Burkholderia pseudomallei kills Caenorhabditis elegans through virulence mechanisms distinct from intestinal lumen colonization

Hirudomacin: a Protein with Dual Effects of Direct Bacterial Inhibition and Regulation of Innate Immunity

Hirudomacin (Hmc) belongs to the Macin family of antimicrobial peptides, which can be used for bactericidal purposes in vitro by cleaving cell membranes. Although the Macin family has broad-spectrum antibacterial properties, few studies have been reported on bacterial inhibition by enhancing innate immunity. To further investigate the mechanism of Hmc inhibition, we chose the classical innate immune model organism Caenorhabditis elegans as the study subject. In this investigation, we found that Hmc treatment directly reduced the number of Staphylococcus aureus and Escherichia coli in the intestine of infected wild-type nematodes and infected pmk-1 mutant nematodes. Hmc treatment significantly prolonged the life span of infected wild-type nematodes and increased the expression of antimicrobial effectors (clec-82, nlp-29, lys-1, lys-7), and Hmc treatment still significantly increased the expression of antimicrobial effectors (clec-82, nlp-29, lys-7) in wild-type nematodes in the absence of bacterial stimulation. In addition, Hmc treatment significantly increased the expression of key genes of the pmk-1/p38 MAPK pathway (pmk-1, tir-1, atf-7, skn-1) under both infected and uninfected conditions but failed to increase the life span of infected pmk-1 mutant nematodes as well as the expression of antimicrobial effector genes. Western blot results further demonstrated that Hmc treatment significantly elevated pmk-1 protein expression levels in infected wild-type nematodes. In conclusion, our data suggest that Hmc has both direct bacteriostatic and immunomodulatory effects and may upregulate antimicrobial peptides in response to infection via the pmk-1/p38 MAPK pathway. It has the potential to serve as a new antibacterial agent and immune modulator. IMPORTANCE In today's world, bacterial drug resistance is becoming increasingly serious, and natural antibacterial proteins are attracting attention because of advantages such as their diverse and complex antibacterial modes, lack of residue, and harder-to-develop drug resistance. Notably, there are few antibacterial proteins with multiple effects such as direct antibacterial and innate immunity enhancement at the same time. We believe that an ideal antimicrobial agent can be developed only through a more comprehensive and in-depth study of the bacteriostatic mechanism of natural antibacterial proteins. The significance of our study is that based on the known in vitro bacterial inhibition of Hirudomacin (Hmc), we further clarified its mechanism in vivo, which can be subsequently developed as a natural bacterial inhibitor for various applications in medicine, food, farming, and daily chemicals.

Exposure to polystyrene nanoparticles at predicted environmental concentrations enhances toxic effects of Acinetobacter johnsonii AC15 infection on Caenorhabditis elegans

Nanoplastics and microbial pathogens are both widely distributed in the environment; however, their combined toxicity remains largely unclear. Using Caenorhabditis elegans as an animal model, we examined the possible effect of exposure to polystyrene nanoparticle (PS-NP) in Acinetobacter johnsonii AC15 (a bacterial pathogen) infected animals. Exposure to PS-NP at the concentrations of 0.1-10 μg/L significantly enhanced the toxicity of Acinetobacter johnsonii AC15 infection on lifespan and locomotion behaviors. In addition, after exposure to 0.1-10 μg/L PS-NP, the accumulation of Acinetobacter johnsonii AC15 in body of nematodes was also increased. Meanwhile, the innate immune response indicated by the increase of antimicrobial gene expressions in Acinetobacter johnsonii AC15 infected nematodes was suppressed by exposure to 0.1-10 μg/L PS-NP. Moreover, expressions of egl-1, dbl-1, bar-1, daf-16, pmk-1, and elt-2 governing the bacterial infection and immunity in Acinetobacter johnsonii AC15 infected nematodes were further inhibited by exposure to 0.1-10 μg/L PS-NP. Therefore, our data suggested the possible exposure risk of nanoplastic at predicted environmental concentrations in enhancing the toxic effects of bacterial pathogens on environmental organisms.

High-resolution imaging and analysis of the intestinal bacterial load of Caenorhabditis elegans during early adulthood

We study the presence within the worm Caenorhabditis elegans (C. elegans) of a fluorescent strain of the worm's bacterial food (Escherichia coli (E. coli) OP50) during early adulthood. Use of a microfluidic chip based on a thin glass coverslip substrate allows investigation of the intestinal bacterial load using a Spinning Disk Confocal Microscope (SDCM) equipped with a high-resolution objective (60×). High-resolution z-stack fluorescence images of the gut bacteria in adult worms, which were loaded in the microfluidic chip and subsequently fixed, were analyzed using IMARIS software and 3D reconstructions of the intestinal bacterial load in the worms were obtained. We present an automated bivariate histogram analysis of the volumes and intensities of the bacterial spots for each worm and find that, as the worms age, the bacterial load in their hindguts increases. We show the advantage of single-worm resolution automated analysis for bacterial load studies and anticipate that the methods described in our work can be easily implemented in existing microfluidic solutions to enable thorough studies of bacterial proliferation.

Salmonella enterica serovar Paratyphi A-induced immune response in Caenorhabditis elegans depends on MAPK pathways and DAF-16

Salmonella enterica serovar Paratyphi A (S. Paratyphi A) is a pathogen that can cause enteric fever. According to the recent epidemic trends of typhoid fever, S. Paratyphi A has been the major important causative factor in paratyphoid fever. An effective vaccine for S. Paratyphi A has not been developed, which made it a tricky public health concern. Until now, how S. Paratyphi A interacts with organisms remain unknown. Here using lifespan assay, we found that S. Paratyphi A could infect Caenorhabditis elegans (C. elegans) at 25°C, and attenuate thermotolerance. The immune response of C. elegans was mediated by tir-1, nsy-1, sek-1, pmk-1, mpk-1, skn-1, daf-2 and daf-16, suggesting that S. Paratyphi A could regulate the MAPK and insulin pathways. Furthermore, we observed several phenotypical changes when C. elegans were fed S. Paratyphi A, including an accelerated decline in body movement, reduced the reproductive capacity, shortened spawning cycle, strong preference for OP50, arrested pharyngeal pumping and colonization of the intestinal lumen. The virulence of S. Paratyphi A requires living bacteria and is not mediated by secreting toxin. Using hydrogen peroxide analysis and quantitative RT-PCR, we discovered that S. Paratyphi A could increase oxidative stress and regulate the immune response in C. elegans. Our results sheds light on the infection mechanisms of S. Paratyphi A and lays a foundation for drugs and vaccine development.

Genome-wide transposon mutagenesis analysis of Burkholderia pseudomallei reveals essential genes for in vitro and in vivo survival

Introduction

Burkholderia pseudomallei, a soil-dwelling microbe that infects humans and animals is the cause of the fatal disease melioidosis. The molecular mechanisms that underlie B. pseudomallei's versatility to survive within a broad range of environments are still not well defined.

Methods

We used the genome-wide screening tool TraDIS (Transposon Directed Insertion-site Sequencing) to identify B. pseudomallei essential genes. Transposon-flanking regions were sequenced and gene essentiality was assessed based on the frequency of transposon insertions within each gene. Transposon mutants were grown in LB and M9 minimal medium to determine conditionally essential genes required for growth under laboratory conditions. The Caenorhabditis elegans infection model was used to assess genes associated with in vivo B. pseudomallei survival. Transposon mutants were fed to the worms, recovered from worm intestines, and sequenced. Two selected mutants were constructed and evaluated for the bacteria's ability to survive and proliferate in the nematode intestinal lumen.

Results

Approximately 500,000 transposon-insertion mutants of B. pseudomallei strain R15 were generated. A total of 848,811 unique transposon insertion sites were identified in the B. pseudomallei R15 genome and 492 genes carrying low insertion frequencies were predicted to be essential. A total of 96 genes specifically required to support growth under nutrient-depleted conditions were identified. Genes most likely to be involved in B. pseudomallei survival and adaptation in the C. elegans intestinal lumen, were identified. When compared to wild type B. pseudomallei, a Tn5 mutant of bpsl2988 exhibited reduced survival in the worm intestine, was attenuated in C. elegans killing and showed decreased colonization in the organs of infected mice.

Discussion

The B. pseudomallei conditional essential proteins should provide further insights into the bacteria's niche adaptation, pathogenesis, and virulence.

Salmonella Typhimurium fepB negatively regulates C. elegans behavioral plasticity

OBJECTIVES

Dauer is an alternative developmental stage of Caenorhabditis elegans (C. elegans) that gives survival benefits under unfavorable environmental conditions. Our study aims to decipher C. elegans dauer larvae development upon Salmonella Typhimurium infection and how the bacterial gene regulating the worm's behavioural plasticity for better survival.

METHODS

Age-synchronized L4 C. elegans worms were infected with Salmonella Typhimurium 14028s (WT-STM) strain and mutant strains to check the dauer larvae development using 1% SDS. Besides, bacterial load in animals' gut, pharyngeal pumping rate and viability were checked. Worm's immune genes (e.g., ilys-3, lys-7, pmk-1, abf-2, clec-60) and dauer regulatory genes (e.g., daf-7, daf-11, daf-12, daf-16, daf-3) were checked by performing qRT-PCR under infection conditions.

RESULTS

We found that deletion of the fepB gene in S. Typhimurium strain became less pathogenic with reduced flagellar motility and biofilm-forming ability. Besides, there was decreased bacterial burden in the worm's gut with no damage to their pharynx. The fepB mutant strain was also able to enhance the immune responses for better survival of worms. Infection with mutant strain could activate dauer signaling via the TGF-β pathway leading to a significant increase in dauer formation than WT-STM infection.

CONCLUSION

Our study indicated that the bacteria act as a food source for the growth of C. elegans and development and can act as a signal that might be playing an essential role in regulating the host physiology for their survival. Such a study can help us in understanding the complex host-pathogen interaction benefiting pathogen in host dissemination.

Macrophage Identification In Situ

Understanding the processes of inflammation and tissue regeneration after injury is of great importance. For a long time, macrophages have been known to play a central role during different stages of inflammation and tissue regeneration. However, the molecular and cellular mechanisms by which they exert their effects are as yet mostly unknown. While in vitro macrophages have been characterized, recent progress in macrophage biology studies revealed that macrophages in vivo exhibited distinctive features. Actually, the precise characterization of the macrophages in vivo is essential to develop new healing treatments and can be approached via in situ analyses. Nowadays, the characterization of macrophages in situ has improved significantly using antigen surface markers and cytokine secretion identification resulting in specific patterns. This review aims for a comprehensive overview of different tools used for in situ macrophage identification, reporter genes, immunolabeling and in situ hybridization, discussing their advantages and limitations.

Cinnamomum cassia and Syzygium aromaticum Essential Oils Reduce the Colonization of Salmonella Typhimurium in an In Vivo Infection Model Using Caenorhabditis elegans

The regulation of intestinal colonization in livestock by means of non-bactericidal additives is an important management lever for zoonotic bacteria such as Salmonella spp. Caenorhabditis elegans is proposed here as a model for the evaluation of five essential oils (EOs) as anti-colonization products against Salmonella Typhimurium. An evaluation of the toxicity of EOs for C. elegans showed LD50 values ranging from 74.5 ± 9.6 µg/mL for Cinnamomum cassia (CEO) to 271.6 ± 14.9 µg/mL for Syzygium aromaticum (SyEO). Both EOs significantly inhibited bacterial colonization in the digestive tract of C. elegans with reductions of 0.88 and 0.70 log CFU/nematode at nontoxic concentrations of 50 µg/mL and 150 µg/mL, respectively. With the minimal bactericidal concentrations of CEO and SyEO against S. Typhimurium being 312.5 µg/mL and 625 µg/mL, respectively, an antibacterial effect can be excluded to explain the inhibition of the bacterial load. The anti-colonizing activity of these two EOs could, however, be related to an inhibition of the swimming motility, which was significantly reduced by 23.47% for CEO at 50 µg/mL and 19.56% for SyEO at 150 µg/mL. This study shows the potential of C. elegans as a predictive in vivo model of anti-colonizing activities that is suitable for the evaluation of essential oils.

Evaluation of changes in C. elegans immune response during bacterial infection: A single nematode approach

Routinely, studies were performed using age-synchronized group of C. elegans as host which suggested a collective response by the host system. Here, we report the modulation of immune response in a single nematode against Staphylococcus aureus and Proteus mirabilis. Initially, the survival of wild-type N2 was tested and was found that S. aureus killed single nematode at 42 h while P. mirabilis failed to provoke infection but colonized the nematode's intestine. With this milieu, the pathogenicity of the bacteria was assessed by Fourier Transform Infra-Red (FTIR) spectroscopy and Cyclic Voltammetry (CV) and was found that S. aureus in the presence of host elicited its virulence while P. mirabilis and Escherichia coli OP50 did not show any alteration. Vertical transmission of infection was also deduced by colony forming unit assay using Cyanine dyes. The MALDI-TOF/TOF analysis was also performed to identify the proteome changes in the single nematode that showcased different proteins related to various immune pathways. This study suggested the importance of understanding the infection pathology and traits of individual nematode which could help our understanding on otherwise the disordered processes during host and microbe interactions.

Ubiquitin-related processes and innate immunity in C. elegans

Innate immunity is an evolutionary ancient defence strategy that serves to eliminate infectious agents while maintaining host health. It involves a complex network of sensors, signaling proteins and immune effectors that detect the danger, then relay and execute the immune programme. Post-translational modifications relying on conserved ubiquitin and ubiquitin-like proteins are an integral part of the system. Studies using invertebrate models of infection, such as the nematode Caenorhabditis elegans, have greatly contributed to our understanding of how ubiquitin-related processes act in immune sensing, regulate immune signaling pathways, and participate to host defence responses. This review highlights the interest of working with a genetically tractable model organism and illustrates how C. elegans has been used to identify ubiquitin-dependent immune mechanisms, discover novel ubiquitin-based resistance strategies that mediate pathogen clearance, and unravel the role of ubiquitin-related processes in tolerance, preserving host fitness during pathogen attack. Special emphasis is placed on processes that are conserved in mammals.

Worms eat oil: Alcanivorax borkumensis hydrocarbonoclastic bacteria colonise Caenorhabditis elegans nematodes intestines as a first step towards oil spills zooremediation

The environmental hazards of oil spills cannot be underestimated. Bioremediation holds promise among various approaches to tackle oil spills in soils and sediments. In particular, using oil-degrading bacteria is an efficient and self-regulating way to remove oil spills. Using animals for oil spills remediation is in its infancy, mostly due to the lack of efficient oil-degrading capabilities in eukaryotes. Here we show that Caenorhabditis elegans nematodes survive for extended periods (up to 22 days) on pure crude oil diet. Moreover, we report for the first time the use of Alcanivorax borkumensis hydrocarbonoclastic bacteria for colonisation of C. elegans intestines, which allows for effective digestion of crude oil by the nematodes. The worms fed and colonised by A. borkumensis demonstrated the similar or even better longevity, resistance against oxidative and thermal stress and reproductivity as those animals fed with Escherichia coli bacteria (normal food). Importantly, A. borkumensis-carrying nematodes were able to accumulate oil droplet from oil-contaminated soils. Artificial colonisation of soil invertebrates with oil-degrading bacteria will be an efficient way to distribute microorganisms in polluted soil, thus opening new avenues for oil spills zooremediation.

TOR functions as a molecular switch connecting an iron cue with host innate defense against bacterial infection

As both host and pathogen require iron for survival, iron is an important regulator of host-pathogen interactions. However, the molecular mechanism by which how the availability of iron modulates host innate immunity against bacterial infections remains largely unknown. Using the metazoan Caenorhabditis elegans as a model, we demonstrate that infection with a pathogenic bacterium Salmonella enterica serovar Typhimurium induces autophagy by inactivating the target of rapamycin (TOR). Although the transcripts of ftn-1 and ftn-2 encoding two H-ferritin subunits are upregulated upon S. Typhimurium infection, the ferritin protein is kept at a low level due to its degradation mediated by autophagy. Autophagy, but not ferritin, is required for defense against S. Typhimurium infection under normal circumstances. Increased abundance of iron suppresses autophagy by activating TOR, leading to an increase in the ferritin protein level. Iron sequestration, but not autophagy, becomes pivotal to protect the host from S. Typhimurium infection in the presence of exogenous iron. Our results show that TOR acts as a regulator linking iron availability with host defense against bacterial infection.

Iron, an essential nutrient for both hosts and pathogens, can impact host-pathogen interactions. Thus, depending on availability of iron, hosts may use distinct strategies to defend against bacterial infections. Using the model organism Caenorhabditis elegans, we show that autophagy is activated and required for resistance to S. Typhimurium infection under normal conditions. In the presence of exogenous iron, autophagy is suppressed, leading to an increase in the protein level of ferritin, which in turn protects worms against S. Typhimurium infection. Finally, our data demonstrate that TOR is a molecular switch for turning autophagy and ferritin on and off. Thus, our study reveals a previously unknown role of TOR in modulating the transition between autophagy and iron sequestration in response to a bacterial infection via sensing an iron cue.

Burkholderia pseudomallei OMVs derived from infection mimicking conditions elicit similar protection to a live-attenuated vaccine

Burkholderia pseudomallei is a Gram-negative, facultative intracellular bacillus that causes the disease melioidosis. B. pseudomallei expresses a number of proteins that contribute to its intracellular survival in the mammalian host. We previously demonstrated that immunization with OMVs derived from B. pseudomallei grown in nutrient-rich media protects mice against lethal disease. Here, we evaluated if OMVs derived from B. pseudomallei grown under macrophage-mimicking growth conditions could be enriched with intracellular-stage proteins in order to improve the vaccine. We show that OMVs produced in this manner (M9 OMVs) contain proteins associated with intracellular survival yet are non-toxic to living cells. Immunization of mice provides significant protection against pulmonary infection similar to that achieved with a live attenuated vaccine and is associated with increased IgG, CD4⁺, and CD8⁺ T cells. OMVs possess inherent adjuvanticity and drive DC activation and maturation. These results indicate that M9 OMVs constitute a new promising vaccine against melioidosis.

Cholesterol Regulates Innate Immunity via Nuclear Hormone Receptor NHR-8

Cholesterol is an essential nutrient for the function of diverse biological processes and for steroid biosynthesis across metazoans. However, the role of cholesterol in immune function remains understudied. Using the nematode Caenorhabditis elegans, which depends on the external environment for cholesterol, we studied the relationship between cholesterol and innate immunity. We found that the transporter CHUP-1 is required for the effect of cholesterol in the development of innate immunity and that the cholesterol-mediated immune response requires the nuclear hormone receptor NHR-8. Cholesterol acts through NHR-8 to transcriptionally regulate immune genes that are controlled by conserved immune pathways, including a p38/PMK-1 MAPK pathway, a DAF-2/DAF-16 insulin pathway, and an Nrf/SKN-1 pathway. Our results indicate that cholesterol plays a key role in the activation of conserved microbicidal pathways that are essential for survival against bacterial infections.

Distinct Pathogenic Patterns of Burkholderia pseudomallei Isolates Selected from Caenorhabditis elegans and Dictyostelium discoideum Models

Burkholderia pseudomallei is a selective agent that causes septic melioidosis and exhibits a broad range of lethal doses in animals. Host cellular virulence and phagocytic resistance are pathologic keys of B. pseudomallei. We first proposed Caenorhabditis elegans as the host cellular virulence model to mimic bacterial virulence against mammals and second established the resistance of B. pseudomallei to predation by Dictyostelium discoideum as the phagocytosis model. The saprophytic sepsis-causing Burkholderia sp. (B. pseudomallei, Burkholderia thailandensis, Burkholderia cenocepacia, and Burkholderia multivorans) exhibited different virulence patterns in both simple models, but B. pseudomallei was the most toxic. Using both models, attenuated isolates of B. pseudomallei were selected from a transposon-mutant library and a panel of environmental isolates and reconfirmed by in vitro mouse peritoneal exudate cell association and invasion assays. The distinct pathological patterns of melioidosis were inducted by different selected B. pseudomallei isolates. Fatal melioidosis was induced by the isolates with high virulence in both simple models within 4-5 day, whereas the low-virulence isolates resulted in prolonged survival greater than 30 day. Infection with the isolates having high resistance to D. discoideum predation but a low C. elegans killing effect led to 83% of mice with neurologic melioidosis. By contrast, infection with the isolates having low resistance to D. discoideum predation but high C. elegans killing effect led to 20% cases with inflammation in the salivary glands. Our results indicated that individual B. pseudomallei isolates selected from simple biological models contribute differently to disease progression and/or tissue tropism.

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.

Intra Strain Variation of the Effects of Gram-Negative ESKAPE Pathogens on Intestinal Colonization, Host Viability, and Host Response in the Model Organism Caenorhabditis elegans

In its native environment of rotting vegetation, the soil nematode Caenorhabditis elegans encounters a range of bacteria. This includes species from the ESKAPE group of pathogens that pose a clinical problem in acquired hospital infections. Here, we investigated three Gram-negative members of the ESKAPE group, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii. Pathogenicity profiles as measured by time to kill adult C. elegans showed that P. aeruginosa was the most pathogenic, followed by K. pneumoniae, while C. elegans cultured on A. baumannii exhibited the same survival as those on the standard laboratory food source for C. elegans, Escherichia coli OP50. The pathogenicity was paralleled by a reduction in time that C. elegans resided on the bacterial lawn with the most pathogenic strains triggering an increase in the frequency of food-leaving. Previous reports indicate that gut colonization is a feature of pathogenicity, but we found that the most pathogenic strains were not associated with the highest level of colonization. Indeed, clearance of P. aeruginosa strains from the C. elegans gut was independent of bacterial pathogenicity. We show that this clearance is regulated by neuromodulation as C. elegans mutants in unc-31 and egl-3 have enhanced clearance of P. aeruginosa. Intriguingly this is also not linked to their pathogenicity. It is likely that there is a dynamic balance occurring in the C. elegans intestinal environment between maintaining a healthy, beneficial microbiota and removal of pathogenic bacteria.

Malleilactone Is a Burkholderia pseudomallei Virulence Factor Regulated by Antibiotics and Quorum Sensing

Burkholderia pseudomallei, the causative agent of melioidosis, encodes almost a dozen predicted polyketide (PK) biosynthetic gene clusters. Many of these are regulated by LuxR-I-type acyl-homoserine (AHL) quorum-sensing systems. One of the PK gene clusters, the mal gene cluster, is conserved in the close relative Burkholderia thailandensis The B. thailandensis mal genes code for the cytotoxin malleilactone and are regulated by a genetically linked LuxR-type transcription factor, MalR. Although AHLs typically interact with LuxR-type proteins to modulate gene transcription, the B. thailandensis MalR does not appear to be an AHL receptor. Here, we characterize the mal genes and MalR in B. pseudomallei We use chemical analyses to demonstrate that the B. pseudomallei mal genes code for malleilactone. Our results show that MalR and the mal genes contribute to the ability of B. pseudomallei to kill Caenorhabditis elegans In B. thailandensis, antibiotics like trimethoprim can activate MalR by driving transcription of the mal genes, and we demonstrate that some of the same antibiotics induce expression of B. pseudomallei malR We also demonstrate that B. pseudomallei MalR does not respond directly to AHLs. Our results suggest that MalR is indirectly repressed by AHLs, possibly through a repressor, ScmR. We further show that malleilactone is a B. pseudomallei virulence factor and provide the foundation for understanding how malleilactone contributes to the pathology of melioidosis infections.IMPORTANCE Many bacterially produced polyketides are cytotoxic to mammalian cells and are potentially important contributors to pathogenesis during infection. We are interested in the polyketide gene clusters present in Burkholderia pseudomallei, which causes the often-fatal human disease melioidosis. Using knowledge gained by studies in the close relative Burkholderia thailandensis, we show that one of the B. pseudomallei polyketide biosynthetic clusters produces a cytotoxic polyketide, malleilactone. Malleilactone contributes to B. pseudomallei virulence in a Caenorhabditis elegans infection model and is regulated by an orphan LuxR family quorum-sensing transcription factor, MalR. Our studies demonstrate that malleilactone biosynthesis or MalR could be new targets for developing therapeutics to treat melioidosis.

Genetic Determinants Associated With in Vivo Survival of Burkholderia cenocepacia in the Caenorhabditis elegans Model

A Burkholderia cenocepacia infection usually leads to reduced survival and fatal cepacia syndrome in cystic fibrosis patients. The identification of B. cenocepacia essential genes for in vivo survival is key to designing new anti-infectives therapies. We used the Transposon-Directed Insertion Sequencing (TraDIS) approach to identify genes required for B. cenocepacia survival in the model infection host, Caenorhabditis elegans. A B. cenocepacia J2315 transposon pool of ∼500,000 mutants was used to infect C. elegans. We identified 178 genes as crucial for B. cenocepacia survival in the infected nematode. The majority of these genes code for proteins of unknown function, many of which are encoded by the genomic island BcenGI13, while other gene products are involved in nutrient acquisition, general stress responses and LPS O-antigen biosynthesis. Deletion of the glycosyltransferase gene wbxB and a histone-like nucleoid structuring (H-NS) protein-encoding gene (BCAL0154) reduced bacterial accumulation and attenuated virulence in C. elegans. Further analysis using quantitative RT-PCR indicated that BCAL0154 modulates B. cenocepacia pathogenesis via transcriptional regulation of motility-associated genes including fliC, fliG, flhD, and cheB1. This screen has successfully identified genes required for B. cenocepacia survival within the host-associated environment, many of which are potential targets for developing new antimicrobials.

Melioidosis in Malaysia: Incidence, Clinical Challenges, and Advances in Understanding Pathogenesis

Malaysia is an endemic hot spot for melioidosis; however, a comprehensive picture of the burden of disease, clinical presentations, and challenges faced in diagnosis and treatment of melioidosis is not available. This review provides a nonexhaustive overview of epidemiological data, clinical studies, risk factors, and mortality rates from available literature and case reports. Clinical patterns of melioidosis are generally consistent with those from South and Southeast Asia in terms of common primary presentations with diabetes as a major risk factor. Early diagnosis and appropriate management of Malaysian patients is a key limiting factor, which needs to be addressed to reduce serious complications and high mortality and recurrence rates. Promoting awareness among the local healthcare personnel is crucial to improving diagnostics and early treatment, as well as educating the Malaysian public on disease symptoms and risk factors. A further matter of urgency is the need to make this a notifiable disease and the establishment of a national melioidosis registry. We also highlight local studies on the causative agent, Burkholderia pseudomallei, with regards to bacteriology and identification of virulence factors as well as findings from host⁻pathogen interaction studies. Collectively, these studies have uncovered new correlations and insights for further understanding of the disease.

Nematode Peptides with Host-Directed Anti-inflammatory Activity Rescue Caenorhabditis elegans from a Burkholderia pseudomallei Infection

Burkholderia pseudomallei, the causative agent of melioidosis, is among a growing number of bacterial pathogens that are increasingly antibiotic resistant. Antimicrobial peptides (AMPs) have been investigated as an alternative approach to treat microbial infections, as generally, there is a lower likelihood that a pathogen will develop resistance to AMPs. In this study, 36 candidate Caenorhabditis elegans genes that encode secreted peptides of <150 amino acids and previously shown to be overexpressed during infection by B. pseudomallei were identified from the expression profile of infected nematodes. RNA interference (RNAi)-based knockdown of 12/34 peptide-encoding genes resulted in enhanced nematode susceptibility to B. pseudomallei without affecting worm fitness. A microdilution test demonstrated that two peptides, NLP-31 and Y43C5A.3, exhibited anti-B. pseudomallei activity in a dose dependent manner on different pathogens. Time kill analysis proposed that these peptides were bacteriostatic against B. pseudomallei at concentrations up to 8× MIC90. The SYTOX green assay demonstrated that NLP-31 and Y43C5A.3 did not disrupt the B. pseudomallei membrane. Instead, gel retardation assays revealed that both peptides were able to bind to DNA and interfere with bacterial viability. In parallel, microscopic examination showed induction of cellular filamentation, a hallmark of DNA synthesis inhibition, of NLP-31 and Y43C5A.3 treated cells. In addition, the peptides also regulated the expression of inflammatory cytokines in B. pseudomallei infected macrophage cells. Collectively, these findings demonstrate the potential of NLP-31 and Y43C5A.3 as anti-B. pseudomallei peptides based on their function as immune modulators.

Detection of Burkholderia pseudomallei toxin-mediated inhibition of protein synthesis using a Caenorhabditis elegans ugt-29 biosensor

Toxins are believed to play a crucial role in Burkholderia pseudomallei pathogenicity, however to date, only a few have been identified. The discovery of additional toxic molecules is limited by the lack of a sensitive indicator of B. pseudomallei toxicity. Previously, from a whole genome transcriptome analysis of B. pseudomallei-infected Caenorhabditis elegans, we noted significant overexpression of a number of worm genes encoding detoxification enzymes, indicating the host's attempt to clear bacterial toxic molecules. One of these genes, ugt-29, a family member of UDP-glucuronosyltransferases, was the most robustly induced phase II detoxification gene. In this study, we show that strong induction of ugt-29 is restricted to infections by the most virulent species among the pathogens tested. We also noted that ugt-29 is activated upon disruption of host protein synthesis. Hence, we propose that UGT-29 could be a promising biosensor to detect B. pseudomallei toxins that compromise host protein synthesis. The identification of bactobolin, a polyketide-peptide hybrid molecule, as a toxic molecule of B. pseudomallei further verifies the utilization of this surveillance system to search for bacterial toxins. Hence, a ugt-29 based reporter should be useful in screening for other molecules that inhibit host protein synthesis.

Circumvention of Learning Increases Intoxication Efficacy of Nematicidal Engineered Bacteria

Synthetic biology holds promise to engineer systems to treat diseases. One critical, yet underexplored, facet of designing such systems is the interplay between the system and the pathogen. Understanding this interplay may be critical to increasing efficacy and overcoming resistance against the system. Using the principles of synthetic biology, we engineer a strain of Escherichia coli to attract and intoxicate the nematode Caenorhabditis elegans. Our bacteria are engineered with a toxin module, which intoxicates the nematode upon ingestion, and an attraction module, which serves to attract and increase the feeding rate of the nematodes. When independently implemented, these modules successfully intoxicate and attract the worms, respectively. However, in combination, the efficacy of our bacteria is significantly reduced due to aversive associative learning in C. elegans. Guided by mathematical modeling, we dynamically regulate module induction to increase intoxication by circumventing learning. Our results detail the creation of a novel nematicidal bacterium that may have application against nematodes, unravel unique constraints on circuit dynamics that are governed by C. elegans physiology, and add to the growing list of design and implementation considerations associated with synthetic biology.

Investigating the Role of the Host Multidrug Resistance Associated Protein Transporter Family in Burkholderia cepacia Complex Pathogenicity Using a Caenorhabditis elegans Infection Model

This study investigated the relationship between host efflux system of the non-vertebrate nematode Caenorhabditis elegans and Burkholderia cepacia complex (Bcc) strain virulence. This is the first comprehensive effort to profile host-transporters within the context of Bcc infection. With this aim, two different toxicity tests were performed: a slow killing assay that monitors mortality of the host by intestinal colonization and a fast killing assay that assesses production of toxins. A Virulence Ranking scheme was defined, that expressed the toxicity of the Bcc panel members, based on the percentage of surviving worms. According to this ranking the 18 Bcc strains were divided in 4 distinct groups. Only the Cystic Fibrosis isolated strains possessed profound nematode killing ability to accumulate in worms’ intestines. For the transporter analysis a complete set of isogenic nematode single Multidrug Resistance associated Protein (MRP) efflux mutants and a number of efflux inhibitors were interrogated in the host toxicity assays. The Bcc pathogenicity profile of the 7 isogenic C. elegans MRP knock-out strains functionality was classified in two distinct groups. Disabling host transporters enhanced nematode mortality more than 50% in 5 out of 7 mutants when compared to wild type. In particular mrp-2 was the most susceptible phenotype with increased mortality for 13 out 18 Bcc strains, whereas mrp-3 and mrp-4 knock-outs had lower mortality rates, suggesting a different role in toxin-substrate recognition. The use of MRP efflux inhibitors in the assays resulted in substantially increased (>40% on average) mortality of wild-type worms.

Effects of Colonization of the Roots of Domestic Rice (Oryza sativa L. cv. Amaroo) by Burkholderia pseudomallei

Burkholderia pseudomallei is a saprophytic bacterium that causes melioidosis and is often isolated from rice fields in Southeast Asia, where the infection incidence is high among rice field workers. The aim of this study was to investigate the relationship between this bacterium and rice through growth experiments where the effect of colonization of domestic rice (Oryza sativa L. cv Amaroo) roots by B. pseudomallei could be observed. When B. pseudomallei was exposed to surface-sterilized seeds, the growth of both the root and the aerosphere was retarded compared to that in controls. The organism was found to localize in the root hairs and endodermis of the plant. A biofilm formed around the root and root structures that were colonized. Growth experiments with a wild rice species (Oryza meridionalis) produced similar retardation of growth, while another domestic cultivar (O. sativa L. cv Koshihikari) did not show retarded growth. Here we report B. pseudomallei infection and inhibition of O. sativa L. cv Amaroo, which might provide insights into plant interactions with this important human pathogen.

Curcumin rescues Caenorhabditis elegans from a Burkholderia pseudomallei infection

The tropical pathogen Burkholderia pseudomallei requires long-term parenteral antimicrobial treatment to eradicate the pathogen from an infected patient. However, the development of antibiotic resistance is emerging as a threat to this form of treatment. To meet the need for alternative therapeutics, we proposed a screen of natural products for compounds that do not kill the pathogen, but in turn, abrogate bacterial virulence. We suggest that the use of molecules or compounds that are non-bactericidal (bacteriostatic) will reduce or abolish the development of resistance by the pathogen. In this study, we adopted the established Caenorhabditis elegans-B. pseudomallei infection model to screen a collection of natural products for any that are able to extend the survival of B. pseudomallei infected worms. Of the 42 natural products screened, only curcumin significantly improved worm survival following infection whilst not affecting bacterial growth. This suggested that curcumin promoted B. pseudomallei-infected worm survival independent of pathogen killing. To validate that the protective effect of curcumin was directed toward the pathogen, bacteria were treated with curcumin prior to infection. Worms fed with curcumin-treated bacteria survived with a significantly extended mean-time-to-death (p < 0.0001) compared to the untreated control. In in vitro assays, curcumin reduced the activity of known virulence factors (lipase and protease) and biofilm formation. To determine if other bacterial genes were also regulated in the presence of curcumin, a genome-wide transcriptome analysis was performed on curcumin-treated pathogen. A number of genes involved in iron acquisition and transport as well as genes encoding hypothetical proteins were induced in the presence of curcumin. Thus, we propose that curcumin may attenuate B. pseudomallei by modulating the expression of a number of bacterial proteins including lipase and protease as well as biofilm formation whilst concomitantly regulating iron transport and other proteins of unknown function.

Alterations in Caenorhabditis elegans and Cronobacter sakazakii lipopolysaccharide during interaction

Lipopolysaccharide is one of the pathogen-associated molecular patterns of Gram-negative bacteria which are essential for its pathogenicity. Cronobacter sakazakii is an opportunistic, emergent pathogen, which infects and cause mortality in Caenorhabditis elegans. In this study, modifications in host and C. sakazakii LPS during infections were evaluated. The physiological assays revealed that LPS alone is sufficient to affect the host pharyngeal pumping rate, brood size and cause lethality. FTIR spectra of LPS revealed that C. sakazakii modifies its LPS to escape from the recognition of host immune system. These results indicate that LPS plays a key role in C. sakazakii pathogenicity. qPCR studies revealed that LPS modulated the expression of selected host immune (clec-60, clec-87, lys-7, ilys-3, F08G5.6, atf-7, scl-2, cpr-2) and aging-related genes (skn-1, clk-2, bra-2, age-1, bec-1, daf-16, daf-2). Moreover, it was confirmed that p38 MAPK pathway has a major role in host immune response against LPS-mediated challenges.

Modulation of Caenorhabditis elegans immune response and modification of Shigella endotoxin upon interaction

To analyze the pathogenesis at both physiological and molecular level using the model organism, Caenorhabditis elegans at different developmental stages in response to Shigella spp. and its pathogen associated molecular patterns such as lipopolysaccharide. The solid plate and liquid culture-based infection assays revealed that Shigella spp. infects C. elegans and had an impact on the brood size and pharyngeal pumping rate. LPS of Shigella spp. was toxic to C. elegans. qPCR analysis revealed that host innate immune genes have been modulated upon Shigella spp. infections and its LPS challenges. Non-destructive analysis was performed to kinetically assess the alterations in LPS during interaction of Shigella spp. with C. elegans. The modulation of innate immune genes attributed the surrendering of host immune system to Shigella spp. by favoring the infection. LPS appeared to have a major role in Shigella-mediated pathogenesis and Shigella employs a tactic behavior of modifying its LPS content to escape from the recognition of host immune system.

Orthosiphon stamineus protects Caenorhabditis elegans against Staphylococcus aureus infection through immunomodulation

Amidst growing concerns over the spread of antibiotic-resistant Staphylococcus aureus strains, the identification of alternative therapeutic molecules has become paramount. Previously, we utilized a Caenorhabditis elegans–S. aureus screening platform to identify potential anti-infective agents from a collection of natural extracts and synthetic compounds. One of the hits obtained from the screen was the aqueous extract of Orthosiphon stamineus leaves (UE-12) that enhanced the survival of infected nematodes without interfering with bacterial growth. In this study, we used a fluorescent transgenic reporter strain and observed that the repressed expression of the lys-7 defense gene in infected nematodes was restored in the presence of UE-12. Analysis of a selected panel of PMK-1 and DAF-16-regulated transcripts and loss-of-function mutants in these pathways indicates that the protective role of UE-12 is mediated via the p38 MAP kinase and insulin-like signaling pathways. Further analysis of a panel of known bioactive compounds of UE-12 proposed eupatorin (C₁₈H₁₆O₇) as the possible candidate active molecule contributing to the anti-infective property of UE-12. Taken together, these findings strongly suggest that the O. stamineus leaf extract is a promising anti-infective agent that confers an advantage in survival against S. aureus infection by modulating the immune response of the infected host.

Spread and transmission of bacterial pathogens in experimental populations of the nematode Caenorhabditis elegans

Caenorhabditis elegans is frequently used as a model species for the study of bacterial virulence and innate immunity. In recent years, diverse mechanisms contributing to the nematode's immune response to bacterial infection have been discovered. Yet despite growing interest in the biochemical and molecular basis of nematode-bacterium associations, many questions remain about their ecology. Although recent studies have demonstrated that free-living nematodes could act as vectors of opportunistic pathogens in soil, the extent to which worms may contribute to the persistence and spread of these bacteria has not been quantified. We conducted a series of experiments to test whether colonization of and transmission between C. elegans nematodes could enable two opportunistic pathogens (Salmonella enterica and Pseudomonas aeruginosa) to spread on agar plates occupied by Escherichia coli. We monitored the transmission of S. enterica and P. aeruginosa from single infected nematodes to their progeny and measured bacterial loads both within worms and on the plates. In particular, we analyzed three factors affecting the dynamics of bacteria: (i) initial source of the bacteria, (ii) bacterial species, and (iii) feeding behavior of the host. Results demonstrate that worms increased the spread of bacteria through shedding and transmission. Furthermore, we found that despite P. aeruginosa's relatively high transmission rate among worms, its pathogenic effects reduced the overall number of worms colonized. This study opens new avenues to understand the role of nematodes in the epidemiology and evolution of pathogenic bacteria in the environment.

Discovery of potential anti-infectives against Staphylococcus aureus using a Caenorhabditis elegans infection model

Background

The limited antibiotic options for effective control of methicillin-resistant Staphylococcus aureus infections has led to calls for new therapeutic approaches to combat this human pathogen. An alternative approach to control MRSA is through the use of anti-infective agents that selectively disrupt virulence-mediated pathways without affecting microbial cell viability or by modulating the host natural immune defenses to combat the pathogen.

Methods

We established a C. elegans – S. aureus liquid-based assay to screen for potential anti-infectives against S. aureus. The assay was utilized to screen 37 natural extracts and 29 synthetic compounds for the ability to extend the lifespan of infected nematodes. Disc diffusion and MIC microdilution tests were used to evaluate the anti-microbial properties of these natural extracts and synthetic compounds whilst in vivo bacterial CFU within the C. elegans gut were also enumerated.

Results

We screened a total of 37 natural extracts and 29 synthetic compounds for anti-infective properties. The screen successfully revealed 14 natural extracts from six plants (Nypa fruticans, Swietenia macrophylla, Curcuma longa, Eurycoma longifolia, Orthosiphon stamineus and Silybum eburneum) and one marine sample (Faunus ater) that improved the survival of S. aureus-infected worms by at least 2.8-fold as well as 14 synthetic compounds that prolonged the survival of S. aureus-infected nematodes by 4-fold or greater. An anti-microbial screen of all positive hits demonstrated that 8/28 hits had no effect on S. aureus growth. Of these 8 candidates, 5 of them also protected the worms from MRSA infection. We also noted that worms exposed to N. fruticans root and O. stamineus leaf extracts showed reduced intestinal colonization by live S. aureus. This suggests that these extracts could possibly activate host immunity to eliminate the bacteria or interfere with factor/s that prevents pathogen accumulation.

Conclusion

We have successfully demonstrated the utility of this liquid-based screen to identify anti-infective substances that prolong S. aureus-infected host survival without affecting bacterial cell viability.