholderia mallei tonB Mutant and Its Potential as a Backbone Strain for Vaccine Development

Background

In this study, a Burkholderia mallei tonB mutant (TMM001) deficient in iron acquisition was constructed, characterized, and evaluated for its protective properties in acute inhalational infection models of murine glanders and melioidosis.

Methodology/Principal Findings

Compared to the wild-type, TMM001 exhibits slower growth kinetics, siderophore hyper-secretion and the inability to utilize heme-containing proteins as iron sources. A series of animal challenge studies showed an inverse correlation between the percentage of survival in BALB/c mice and iron-dependent TMM001 growth. Upon evaluation of TMM001 as a potential protective strain against infection, we found 100% survival following B. mallei CSM001 challenge of mice previously receiving 1.5 x 10⁴ CFU of TMM001. At 21 days post-immunization, TMM001-treated animals showed significantly higher levels of B. mallei-specific IgG1, IgG2a and IgM when compared to PBS-treated controls. At 48 h post-challenge, PBS-treated controls exhibited higher levels of serum inflammatory cytokines and more severe pathological damage to target organs compared to animals receiving TMM001. In a cross-protection study of acute inhalational melioidosis with B. pseudomallei, TMM001-treated mice were significantly protected. While wild type was cleared in all B. mallei challenge studies, mice failed to clear TMM001.

Conclusions/Significance

Although further work is needed to prevent chronic infection by TMM001 while maintaining immunogenicity, our attenuated strain demonstrates great potential as a backbone strain for future vaccine development against both glanders and melioidosis.

Burkholderia mallei and B. pseudomallei are the causative agents of glanders and melioidosis, respectively. In addition to the recent rise in cases of glanders and the endemicity of melioidosis worldwide, these pathogens have gained attention as potential bioweapons. Further, these pathogens have huge potential for aerosol delivery and often produce fatal infection amongst untreated individuals. Both pathogens are difficult to treat, and even with antibiotic intervention, patients relapse or get re-infected. A big challenge for vaccine development against these pathogens includes identification of broadly protective antigens and a better understanding of the correlates of protection from both acute and chronic infections. Our study is the first to demonstrate significant protection against a lethal challenge with both Burkholderia species. Because TMM001 persists in immunized mice, we propose that this attenuated organism is a promising backbone-based strain from which a legitimate vaccine candidate can be generated.

Pathogenesis of percutaneous infection of goats with Burkholderia pseudomallei: clinical, pathologic, and immunological responses in chronic melioidosis

Melioidosis is a severe suppurative to granulomatous infection caused by Burkholderia pseudomallei. The disease is endemic to South-East Asia and Northern Australasia and is also of interest as a potential biological weapon. Natural infection can occur by percutaneous inoculation, inhalation or ingestion, but the relative importance of each route is unknown. Experimental infection models using mice have shown inhalation to be the most lethal route of exposure, but few studies have examined the pathogenesis of percutaneous infection despite its presumptive importance in natural disease. Caprine models are useful in the study of melioidosis because goats are susceptible to natural infection by B. pseudomallei, display similar epizootiology/epidemiology to that of humans within the endemic range and develop similar pathologic lesions. Percutaneous inoculation with 10(4)  CFU of B. pseudomallei produced disease in all experimental animals with rapid dissemination to the lungs, spleen and kidneys. Initial fever was brief, but temperatures did not return to pre-infection levels until day 18, concurrent with a dramatic lymphocytosis and the transition to chronic disease. Distribution and appearance of gross pathologic and radiographic lesions in goats were similar to caprine aerosol infection and to reported human disease. The similarities seen despite different routes of infection suggest that host or bacterial factors may be more important than the route of infection in disease pathogenesis. The nature of melioidosis in goats makes it amenable for modelling additional risk factors to produce acute clinical disease, which is important to the study of human melioidosis.

Communication between Bacteria and Their Hosts

It is clear that a dialogue is occurring between microbes and their hosts and that chemical signals are the language of this interkingdom communication. Microbial endocrinology shows that, through their long coexistence with animals and plants, microorganisms have evolved sensors for detecting eukaryotic hormones, which the microbe uses to determine that they are within proximity of a suitable host and to optimally time the expression of genes needed for host colonisation. It has also been shown that some prokaryotic chemical communication signals are recognized by eukaryotes. Deciphering what is being said during the cross-talk between microbe and host is therefore important, as it could lead to new strategies for preventing or treating bacterial infections.

Pseudomonas aeruginosa biofilms perturb wound resolution and antibiotic tolerance in diabetic mice

Diabetic patients are more susceptible to the development of chronic wounds than non-diabetics. The impaired healing properties of these wounds, which often develop debilitating bacterial infections, significantly increase the rate of lower extremity amputation in diabetic patients. We hypothesize that bacterial biofilms, or sessile communities of bacteria that reside in a complex matrix of exopolymeric material, contribute to the severity of diabetic wounds. To test this hypothesis, we developed an in vivo chronic wound, diabetic mouse model to determine the ability of the opportunistic pathogen, Pseudomonas aeruginosa, to cause biofilm-associated infections. Utilizing this model, we observed that diabetic mice with P. aeruginosa-infected chronic wounds displayed impaired bacterial clearing and wound closure in comparison with their non-diabetic littermates. While treating diabetic mice with insulin improved their overall health, it did not restore their ability to resolve P. aeruginosa wound infections or speed healing. In fact, the prevalence of biofilms and the tolerance of P. aeruginosa to gentamicin treatment increased when diabetic mice were treated with insulin. Insulin treatment was observed to directly affect the ability of P. aeruginosa to form biofilms in vitro. These data demonstrate that the chronically wounded diabetic mouse appears to be a useful model to study wound healing and biofilm infection dynamics, and suggest that the diabetic wound environment may promote the formation of biofilms. Further, this model provides for the elucidation of mechanistic factors, such as the ability of insulin to influence antimicrobial effectiveness, which may be relevant to the formation of biofilms in diabetic wounds.

Delayed activation of host innate immune pathways in streptozotocin-induced diabetic hosts leads to more severe disease during infection with Burkholderia pseudomallei

Diabetes mellitus is a predisposing factor of melioidosis, contributing to higher mortality rates in diabetics infected with Burkholderia pseudomallei. To investigate how diabetes alters the inflammatory response, we established a streptozotocin (STZ) -induced diabetic murine acute-phase melioidosis model. Viable B. pseudomallei cells were consistently detected in the blood, liver and spleen during the 42-hr course of infection but the hyperglycaemic environment did not increase the bacterial burden. However, after 24 hr, granulocyte counts increased in response to infection, whereas blood glucose concentrations decreased over the course of infection. A genome-wide expression analysis of the STZ-diabetic murine acute melioidosis liver identified ~1000 genes whose expression was altered in the STZ-diabetic mice. The STZ-diabetic host transcriptional response was compared with the normoglycaemic host transcriptional response recently reported by our group. The microarray data suggest that the presence of elevated glucose levels impairs the host innate immune system by delaying the identification and recognition of B. pseudomallei surface structures. Consequently, the host is unable to activate the appropriate innate immune response over time, which may explain the increased susceptibility to melioidosis in the STZ-diabetic host. Nevertheless, a general 'alarm signal' of infection as well as defence programmes are still triggered by the STZ-diabetic host, although only 24 hr after infection. In summary, this study demonstrates that in the face of a B. pseudomallei acute infection, poor glycaemic control impaired innate responses during the early stages of B. pseudomallei infection, contributing to the increased susceptibility of STZ-induced diabetics to this fatal disease.

Melioidosis vaccines: a systematic review and appraisal of the potential to exploit biodefense vaccines for public health purposes

Background

Burkholderia pseudomallei is a Category B select agent and the cause of melioidosis. Research funding for vaccine development has largely considered protection within the biothreat context, but the resulting vaccines could be applicable to populations who are at risk of naturally acquired melioidosis. Here, we discuss target populations for vaccination, consider the cost-benefit of different vaccination strategies and review potential vaccine candidates.

Methods and Findings

Melioidosis is highly endemic in Thailand and northern Australia, where a biodefense vaccine might be adopted for public health purposes. A cost-effectiveness analysis model was developed, which showed that a vaccine could be a cost-effective intervention in Thailand, particularly if used in high-risk populations such as diabetics. Cost-effectiveness was observed in a model in which only partial immunity was assumed. The review systematically summarized all melioidosis vaccine candidates and studies in animal models that had evaluated their protectiveness. Possible candidates included live attenuated, whole cell killed, sub-unit, plasmid DNA and dendritic cell vaccines. Live attenuated vaccines were not considered favorably because of possible reversion to virulence and hypothetical risk of latent infection, while the other candidates need further development and evaluation. Melioidosis is acquired by skin inoculation, inhalation and ingestion, but routes of animal inoculation in most published studies to date do not reflect all of this. We found a lack of studies using diabetic models, which will be central to any evaluation of a melioidosis vaccine for natural infection since diabetes is the most important risk factor.

Conclusion

Vaccines could represent one strand of a public health initiative to reduce the global incidence of melioidosis.

The designation of Burkholderia pseudomallei as a category B select agent has resulted in considerable research funding to develop a protective vaccine. This bacterium also causes a naturally occurring disease (melioidosis), an important cause of death in many countries including Thailand and Australia. In this study, we explored whether a vaccine could be used to provide protection from melioidosis. An economic evaluation based on its use in Thailand indicated that a vaccine could be a cost-effective intervention if used in high-risk populations such as diabetics and those with chronic kidney or lung disease. A literature search of vaccine studies in animal models identified the current candidates, but noted that models failed to take account of the common routes of infection in natural melioidosis and major risk factors for infection, primarily diabetes. This review highlights important areas for future research if biodefence-driven vaccines are to play a role in reducing the global incidence of melioidosis.

Screening microorganisms for insulin binding reveals binding by Burkholderia multivorans and Burkholderia cenocepacia and novel attachment of insulin to Aeromonas salmonicida via the A-layer

Exposure to microorganisms is considered an environmental factor that can contribute to Type 1 diabetes. Insulin-binding proteins (IBPs) on microorganisms may induce production of antibodies that can react with the human insulin receptor (HIR) with possible consequences in developing a diabetic autoimmune response against HIR and insulin. The interaction of insulin with microorganisms was studied by screening 45 microbial species for their ability to bind insulin. Binding assays were performed using labelled insulin to identify insulin-binding components on the microorganisms. Burkholderia multivorans and Burkholderia cenocepacia isolated from patients with cystic fibrosis (CF) and the fish pathogen Aeromonas salmonicida were the only strains of those tested, which showed insulin-binding components on their cell surfaces. Further work with A. salmonicida suggested that the insulin-binding activity of A. salmonicida is due to the A-layer. A mutant of A. salmonicida lacking the A-layer showed binding, but at a much reduced rate suggesting another insulin-binding component in addition to the high affinity of the A-protein. Soluble protein lysates were subjected to Western ligand blotting using peroxidase-labelled insulin to detect IBPs. Two positive IBPs were apparent at approximately 30 and 20 kDa in lysates from Burkholderia strains, but no IBP was detected in A. salmonicida lysates.

Microbial Endocrinology

Microbial Endocrinology is a new microbiology research discipline that represents the intersection of microbiology and endocrinology with neurophysiology. It has as its main tenet that through their long co-existence with animals and plants, micro-organisms have evolved sensory systems for detecting host-associated hormones. These sensing systems allow the microbe to determine that they are within proximity of a suitable host, and that is time to initiate expression of genes involved in host colonisation. Microbial Endocrinology therefore provides a new paradigm with which to examine and understand the interactions of micro-organisms with their host under conditions present in both health and disease. This article will focus on microbial interactions with the fight and flight family of catecholamine stress hormones.

Evaluation of surrogate animal models of melioidosis

Burkholderia pseudomallei is the Gram-negative bacterial pathogen responsible for the disease melioidosis. B. pseudomallei establishes disease in susceptible individuals through multiple routes of infection, all of which may proceed to a septicemic disease associated with a high mortality rate. B. pseudomallei opportunistically infects humans and a wide range of animals directly from the environment, and modeling of experimental melioidosis has been conducted in numerous biologically relevant models including mammalian and invertebrate hosts. This review seeks to summarize published findings related to established animal models of melioidosis, with an aim to compare and contrast the virulence of B. pseudomallei in these models. The effect of the route of delivery on disease is also discussed for intravenous, intraperitoneal, subcutaneous, intranasal, aerosol, oral, and intratracheal infection methodologies, with a particular focus on how they relate to modeling clinical melioidosis. The importance of the translational validity of the animal models used in B. pseudomallei research is highlighted as these studies have become increasingly therapeutic in nature.

Use of a low-dose steroid as an adjunct in the treatment, in mice, of severe sepsis caused by Burkholderia pseudomallei

Human melioidosis caused by Burkholderia pseudomallei is a severe septic disease that is associated with high mortality, even under appropriate antibiotic treatment. The therapeutic effects of low-dose hydrocortisone plus ceftazidime, and of ceftazidime alone, have recently been investigated in the treatment of acute, severe sepsis caused by B. pseudomallei, both in normal BALB/c mice and in BALB/c mice with streptozotocin-induced diabetes. The mice were infected and then treated intravenously, from day 1 or day 2 post-infection, with saline (as a control, given twice daily for 10 days), low-dose hydrocortisone (given in twice-daily doses of 5 mg/kg, for 5 days) plus ceftazidime (given in twice-daily doses of 1200 mg/kg, for 10 days), or the same doses of ceftazidime alone. Although the infected, untreated mice all died within 14 days, almost all of the treated animals were still alive at the end of the follow-up, 30 days post-infection. The addition of the steroid appeared to have no benefit, with bacterial loads and plasma concentrations of tumour necrosis factor, aspartate aminotransferase, alanine aminotransferase and creatinine decreasing similarly in all the treated groups. The infected diabetic mice given hydrocortisone-ceftazidime from day 1 (but not those given just ceftazidime from day 1) showed an increase in their blood glucose concentrations. When infected mice were treated with the low-dose steroid and lower doses of the antibiotic (in twice-daily doses of 120-600 mg/kg), the steroid not only offered no apparent benefit but seemed to reduce survival. It therefore appears that low-dose hydrocortisone, as an adjunct to antibiotic treatment, does not provide benefit in the treatment of murine melioidosis and may have negative effects on human cases of the disease who have diabetes mellitus.

Burkholderia pseudomallei: animal models of infection

A range of animal models of Burkholderia pseudomallei infection have been reported, and the host species differ widely both in their susceptibility to infection and in the pathogenesis of disease. In mice, and depending on the route of infection, dose, and mouse strain, the disease can range from a chronic, and in some cases, an apparently latent infection to an acute fulminant disease. Alternative small animal models of infection include diabetic rats or hamsters. Larger animal models of disease have not yet been fully developed. It is not clear which of the small animal models of melioidosis most accurately reflect disease in humans. However, the findings that diabetic rats are susceptible to infection, that some strains of mice can develop persistent subclinical infections that can spontaneously reactivate, and that inhalation exposure generally results in more acute disease suggest that these different models mimic different aspects of human melioidosis.

Gram-negative bacterial sensors for eukaryotic signal molecules

Ample evidence exists showing that eukaryotic signal molecules synthesized and released by the host can activate the virulence of opportunistic pathogens. The sensitivity of prokaryotes to host signal molecules requires the presence of bacterial sensors. These prokaryotic sensors, or receptors, have a double function: stereospecific recognition in a complex environment and transduction of the message in order to initiate bacterial physiological modifications. As messengers are generally unable to freely cross the bacterial membrane, they require either the presence of sensors anchored in the membrane or transporters allowing direct recognition inside the bacterial cytoplasm. Since the discovery of quorum sensing, it was established that the production of virulence factors by bacteria is tightly growth-phase regulated. It is now obvious that expression of bacterial virulence is also controlled by detection of the eukaryotic messengers released in the micro-environment as endocrine or neuro-endocrine modulators. In the presence of host physiological stress many eukaryotic factors are released and detected by Gram-negative bacteria which in return rapidly adapt their physiology. For instance, Pseudomonas aeruginosa can bind elements of the host immune system such as interferon-γ and dynorphin and then through quorum sensing circuitry enhance its virulence. Escherichia coli sensitivity to the neurohormones of the catecholamines family appears relayed by a recently identified bacterial adrenergic receptor. In the present review, we will describe the mechanisms by which various eukaryotic signal molecules produced by host may activate Gram-negative bacteria virulence. Particular attention will be paid to Pseudomonas, a genus whose representative species, P. aeruginosa, is a common opportunistic pathogen. The discussion will be particularly focused on the pivotal role played by these new types of pathogen sensors from the sensing to the transduction mechanism involved in virulence factors regulation. Finally, we will discuss the consequence of the impact of host signal molecules on commensally or opportunistic pathogens associated with different human tissue.

Strategies toward vaccines against Burkholderia mallei and Burkholderia pseudomallei

Burkholderia mallei and Burkholderia pseudomallei are Gram-negative, rod-shaped bacteria, and are the causative agents of the diseases glanders and melioidosis, respectively. These bacteria have been recognized as important pathogens for over 100 years, yet a relative dearth of available information exists regarding their virulence determinants and immunopathology. Infection with either of these bacteria presents with nonspecific symptoms and can be either acute or chronic, impeding rapid diagnosis. The lack of a vaccine for either bacterium also makes them potential candidates for bioweaponization. Together with their high rate of infectivity via aerosols and resistance to many common antibiotics, both bacteria have been classified as category B priority pathogens by the US NIH and US CDC, which has spurred a dramatic increase in interest in these microorganisms. Attempts have been made to develop vaccines for these infections, which would not only benefit military personnel, a group most likely to be targeted in an intentional release, but also individuals who may come in contact with glanders-infected animals or live in areas where melioidosis is endemic. This review highlights some recent attempts of vaccine development for these infections and the strategies used to improve the efficacy of vaccine approaches.

Development of novel animal infection models for the study of acute and chronic Burkholderia pseudomallei pulmonary infections

Burkholderia pseudomallei causes the disease melioidosis. The most common clinical presentation of melioidosis is pneumonia which can occur in acute and chronic forms. The tsunami of 2004 demonstrated a new risk factor for the acquisition of melioidosis and resulted in the proposal that direct delivery of B. pseudomallei into the lungs may result in the enhanced ability of this pathogen to cause disease. In the present studies, we present the development and characterization of rat models of acute and chronic pulmonary melioidosis, and we have utilized these models to demonstrate that direct delivery of B. pseudomallei into the lungs does indeed result in the enhanced ability of this pathogen to cause disease. Importantly, the rat lung infection models for melioidosis can quantify differences in virulence between individual B. pseudomallei wild type strains during both acute and chronic infections. Further, the histopathology associated with pulmonary melioidosis in the rat resembles that seen in tuberculosis. B. pseudomallei microarrays were used to characterize gene expression patterns during chronic pulmonary infections. Transcriptional profiling at several time points during chronic infection revealed that a wide range of genes associated with virulence and metabolic functions are differentially regulated in vivo during chronic infections.

Melioidosis: epidemiology, pathophysiology, and management

Melioidosis, caused by the gram-negative saprophyte Burkholderia pseudomallei, is a disease of public health importance in southeast Asia and northern Australia that is associated with high case-fatality rates in animals and humans. It has the potential for epidemic spread to areas where it is not endemic, and sporadic case reports elsewhere in the world suggest that as-yet-unrecognized foci of infection may exist. Environmental determinants of this infection, apart from a close association with rainfall, are yet to be elucidated. The sequencing of the genome of a strain of B. pseudomallei has recently been completed and will help in the further identification of virulence factors. The presence of specific risk factors for infection, such as diabetes, suggests that functional neutrophil defects are important in the pathogenesis of melioidosis; other studies have defined virulence factors (including a type III secretion system) that allow evasion of killing mechanisms by phagocytes. There is a possible role for cell-mediated immunity, but repeated environmental exposure does not elicit protective humoral or cellular immunity. A vaccine is under development, but economic constraints may make vaccination an unrealistic option for many regions of endemicity. Disease manifestations are protean, and no inexpensive, practical, and accurate rapid diagnostic tests are commercially available; diagnosis relies on culture of the organism. Despite the introduction of ceftazidime- and carbapenem-based intravenous treatments, melioidosis is still associated with a significant mortality attributable to severe sepsis and its complications. A long course of oral eradication therapy is required to prevent relapse. Studies exploring the role of preventative measures, earlier clinical identification, and better management of severe sepsis are required to reduce the burden of this disease.

Melioidosis: epidemiology, pathophysiology, and management

Melioidosis, caused by the gram-negative saprophyte Burkholderia pseudomallei, is a disease of public health importance in southeast Asia and northern Australia that is associated with high case-fatality rates in animals and humans. It has the potential for epidemic spread to areas where it is not endemic, and sporadic case reports elsewhere in the world suggest that as-yet-unrecognized foci of infection may exist. Environmental determinants of this infection, apart from a close association with rainfall, are yet to be elucidated. The sequencing of the genome of a strain of B. pseudomallei has recently been completed and will help in the further identification of virulence factors. The presence of specific risk factors for infection, such as diabetes, suggests that functional neutrophil defects are important in the pathogenesis of melioidosis; other studies have defined virulence factors (including a type III secretion system) that allow evasion of killing mechanisms by phagocytes. There is a possible role for cell-mediated immunity, but repeated environmental exposure does not elicit protective humoral or cellular immunity. A vaccine is under development, but economic constraints may make vaccination an unrealistic option for many regions of endemicity. Disease manifestations are protean, and no inexpensive, practical, and accurate rapid diagnostic tests are commercially available; diagnosis relies on culture of the organism. Despite the introduction of ceftazidime- and carbapenem-based intravenous treatments, melioidosis is still associated with a significant mortality attributable to severe sepsis and its complications. A long course of oral eradication therapy is required to prevent relapse. Studies exploring the role of preventative measures, earlier clinical identification, and better management of severe sepsis are required to reduce the burden of this disease.

The capsular polysaccharide of Burkholderia pseudomallei contributes to survival in serum by reducing complement factor C3b deposition

Burkholderia pseudomallei produces an extracellular polysaccharide capsule -3)-2-O-acetyl-6-deoxy-beta-D-manno-heptopyranose-(1- which has been shown to be an essential virulence determinant. The addition of purified capsule was shown to increase the virulence of a capsule mutant strain in the Syrian hamster model of acute melioidosis. An increase in the number of wild-type B. pseudomallei cells in the blood was seen by 48 h, while the number of capsule mutant cells in the blood declined by 48 h. Capsule expression was shown to be induced in the presence of serum using a lux reporter fusion to the capsule gene wcbB. The addition of purified B. pseudomallei capsule to serum bactericidal assays increased the survival of B. pseudomallei SLR5, a serum-sensitive strain, by 1,000-fold in normal human serum. Capsule production by B. pseudomallei contributed to reduced activation of the complement cascade by reducing the levels of complement factor C3b deposition. An increase in phagocytosis of the capsule mutant compared to the wild type was observed in the presence of normal human serum. These results suggest that the production of this capsule contributes to resistance to phagocytosis by reducing C3b deposition on the surface of the bacterium, thereby contributing to the persistence of bacteria in the blood of the infected host. Continued studies to characterize this capsule are essential for understanding the pathogenesis of B. pseudomallei infections and the development of preventive strategies for treatment of this disease.

Evaluation of lipopolysaccharide and capsular polysaccharide as subunit vaccines against experimental melioidosis

Burkholderia pseudomallei is the causative agent of melioidosis, which is a major cause of morbidity and mortality in endemic regions. Currently there is no human vaccine against melioidosis. In this study, LPS or capsular polysaccharide was used to immunize BALB/c mice. The different polysaccharide antigens induced antibody responses. Mice vaccinated with LPS developed predominantly IgM and IgG3 responses. Contrastingly, mice vaccinated with capsular polysaccharide developed a predominantly IgG2b response. After immunization, mice were challenged by the intra-peritoneal route and an increased mean time to death was observed compared with unvaccinated controls. Immunization with LPS provided an optimal protective response. Mice challenged by the aerosol route showed a small increase in the mean time to death compared with the unvaccinated controls. The passive transfer of antigen from immunized into naïve mice provided protection against a subsequent challenge. This study is the first time antigens protective by active immunization have been identified and suggests that polysaccharides have potential as vaccine candidates against melioidosis.

The effects of streptozotocin-induced diabetes on brucellosis of rats

It is believed that an infection is more common and runs a more protracted course in people with diabetes. In clinical practice, it is important to be aware of these associations, as the prognosis is often dependent upon prompt recognition and appropriate treatment. To show the course of brucellosis in the diabetic state, a model of Brucella melitensis infection was used in the setting of streptozotocin-induced diabetes in rat. B. melitensis infection proceeded more severely in diabetic rats and the severity of diabetes affected the prognosis. However, no association was found between B. melitensis and insulin using in vitro and in vivo experiments. Our study illustrates that B. melitensis infection in diabetes should be taken seriously.

Diabetes mellitus, insulin, and melioidosis in Thailand

A review of case records for 1817 Thai patients with melioidosis revealed that <10% of the 382 patients with diabetes mellitus were insulin dependent. This provides evidence against the hypothesis that insulin deficiency contributes to the known susceptibility to melioidosis in patients with diabetes mellitus.

The growth response of Escherichia coli to neurotransmitters and related catecholamine drugs requires a functional enterobactin biosynthesis and uptake system

The neurotransmitter norepinephrine (NE) stimulates the growth of low inocula of Escherichia coli in a minimal medium (SAPI) supplemented with serum (SAPI+serum) and induces the production of an "autoinducer" (AI) which, in turn, promotes E. coli growth in the absence of NE. Given the importance of NE, epinephrine, and their corresponding adrenergic agonists and antagonists in clinical medicine, we sought to investigate the molecular basis for these observations. Using a variety of NE precursors, metabolites, and therapeutic agents, we demonstrated that their ability to stimulate E. coli growth in SAPI+serum is dependent on the presence of a catechol (1,2-dihydroxybenzene) moiety with maximal activity requiring a two-carbon substituent incorporating a terminal primary amine. Serum contains the iron-binding glycoprotein, transferrin, and when SAPI+serum was supplemented with sufficient Fe(3+) to saturate transferrin, growth inhibition was relieved. Other metal cations, including Mg(2+), Ca(2+), and Zn(2+), had no effect. These data suggested that the stimulation of E. coli growth by NE in SAPI+serum may involve the catecholate siderophore, enterobactin, a cyclic triester of 2,3-dihydroxybenzoylserine. Consistent with this hypothesis, E. coli strains with mutations in ferrienterobactin transport (fepA or tonB) or enterobactin biosynthesis (entA) did not respond to NE. Furthermore, NE induced expression of the ferrienterobactin receptor, FepA, during growth in SAPI+serum. The enterobactin degradation product, 2,3-dihydroxybenzoylserine (DBS) was as effective as NE in stimulating the growth of E. coli and mutations in fepA or tonB abolished the DBS-dependent growth stimulation. In contrast to NE, however, DBS stimulated the growth of the entA mutant. Moreover, after growth in an iron-limited M9 medium in the absence of NE, ethyl acetate extracts of the E. coli entA(+) parent but not of the entA mutant contained AI, i.e., stimulated the growth of E. coli in SAPI+serum. Taken together, these data show that when low numbers of E. coli are inoculated into SAPI+serum, NE, DBS, and related catecholamines induce the enterobactin iron uptake system. This, in turn, facilitates iron sequestration from transferrin and indicates that the AI present in NE-conditioned SAPI+serum medium is enterobactin and its DBS breakdown products.

The mammalian neuroendocrine hormone norepinephrine supplies iron for bacterial growth in the presence of transferrin or lactoferrin

Norepinephrine stimulates the growth of a range of bacterial species in nutritionally poor SAPI minimal salts medium containing 30% serum. Addition of size-fractionated serum components to SAPI medium indicated that transferrin was required for norepinephrine stimulation of growth of Escherichia coli. Since bacteriostasis by serum is primarily due to the iron-withholding capacity of transferrin, we considered the possibility that norepinephrine can overcome this effect by supplying transferrin-bound iron for growth. Incubation with concentrations of norepinephrine that stimulated bacterial growth in serum-SAPI medium resulted in loss of bound iron from iron-saturated transferrin, as indicated by the appearance of monoferric and apo- isoforms upon electrophoresis in denaturing gels. Norepinephrine also caused the loss of iron from lactoferrin. The pharmacologically inactive metabolite norepinephrine 3-O-sulfate, by contrast, did not result in iron loss from transferrin or lactoferrin and did not stimulate bacterial growth in serum-SAPI medium. Norepinephrine formed stable complexes with transferrin, lactoferrin, and serum albumin. Norepinephrine-transferrin and norepinephrine-lactoferrin complexes, but not norepinephrine-apotransferrin or norepinephrine-albumin complexes, stimulated bacterial growth in serum-SAPI medium in the absence of additional norepinephrine. Norepinephrine-stimulated growth in medium containing (55)Fe complexed with transferrin or lactoferrin resulted in uptake of radioactivity by bacterial cells. Moreover, norepinephrine-stimulated growth in medium containing [(3)H]norepinephrine indicated concomitant uptake of norepinephrine. In each case, addition of excess iron did not affect growth but significantly reduced levels of radioactivity ((55)Fe or (3)H) associated with bacterial cells. A role for catecholamine-mediated iron supply in the pathophysiology of infectious diseases is proposed.

Endemic melioidosis in tropical northern Australia: a 10-year prospective study and review of the literature

In a prospective study of melioidosis in northern Australia, 252 cases were found over 10 years. Of these, 46% were bacteremic, and 49 (19%) patients died. Despite administration of ceftazidime or carbapenems, mortality was 86% (43 of 50 patients) among those with septic shock. Pneumonia accounted for 127 presentations (50%) and genitourinary infections for 37 (15%), with 35 men (18%) having prostatic abscesses. Other presentations included skin abscesses (32 patients; 13%), osteomyelitis and/or septic arthritis (9; 4%), soft tissue abscesses (10; 4%), and encephalomyelitis (10; 4%). Risk factors included diabetes (37%), excessive alcohol intake (39%), chronic lung disease (27%), chronic renal disease (10%), and consumption of kava (8%). Only 1 death occurred among the 51 patients (20%) with no risk factors (relative risk, 0.08; 95% confidence interval, 0.01-0.58). Intensive therapy with ceftazidime or carbapenems, followed by at least 3 months of eradication therapy with trimethoprim-sulfamethoxazole, was associated with decreased mortality. Strategies are needed to decrease the high mortality with melioidosis septic shock. Preliminary data on granulocyte colony-stimulating factor therapy are very encouraging.

Pathogenesis of and immunity to melioidosis

While Burkholderia pseudomallei, the causative agent of melioidosis, is becoming increasingly recognized as a significant cause of morbidity and mortality in regions to which it is endemic, no licensed vaccine preparation currently exists for immunization against the disease. Therefore, one of the primary goals of our research has been to identify and characterize antigens expressed by B. pseudomallei isolates for the intended purpose of developing a vaccine construct that can be used to actively immunize specific high risk populations against the disease. By utilizing a combination of biochemical, immunological and molecular approaches, our studies now indicate that some of the most promising candidates for this task include flagellin proteins and the endotoxin derived O-polysaccharide (PS) antigens expressed by the organism. In this review, we have attempted to summarize the current status of B. pseudomallei research while endeavoring to provide a rationale for our approach towards the development of a melioidosis vaccine.

Ecology of Burkholderia pseudomallei and the interactions between environmental Burkholderia spp. and human-animal hosts

Early workers thought that melioidosis was a zoonosis with a reservoir in rodents, but we now know that Burkholderia pseudomallei is a widely distributed environmental saprophyte. In northeast Thailand, two thirds of paddy fields yield the organism, and 80% of children have antibodies by the time they are 4 years old. However, interpretation of these results has been complicated by the recent recognition of avirulent, antigenically cross-reacting environmental organisms for which the name B. thailandensis has been proposed. We still know very little about the climatic, physical, chemical and biological factors which control the proliferation and survival of Burkholderia spp. in the environment, although epidemiological studies show space-time clustering of melioidosis. It is assumed that most human and animal melioidosis arises through exposure to contaminated soil or muddy water, although only 6% of human cases have a clear history of inoculation, and a further 0.5% of cases follow near-drowning. Laboratory animals have also been infected by ingestion, inhalation and insect bites, but evidence of infection acquired naturally by these routes remains anecdotal. Sporadic cases have resulted from iatrogenic inoculation, laboratory accidents, and person-to-person or animal-to-person spread. Whether exposure to B. pseudomallei will result in disease probably depends on the balance between the virulence of the strain, the immune status of the host (e.g. diabetes mellitus) and the size of the inoculum.

Interaction of insulin with Burkholderia pseudomallei may be caused by a preservative

AIM

To re-examine the previously reported in vitro interaction of insulin with Burkholderia pseudomallei, in the light of a suggestion that the interaction may have resulted from the presence of the preservative m-cresol in commercial preparations.

METHODS

Broth culture studies of B pseudomallei were performed with and without the addition of m-cresol and various preparations of insulin.

RESULTS

Growth of B pseudomallei was inhibited by m-cresol at the concentrations found in pharmaceutical insulin preparations, and by the insulin preparation Humulin R, but not by pure insulin.

CONCLUSIONS

The results of previous experiments may have been confounded by the presence of the preservative m-cresol.

Characterization of a murine model of melioidosis: comparison of different strains of mice

Melioidosis is an infectious disease caused by the saprophytic gram-negative rod Burkholderia pseudomallei. The aim of this study was to establish and characterize a murine model of melioidosis to provide a basis for further investigations on the pathogenesis of the disease. After intravenous infection with B. pseudomallei, C57BL/6 mice were found to be significantly more resistant than BALB/c mice. There was a marked organotropism of B. pseudomallei for the spleen and liver in both strains of mice, with the highest bacterial load in the spleen. Electron microscopic investigations of the spleen clearly demonstrated intracellular replication within membrane-bound phagosomes. Electron micrographs of the liver provided evidence that B. pseudomallei-containing phagosomes in hepatocytes fuse with lysosomes, leading to degradation of bacteria. In both strains of mice, the course of infection was highly dependent on the infective dose and the bacterial strain used, ranging from death within a few days to death after several weeks. In comparison with BALB/c mice, the bacterial counts in C57BL/6 mice were decreased 12 h after infection, which is suggestive of an innate immune mechanism against B. pseudomallei in this early phase of infection contributing to the lower susceptibility of C57BL/6 mice. BALB/c mice developed a more pronounced lymphopenia, granulocytosis, and splenomegaly at a lower infective dose compared to C57BL/6 mice. Analysis of the antibody response against B. pseudomallei 11 days after infection revealed a significantly higher immunoglobulin G2A (IgG2a)/IgG1 ratio in C57BL/6 mice than in BALB/c mice, indicating that a T helper type 1 immune response is associated with resistance to infection with B. pseudomallei.

Current studies on the pathogenesis of melioidosis

Burkholderia pseudomallei is a major cause of bacterial septicemias in many parts of the world, particularly Thailand; the known geographic range of the organism appears to be enlarging as awareness of the organism and the disease it causes--melioidosis--increases. B. pseudomallei is intrinsically resistant to most antibiotics, and our knowledge of B. pseudomallei pathogenesis is lacking. Thus, the long-term objective of our research is to define at a molecular level the pathogenesis by combining genetic, immunologic, and biochemical approaches with animal model studies. Basic studies on B. pseudomallei pathogenesis are acutely needed to provide a knowledge base to rationally design new modes of therapy directed against this organism.

The type II O-antigenic polysaccharide moiety of Burkholderia pseudomallei lipopolysaccharide is required for serum resistance and virulence

Melioidosis, an infection caused by the gram-negative bacterial pathogen Burkholderia pseudomallei, is endemic in south-east Asia and northern Australia. Acute septicaemic melioidosis is a major cause of morbidity and mortality, especially in north-east Thailand. B. pseudomallei is highly resistant to the bactericidal activity of normal human serum (NHS), and we have found that B. pseudomallei 1026b multiplies in 10-30% NHS. We developed a simple screen for the identification of serum-sensitive mutants based on this novel phenotype. Approximately 1200 Tn5-OT182 mutants were screened, and three serum-sensitive mutants were identified. The type II O-antigenic polysaccharide (O-PS) moiety of lipopolysaccharide was not present in the serum-sensitive mutants. A representative serum-sensitive mutant, SRM117, was killed by the alternative pathway of complement and was less virulent than 1026b in three animal models of melioidosis. The Tn5-OT182 integrations in the serum-sensitive mutants were physically linked on the B. pseudomallei chromosome, and further genetic analysis of this locus revealed a cluster of 15 genes required for type II O-PS production. The proteins encoded by these genes were similar to proteins involved in bacterial polysaccharide biosynthesis. The results presented here demonstrate that type II O-PS is essential for B. pseudomallei serum resistance and virulence.

Identification and characterization of a two-component regulatory system involved in invasion of eukaryotic cells and heavy-metal resistance in Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a disease increasingly recognized as an important cause of morbidity and mortality in many regions of the world. B. pseudomallei is a facultative intracellular pathogen capable of invading eukaryotic cells. We used Tn5-OT182 mutagenesis to generate mutants deficient in the ability to invade a human type II pneumocyte cell line (A549 cells). One of these mutants, AJ1D8, exhibited approximately 10% of the ability of the parental strain, 1026b, to invade A549 cells. There was no difference in the abilities of 1026b and AJ1D8 to resist killing by RAW macrophages or the human defensin HNP-1. The nucleotide sequence flanking the Tn5-OT182 integration in AJ1D8 was determined, and two open reading frames were identified. The predicted proteins shared considerable homology with two-component regulatory systems involved in the regulation of heavy-metal resistance in other organisms. AJ1D8 was 16-fold more sensitive to Cd2+ and twofold more sensitive to Zn2+ than was 1026b but was not sensitive to any of the other heavy metals examined. The B. pseudomallei two-component regulatory system, termed irlRS, complemented the invasion-deficient and heavy-metal-sensitive phenotype of AJ1D8 in trans. There was no significant difference between the virulence of AJ1D8 and that of 1026b in infant diabetic rats and Syrian hamsters, suggesting that the irlRS locus is probably not a virulence determinant in these animal models of acute B. pseudomallei infection.

Mutagenesis of Burkholderia pseudomallei with Tn5-OT182: isolation of motility mutants and molecular characterization of the flagellin structural gene

Burkholderia pseudomallei is a human and animal pathogen in tropical regions, especially Southeast Asia and northern Australia. Currently little is known about the genetics and molecular biology of this organism. In this report, we describe the mutagenesis of B. pseudomallei with the transposon Tn5-OT182. B. pseudomallei 1026b transposon mutants were obtained at a frequency of 4.6 x 10(-4) per initial donor cell, and the transposon inserted randomly into the chromosome. We used Tn5-OT182 to identify the flagellin structural gene, fliC. We screened 3,500 transposon mutants and identified 28 motility mutants. Tn5-OT182 integrated into 19 unique genetic loci encoding proteins with homology to Escherichia coli and Salmonella typhimurium flagellar and chemotaxis proteins. Two mutants, MM35 and MM36, contained Tn5-OT182 integrations in fliC. We cloned and sequenced fliC and used it to complement MM35 and MM36 in trans. The fliC transcriptional start site and a sigmaF-like promoter were identified by primer extension analysis. We observed a significant difference in the expression of two distinct fliC-lacZ transcriptional fusions during bacterial growth, suggesting the presence of a latent intragenic transcriptional terminator in fliC. There was no significant difference in the virulence of 1026b compared to that of MM36 in diabetic rats or Syrian hamsters, suggesting that flagella and/or motility are probably not virulence determinants in these animal models of B. pseudomallei infection. A phylogenetic analysis based on the flagellins from a variety of bacterial species supported the recent transfer of B. pseudomallei from the genus Pseudomonas to Burkholderia.

Burkholderia pseudomallei activates complement and is ingested but not killed by polymorphonuclear leukocytes

The mechanism by which Burkholderia pseudomallei is resistant to lysis by human serum is unknown but may include interference with complement activation, effective opsonization, or complement-mediated lysis. We investigated the interaction of B. pseudomallei with complement in the presence and absence of specific antibody to determine potential mechanisms of serum resistance. We demonstrated rapid activation and consumption of complement by B. pseudomallei which, in the absence of specific antibody, occurred predominantly via the alternative pathway. Complement activation was associated with deposition of the opsonically active C3b and iC3b fragments on the bacterial surface. C5b-9, detected on the bacterial surface after opsonic periods of 1 to 60 min, was susceptible to elution by 1 M NaCl, indicating that resistance to complement-mediated lysis may result from deposition of the membrane attack complex in a nonmicrobicidal location. To define the role of opsonins, we investigated the ability of polymorphonuclear leukocytes (PMNL) to phagocytose B. pseudomallei. Phagocytosis of bacteria by PMNL, and the observed oxidative response, was significantly increased by opsonization of organisms with complement and/or specific antibody. Despite opsonophagocytosis by PMNL and the production of an oxidative response, no significant bacterial killing was observed.

Structural and immunological characterization of Burkholderia pseudomallei O-polysaccharide-flagellin protein conjugates

The O-polysaccharide moiety of Burkholderia pseudomallei 319a lipopolysaccharide was covalently linked to flagellin protein isolated from the same strain. A glycoconjugate incorporating adipic acid dihydrazide as a spacer molecule elicited high-titer immunoglobulin G responses to both the protein and carbohydrate components of the construct. This immunoglobulin G was capable of protecting diabetic rats from challenge with a heterologous B. pseudomallei strain.

Isolation and characterization of Pseudomonas pseudomallei flagellin proteins

Flagellin proteins from several different strains of Pseudomonas pseudomallei have been isolated and purified to homogeneity by mechanical shearing and differential centrifugation techniques. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis yielded flagellin monomer protein bands with an estimated M(r) of 43,400. No lipopolysaccharide contamination of the purified protein preparations was detectable by silver staining of flagellin displayed on polyacrylamide gels and by Western immunoblotting with P. pseudomallei antilipopolysaccharide monoclonal antibody. NH2-terminal amino acid sequence analysis of the flagellin protein of P. pseudomallei 319a revealed significant homology with flagellins from Proteus mirabilis, Bordetella bronchiseptica, and Pseudomonas aeruginosa PAK. Rabbit polyclonal antiserum raised against the 319a flagellin protein reacted with 64 of 65 P. pseudomallei strains tested. The polyclonal antiserum proved effective in inhibiting the motility of these organisms in motility agar plates. Passive immunization studies demonstrated that 319a flagellin-specific antiserum was capable of protecting diabetic rats from challenge with a heterologous P. pseudomallei strain.